I have. Many times. In fact I've done it when making a video with Derek of Veritasium. Your explanation of how a bike rights itself, due mostly to its steering geometry, is correct. The low center of mass in that case is a red herring. In the case of the tightrope walker, I'm 99.3% certain it's not a center of mass issue either. It would be very easy to test this with either a rigid pole, or a pole that's supported with cables as we used to do with the leading edges of hang gliders. In this case I'm nearly certain the tightrope walker is simply utilizing a device with a very high moment of inertia. If you Google tightrope, you'll see that, not only is the C.G. above the tightrope, but no part of the system is lower than the tightrope. Just as it's much easier to balance a long pole vertically on your hand, it's also easier to balance on a tightrope with a long pole (both reduce the natural response of the system down to something easily within human limits).
Also, contrary to intuition, a higher C.G. on a bike or motorcycle will not make it less stable. We have to be careful here to distinguish between static and dynamic stability. In the case of tightrope walkers and bikes, it's always dynamic stability we're considering. Neither will be stable if simply "balanced" at rest and left rigid. But in the case of the bike, the higher C.G. will give the system a greater moment of inertia about its rolling pivot point (where the tires touch the ground) and therefore slow the response down further, making it easier to ride.
@@Riccofiori >> If the rider has a comparatively high center of mass, as the bicycle goes slower, it becomes even more unstable A higher center of mass does not make the system more unstable. >> The rider can easily lower the center of mass by standing on the pedals Standing on the pedals only changes where the rider's weight is applied to the bike. It does not change the center of mass of the system.
@@Rick_Cavallaro The issue isn't mass location, but the fact that when the bicycle leans, steering into the fall allows the CM to stay low. NOT steering in that direction raises the CM, so the natural tendency of the bike is to steer in a way that doesn't raise it. It's like saying "water doesn't flow uphill." Duh!
I believe the angle of the forks on the bicycle also factor into the 'Ghost Bike' ability to self steer. Kinda like the toe and caster on the steering on a motor vehicle to self straighten the steering wheel (when properly set up). Great topic, thanks for sharing.
@@lesliefranklin1870 have to check my bicycles because I usually can ride handsfree but I have a single bike which is quite hard to control. Maybe it's because the handlebar isn't centered correctly
The rake (amount of forward bend) on a bike differs for thw use. A cruising bicycle has a long rake and is more stable. A racing bike tends to has a short fork so that you can turn quicker . A bike specially designed for bicycle polo has straight forks - you can turn very fast - and a fixed gear so you can pedal backwards
You are close but no cigar. It is trail that give stability. Angling forks is one way to achieve trail, but you can have vertical forks and still bui,d in trail to the bike geometry.
Studying advanced motorcycle handling teaches a combination of how center mass, speed, and friction points interact to keep a bike on track and in control for given conditions. There are all kinds of forces at work, then you add in what changes with applying the brakes or throttle at any given lean angle plus speed. All of this is acting on a contact point between the tire and the road about the size of a credit card. It is really a great exercise in physics and engineering once you realize how complex a system it really is.
The tendency of a bike to balance itself is partly due to the rake of the front wheel that is, the angle at which the fork holds the wheel. I experimented with my bike by turning the front wheel completely around thereby placing the centre of the wheel directly under the handlebars. This made it practically impossible to ride. Try it!
Yeah, that's what causes the wheel to steer in the direction of a roll disturbance which is what he was essentially describing. The main reason it works is because the vertical tire force (wheel load) is behind the steer axis. So if the bike is not perfectly vertical with perfectly centered steering, that vertical force causes a torque to act around that axis which steers the wheel in the direction of turn. There is some gyro effect adding to this in the same direction though which might be negligible at low speeds, but might not be on a racing bike at high speeds. It's not really just one or the other, it's both. Just a question of which one is stronger at which speed and with which tire and which motorcycle... In your very cool experiment you reversed it and put the load ahead of the steering axis so the torque is reversed. Leaning to the right would cause it to steer left and make you lean to the right even faster. Totally unstable as you know. It'd take some serious brain rewiring to control that! 😁
@@toddwasson3355 Yeah the lowest center of balance occurs when the bike is laying down and really has nothing to do with balancing a bike. That's why they fall over when they're not moving Road racing bikes have the engine higher in the frame to allow the steering geometry to be more responsive and enables the change from a left turn lean to a right turn lean to happen quicker, and to have the bottom of the bike high enough for the extreme lean. One interesting point is the method used to initiate a turn on a racing motorcycle or any bike really, if you just lean the bike, you start to turn that direction. If you want to start a lean quickly, you push the handlebars in the opposite direction. As soon as the front wheel goes that way, the lean is happening real quick and the steering balancing geometry takes over. It sounds crazy, but there you go. You can try it, it works. Especially if you're leaned into a turn real hard already, think about it. To switch up, and turn the other way, you've gotta get the wheels on the other side of the bike so you turn them that way, harder into the turn, the bike stands up and leans the other way. I had to try, I didn't believe it either. It changes things really quick. Wheels and tires are lighter than engines, they can switch sides faster. If the center of gravity is too low, it's too stable, and you turn slower, and can't lean far enough. All that being said, steering geometry does lower the center of gravity when it's upright and straight until it leans, whereupon it turns to balance.
Its less about center of mass and more due to a few aspects of bike geometry, specifically head tube angle and the mechanical trail measurement. These can vary greatly between bikes but they have the most to do with a bike’s tendency to right itself.
When the bike is *moving*. Neither of these were discussed in the video. The question was never answered. I'm learning quite a bit from the comments though; never occurred to me that frame geometry had to do with stability.
Neil on your example of the bike with the wheels that spin backwards it doesn't cancel out the gyroscope factor. Because that video with the wheel spinning and you can levitate it from one side. That wheel doesn't care which direction it spins the angular momentum still holds it upright. So the bike no matter which direction the wheels spin it would still hold it up. You were onto something with the front wheel turning into the lean though. Because fyi if you try to do that with the bike backwards down a hill it won't work the bike will tweet and fall over every time. The reason the bike steers into the turn going forward is the caster angle of the front forks. That's the reason a car wants to go straight if it's aligned right. But in reverse if you don't touch the wheel the wheel will try to turn full lock one way or the other. It's the caster angle. If that bike goes backwards down the hill the caster is backwards and it will fall. If the forks were perfectly vertical the bike would not be as stable. If the forks went the other way it would want to crash all the time. The caster is what keeps the bike going down the hill.
Lots of really good comments on this one! I want to try to clarify the correct answer to the original question as best as possible. "HOW" does a bike self-balance? Well, "Trail" is the distance from the tire's contact point with the ground to an invisible line through the front wheel's steering axis where it intersects the ground. Tilted steering axis or not, if the tire's contact with Earth is behind the steering axis, you have trail. If the bike is pointing straight ahead, and both tires are directly along the bike's centerline beneath the center of mass (COM), then there is no effect on steering from the mass of the bike or how it relates to the COM of the bike/rider system. But if the bike leans, the contact point of the tire is no longer directly under the COM, and TRAIL becomes a lever arm that turns the bike's steering in the direction of the lean. Here's how you can visualize that. Let's go the the extreme: If you lay a bike horizontally on its side and push up on the front tire where it would make contact with Earth, it easily turns the steering towards the ground, towards the side you laid it down on. Another thing you can do with your bicycle (or motorbike if you're really strong). Hold it upright, not moving, steering straight ahead. Then without touching the handlebars at all, lean it to one side. Because of friction it may not steer right away, but pretty soon, that steering will turn toward the direction of the lean, and that's because of the lever arm of this magic thing called "trail." And you're not even going anywhere! So if you're riding, at most realistic lean angles the force from that invisible lever arm causes the wheel to turn toward the direction of lean which moves the tire contact points back underneath the COM hence balancing the bike. But COM does matter. A high center of mass will require different steering geometry (angles) to automatically balance the bike, and as some here have said, you may want more or less balancing depending on what the purpose of the bike is. Fast and nimble - maybe less trail (a shorter lever arm), or more for comfort and easy riding - go for more trail. Finally, if you have an old bike where you can spin the steering around backward, this gives you negative trail, and all the balancing effects are reversed and good luck giving that a go!
Neil, I'm with you on most things but you're wrong on much of this. You're right that gyroscopic force isn't a big factor but a motorcycle can still be ridden if it's 10 feet tall with the center of mass several feet above the ground. You're right that the front wheel wants to turn as the bike leans and that has to do with the geometry of the front forks, "trail" specifically.
I don't get where he says wheels rotating in opposite directions cancels out gyroscopic effect. I'd like to see that in action. I don't think it'd have any effect.
@@alanbiker5838yeah I'm sure a pair of counter rotating wheels would still have gyroscopic properties ie resisting axial movement perpendicular to wheel rotation because they don't occupy the same space but are a couplet
When I was a young man, I had a go on a 'high wire' training bicycle. It didn't have the big floppy pole (pfffft!) but it had a framework attached to the rear wheel hub that hung 4 ft below the wire at an angle which put it directly below the saddle. There was a 28lb weight in it and that made it almost impossible to fall off! A perfect example of stability through the lowering of the centre of mass. (The wire itself was a nosebleed -inducing SIX FEET off of the ground!)😅 💚🐇🐴💚
It only works if the CM is below the tightrope which is why the framework is there. If that were the case in the typical tightrope walker case without the framework, tightrope walking would be about as easy without the framework as it is with it. In other words, they wouldn't bother with the extra training gear.
@@toddwasson3355 For the walking, yes. But for a clumsy teenager riding a bike 30ft along a wire in the mid 1980s - that framework was very much required! 💚🐇🐴💚
I saw Veritasim's video on the same topic couple of months ago. He experimented riding bicycle with locked steering wheel and it was impossible to balance. Nice to see further explanation and example of highwire.
Someone did a video where they made the steering work the opposite way (turn left and the wheel went right). It took him a very long time to re-learn how to ride (like months iirc). Once he finally got it, he then could not ride a regular bike. Crazy stuff.
@@lemongavine No need for a different bike. Just cross your arms and grab the handlebars carefully (of course much nearer to the center) Be careful you might fall, but after some practice you will get the hang of it. In about ten minutes, not months.
Not often is Neil off the answer, but he completely is this time. Of course wobbling brings the center of mass CLOSER to the ground, which is opposite to what Neil is saying. The reason the system is stable is that the caster angle makes the steering wheel a self-balancing system that compensates instability with a turn to the proper direction due to the gravity pull.
Agreed. I've watched a lot of his videos. He was also wrong about an airplane in one of his videos. As an astrophysicist, I absolutely love his way of explaining things. Bicycle (or airplane) builder, he is not. His overconfidence in his incorrect understanding of some of the tips he speaks about is a little bothersome for me.
I'm a vehicle dynamics model programmer and agree with you. If the bike rolls (leans), you'll get some camber force immediately. Since the tire vertical force is behind the steer axis (mechanical trail), the tire will naturally steer in the direction of lean. That causes a non-zero slip angle and lateral force to develop in the direction of steer and lean which rights the bike again. There's some gyroscopic effect there too (the bike having a roll (leaning) velocity will tend to steer the wheel in that direction too because there's a reaction torque at 90 degrees to the added angular momentum vector) but it might be minor in comparison. So strictly speaking, it's some of both, but the primary stabilizer is most likely the steering as he described. On a racing bike at high speeds this might not be such a minor effect though. I don't like Neil's description of the bike other than the part about the wheel steering and trying to right itself. That part is good enough for the layman, but all this stuff about a low CM is irrelevant, including what he described with race cars which has nothing whatsoever to do with roll stability and everything to do with lateral load transfer and tire load-mu sensitivity to increase maximum lateral acceleration. I wish Neil would talk to some vehicle dynamics engineers before going on about these subjects because they often don't work quite the way he imagines. He's an astrophysicist, not a vehicle dynamics engineer with any expertise in that area.
I don’t doubt that low CG and the so called weeble effect can help prevent tipping, but as a long time gyro guy it’s the spinning motion of tires( whether forward or backward) that resists ANY change. It wants to remain where it is. Ask yourself if the bike isn’t moving will it stay up? No. So the forward motion , and thus wheel spin, is the key. Used to have a bicycle tire in my office and a lazy Susan. Would have customers spin the wheel holding in both hands on axle while standing on lazy Susan. Try turning tire in any direction and see what happens.
@@ericbailey872 the centrifugal force, while it's tangible, is not the key factor. You can replace the wheels with as something as light as possible with no significant difference for the rider's ability to balance the moving bycicle as long as other properties of the wheel are not impacted. The opposite is also true: it's impossible to ride a bycicle with its wheels as heavy as one can imagine, even along a significantly long straight line, if the system doesn't have the key element: a steering wheel with the caster angle which allows for a self-balancing ride.
As a long time bike person I can also comment on that the amazing thing about a bike is that when you make a turn, you actually first slightly turn the handlebars in the opposite direction of your turn (so called "counter steering"). Many bike riders deny this because it is so counter-intuitive, but it is actually for real.
I'm also a many decades long rider of bicycles and motorcycles still, and I agree. You are correct, weird as it may be to the inexperienced. Only a nuance I'd add is (of course) it applies at a decent rolling speed once it 'kicks in as gyroscope effect. In a parking lot, one actually turns the bars from the get go in the direction desired, not so at at speed (10mph say) initially counter steer for sure to start the lean, which is how the bike turns at speed. Once started, of course the hands/eyes/brain combo finesses to smoothly, almost imperceptibly, align the bars again in the travel direction a bit, the leaning doing most of the work. (And providing fun... .) Ride safe... .
@@darcyoneill9377 >> In a parking lot, one actually turns the bars from the get go in the direction desired,\ Most riders believe this to be the case, but I assure you it's not. I made a video with Derek of Veritasium that demonstrates this.
@@darcyoneill9377 While riding in a peloton and someone is about to cross your front wheel you may find a reflex front wheel turn away from their wheel while leaning/ throwing opposite direction then flicking your wheel back will save a spill. You watch yourself do it by reflex.
@@olmalone I'm sure, although my experiences don't include riding in a peloton, I relate from single track dirt trail, close following/followed sometimes, trail rocks/roots to deal with 'on the fly', sharp turns, along, up, and down hills, and, when not that a big city bicycle commuter to work... !
Came as soon as I got the notification popped up and just as a convenience i was just thinking about this very topic. Amazing work startalk team keep up the good work.
In other words bikes have a very cool mechanical auto-steering feedback system for maintaining balance. They self correct their balance with clever mass distribution and head tube angulation.
If there is no external force to cause acceleration of one half of the tyre and an equal but opposite deacceleration of the other half then the front tyre will remain straight , i.e stability while in motion.
The balance pole seems to be more of an inertia thing, if you start leaning to the left, you push the left down which will push you up, you can tightrope walk with no pole by swinging your arms above your head, much like in Tom Scotts tightrope challenge A centre of gravity demo would be the balancing toys where a weight is physically below the balance point
@sparkyprojects Yep, exactly. As I wrote elsewhere: What you have is a system where angular momentum is perhaps 0 to start, along with a (simplifying a bit) single vertical force at the walker's feet. If there's a little disturbance and his CM shifts left a little bit, the vertical force is now to the right of his CM which will cause a torque to the left. The walker will start rotating in that direction. By applying an opposite torque to the bar directly, he can induce an opposite rotation in his body and momentarily shift his CM back to the opposite side of the wire. If he moves it far enough and holds it there long enough he can then apply an opposite torque to reverse the rotation of the bar, then stop it when it's level again, thereby canceling the angular momentum in the system. His CM is now back to center and with 0 torque and angular momentum in the walker+bar system until it's disturbed again. All this stuff about lowering the CM is irrelevant in the tightrope example. If the CM is above the wire at all by any amount, the system is unstable regardless.
@@dre3k78 Lowering the CM is irrelevant if the CM stays above the wire the whole time, which it does in this case. Neil is talking out of his lane on this one. Seems to have been doing that a lot lately unfortunately.
@@toddwasson3355 Right! It just gives something to momentarily rotate and push against to get centered again. Unless the pole is heavier than the walker, and totally below the rope, the center of mass won't balance you. I'll bet the hardest thing would be keeping the rope directly beneath you. That's gotta take a lot of control and core strength.
@@edbruder9975 Yep. I'm sure there's more to it than what I suggested, like altering the force direction on the wire away from vertical to reverse the torque and translations, but yeah, nothing to do with the CM being below the wire. I'm a little surprised Neil thinks the CM is below it given no part of the bar or walker is below the wire when the bar is level.... There was another commenter here somewhere who said as a kid he did a training class on tightwire rope walking. They had a cradle attached with a 28 pound weight hanging way under the wire somewhere that'd move. He said it was basically impossible to fall off. If the CM was really below the wire like Neil thinks, it'd be a stable system and require no skill whatsoever to do.
Fun Fact: What a lot of people do not know is that high wire bicycles are what we bicycle people call a “fixie”. They have the rear wheel mounted in reverse so the caliper and or disc brakes can be removed and the way you break is by back pedaling. Which is why they are able to go forward and backwards. Fixies are bikes that not only have a single gear, but that single gear is fixed at all times. So no matter if you go forward or backwards, up hill or down hill you are constantly pedaling and your speed is determined by how fast your legs can move. You can control your speed by controlling your cadence. There is no coasting. Also a ‘secret weapon’ that fixies have is bringing your feet up off the pedals and resting them on the handlebars so the pedals can freely spin when going down hill. They pick up incredible amounts of speed this way but the only way they have to break is to put their feet down, back pedal or skid the rear tire left and right. Fixies rarely get stolen because you can tell they have no brakes. My ultimate goal on a bicycle is to be skilled with my own fixie one day.
Always good to hear from any of the CPFC faithful. 90' was a vintage that will forever be a year of years. So glad you love our show. From Selhurst to Startalk That's Life !
Neil didn't quite hit the nail on its head here in my opinion. - Trail (contact patch behind axis of rotation of the wheel) is the mechanism that auto-centers the wheel at speed, though it doesn't have any effect at a standstill. Think of caster wheels on a shopping cart. - Rake (inclination of the head stock) inevitably produces trail on a bike with straight forks but it also creates another effect: if you lean the bike a bit and then rotate the wheel slowly, the whole bike will shift side to side slightly but also there will be an angle where the bike sits the lowest on the ground. Now whenever you leave that angle, it raises the bike up very slightly. If we eliminate friction, gravity will always keep the wheel angled in that special angle, which seems to be the correct steering angle for a given lean of the bike, yet I wouldn't be too sure about that. This effect is similar to Neil's balancing stick example, but it's probably only relevant at a standstill, when friction of the tire on the ground is minimal. - Gyroscopic effect is an apparent force that pushes the bike up when it's in motion and leaning to one side. But it can't be the only relevant effect here, because then the bike would just come to rest at a certain lean angle but roll straight on. - When the bike leans, the contact patch shifts to the inside of the curve. This might put leverage on the front wheel to steer it into the turn. The rolling circumference also gets smaller as the bike leans, prompting the wheel to accelerate its rotation, but because of inertia the wheel will instead react by putting leverage on the steering axis, steering the wheel into the turn. If this effect is significant or not, I don't know. I'd be curious to see a computer simulation which can eliminate various factors to see which ones truly contribute to the self-stabilizing of a bike.
Yeah it's called counter streeting, that's how all bikes are steered by riders. Much more obvious on a motorbike at speed. So you initiate a fall to the side in the direction you want to steer in then you steer the opposite direction so that the front tyre tracts the desired radius.
An interesting fact about motorcycles is that a bike set up for Trials, which involves a lot of low speed maneuvering, has a steeper rake then a bike set up for road racing which has a gentler rake angle. Motorcycles set up for drag racing which aren't intended to turn, have their front wheel extended out a few feet to keep them going straight. To get a motorcycle to turn left you actually press down on the left handlebar and to turn right you press down on the right handlebar. Having a single disk brake on the front wheel doesn't make the motorcycle veer to what ever side of the wheel it's mounted on. And you don't lean on a motorcycle in curves, you keep your body straight to maintain stability! Understanding the physics of a motorcycle is important!
Another fun counterintuitive thing about bikes: the weight of the rider is not actually supported by the bottom of the wheels; instead the rider is dangling from the top of the wheels. The spokes are very thin and have very little strength under compression, but they have great strength under tension.
Neil, would love to hear about the physics of a motorcycle jump. Especially what looks like rotational control in mid air. Is the rider, for example, applying the brakes a bit to rotate the nose downwards?
You're really smart just because you guessed that! Yeah, when you accelerate the wheels, nose goes slightly up, when apply brakes - slightly down. I certainly wouldn't have guessed that, I wouldn't have thought that's something even possible
@@ldmtagSounds exactly like what happens on the ground 🧐. It's almost as if angular momentum is the reason why it behaves that way in both cases 🤷♂️. Idk if that's the (only) reason or if it's a pure coincidence 🤔.
@@louisrobitaille5810 no, it's not the only reason, and no, it's not a coincidence. On the ground the main force that makes you do a wheelie is tire friction. It's applied not directly to the center of mass, hence it generates rotational torque. Normally it just slightly squats the bike during acceleration making the center of the rear wheel the center of bike's rotation, around which now the force of gravity is off-centered and too makes the torque, and this torque is what keeps the bike from doing a wheelie.
Interesting... I was convinced by the gyroscopic effect explanation - because this is what I learned while training for my motorcycle license... But if that is not the proper explanation, then there is something else my instructor said that does not make sense anymore: he told me that past a certain speed (around 30 km/h so about 18mph) the gyroscopic effect makes it so the bike simply cannot fall - unless you lose traction or hit something of course. And it does indeed require a conscious effort from the pilot to keep the bike up at low speed - some bikes require more effort than others, due to their center of mass of course... So here's my question (to which I'd bet you have the answer). If the gyroscopic effect is not responsible for this... Why does the bike have to reach a certain speed to be automatically stabilized? Thanks for everything you teach us through your videos btw!
The gyro effect acts in a similar way to the effect Neil talked about: When the bike has some roll velocity (meaning it starts leaning left/right) the gyro effect also acts to steer the front wheel in the direction of the turn. So it's not one or the other, it's both. As you go faster, the gyro effect becomes stronger. I'm not sure if there's a point where the gyro effect is stronger or what, it'd depend on the particular bike, I suppose. Maybe it's always less, maybe it's sometimes more. Not sure, but it should get stronger as bike speed increases anyway.
A UC Berkeley professor hosted a "Bicycling Science" lecture at open house. He had a bicycle made with geometry that caused it to steer left when leaning right, and steer right when leaning left. (But not a reverse-steering bike via the handlebars like "Smarter Every Day.") He let me ride it and was impressed that I had no problem first try. But it was horrible how you had to fight the natural geometry to get it to "steer into the fall."
We gotta "know" the 'critical' center of mass in the system we desire - like on our bikes or up on the wire. 'Critical mass' like the moment between not boiling and boiling - there's a point of no return - the center of mass can bring you down to the ground because it is so heavy- it ain't easy till it's easy, then it balances. You don't forget how to ride a bike, so they say. As an off-road rider I grok this, the bike will just stay up if you keep yourself flexible, strong and ready to go with the bike. Use the throttle and clutch to keep momentum and brakes to control speed. You put the body's momentum toward the line you want while looking fairly far ahead. Hitting huge rocks will fling you one way or the other and the bike will just stay up if you can hang on and stay ahead maintaining correction momentum for the body to match the center of mass toward the lowest point in the system. It's an amazing fun ride.. hang on!
Thank you, Mr. Tyson for the explanation. As a motorbike messenger for 6 years straight, I've always thought about the Center of Mass and Gravity's effect on my motorbike. And I've taken a pro riding course that involves lowering the CMG during cornering too few years ago, which makes me believe on it even more. Thank you, Sir. Once again.
I am no physicist but I think that for a bike (or any comparable vehicle) to stay upright, the center of mass must actually be ABOVE the center of the wheels. And if you think it through that kind of makes sense: A system of any kind is only stable if it is able to counteract any disturbances from its equilibrium (like those wiggle-dolls you mentioned). For a bike that means that if it is going in a straight line and upright, it must counteract any motion that drives it away from that configuration. Imagine the bike (going at some speed) tilting slightly to the right (or left). What happens initially is that the front wheel will - due to its gyroscopic precession - turn inwards to the right, ultimately forcing the bike to turn. Now, because for most bikes the center of mass is located pretty high up relative to the center of wheels (plus rider even more so) that turn will induce, loosely speaking, a centripetal force on that center of mass to the opposite side, resulting in the bike leaning back to the left. This will go back and forth until the front wheel is aligned again. If that doesn’t appears convincing, picture a bike where the upper half of the frame is made of light plastic and the lower half is made of heavy lead. If that bike, rider excluded, were to go downhill starting in a stable configuration and started to tilt to the right, I would not expect the bike to realign itself. But that’s just my theory and I could be totally wrong haha. Cool video anyway✌️
I have a different explanation: centrifugal force. Every time a rolling wheel begins to fall over, it's path to motion curves to the direction of the fall. At that moment, centrifugal force kicks in and puts the wheel back upright. Watch a rolling coin carefully and you will notice that it wiggles as it slows down before losing balance and falling over. As it slows down, the centrifugal force weakens hence the wriggling. Eventually got, the centrifugal force is to weak to counter the fall and the coin falls. It's the same effect with a bike.
Is there any way an unsteady walker could use any of the principles to make it harder to fall? Or does a walker do that, even though the idea seems simpler? I'm not there yet, but I wanted to know if there's any way to improve walking around for those who have trouble.
Neil, I would suggest you buy a book called The Racing Motorcycle by JohnBradley. In it, you will find all the correct information and science as to the dynamics of a bike and how they steer and stay up. It is not how you have been misled and put in this video. This info has been known by bike desugners for decades.
Having your center of mass on a 2-wheeler low is not actually helping with stability, it might be doing the opposite. I have riden very low lowracer recumbents with center of mass of the rider just about 40cm above ground and tall 29" upright bikes with their center of mass maybe 110cm above ground. The low bike requires faster reactions to anything that destabilizes you. It like balancing a short stick on the palm of your hand versus a long stick. Other things being equal lower bike will require faster reflexes and therefore feel less stable. But you need a big heigth difference to notice this. Among just upright bikes for example other differences will be so much more noticable they will hide this one. To make the bike balance without rider you need the steering assembly (front wheel, fork, stem, handlebars) to lean to the side the bike wants to fall to faster then the rest of the bike. Then any disturbance of the initial upright position will result in a turn that corrects it back to upright. It does not matter if your design uses the gyroscopic effect to help this or not, or how much exactly the front wheel trails behind the steering axis. As long as the steering falls into a turn fast enough (but not crazy fast, that could induce fast oscilations) it will work and you can send that bike down the hill and see it keep going. However once you put a human on it it becomes more complicated. The weight of the arms on the handlebars by itself already changes the behaviour of the bike. The overall mass is now much greater and center of mass is in different place. And the mass is no longer rigid, most of it is somewhat flexible and actively moves too. And the person steers actively to keep balance but with the limitations of human reaction times. So now describing the stability of this bike-human system becomes crazy complicated. To my knowledge nobody ever built a good mathematical model for this that would let you predict what bike geometry would be nice to ride. The only way is to build the bike and test it. As a result I have seen bicycles that were not only not stable to ride themselves down a hill but you could not even push the bike just by the saddle, you had to have a hand on the handlebars when pushing it. And yet it was perfectly nice bike once you sit on it and ride it. Just the change in mass and its distribution did the trick. And I have seen a bike that is nicely stable when you push it by saddle alone and yet it is unpleasant to ride - steering feels rubbery and like it is fighting you.
I remember when I was a kid, I did this experiment (I heard or read about it somewhere, but I don't remember where) where you put a fork and spoon together with a toothpick pushed between them, balanced on the edge of a glass, then you light the inward-facing end of the toothpick on fire. It burns to the edge of the glass and stops, and you're left with this thing standing on the edge of a glass, barely touching it, yet staying put. Pretty simple physics, since it's just keeping the center of gravity so low, it can't tip in either direction, but it looks completely counterintuitive.
Here's my counter question: If the pole is for lowering the center of mass, the highwire walkers who do this without a pole usually use their outstretched arms to balance...If lowering the center of mass is the way to go, why lift your arms up to shoulder height, instead just keep them low by your waist?
Its a weird answer alltogether. If you even tried balancing a broom or something on your hand, you will notice it gets easier with the heavy side up instead of the other way around.
A balance pole need not be "floppy" or hang down to be effective. What the pole does is give the arealist a source of torque. Example: From the arealist's viewpoint, if they should lean to the left they can impart a quick counterclockwise torque on the balace pole. The counteracting torque enables them to shift the center of gravity to the right, to a point directly over the wire. Of course, you could have a pole whose ends were below the wire and were heavy enough to make balancing automatic, but you won't see that in a circus act. The length of the pole also increases the polar moment inertia, making the arealist less likely to rotate off center.
It's the curve in the front forks that makes the bike go straight. if you analyze the geometry you can see that when the handle bars are turned that lifts the front of the bike slightly. therefore, in order to turn you have to put energy into the system. i.e. the bike goes straight unless some force acts on it.
ohhh that's interesting. I've mentioned in another comment that I have a single bike which is quite hard to ride handsfree. I have to shift my weight and lean to one side to drive straight. The handlebar is not exactly straight adjusted. Maybe that's the reason. I got this bike for free but wasn't able to center the handlebar due to corrosion
Nope. That helps you come out of corners without applying force to the handlebars to go straight. What keeps the bike up is the self aligning torque of the wheels, the natural tendency of them to roll straight and true on their own. I ride motorcycles, and used to do it competitively, and that takes a LOT of science.
@@macgoryeo Sometimes if a bike is crashed the forks are bent slightly to one side, so the front wheel isn't centered properly. That throws off the alignment. Handlebar setting is more cosmetic, particularly when no hands are involved.
When I sold bicycles, someone came in and paid cash ($600 on 1973) for a Lejune Pro. I had it put together and rode it before the customer picked it up. All you had to do to turn that bike (a really good one at that time) was to lean one direction or another.
This is the first time I have seen Tyson get something wrong. For a Motorbike it is all about Castor not Centre of Mass. The contact point on the ground is dragging behind the pivot point of the wheel. This is why it only works when moving, Tyson’s description would also work when standing still. Other than that his description of it turning into the lean was correct.
Question, if you drive a bicycle without your hands, what is the difference between hit a bump and hit a hole of same size and same shape knowing that the back wheel, will push forward? Do you have more chance to fall because of a hole of a bump?
On my Dad's desk was a metal highwire figurine that stood on a metal cylinder. I would watch that thing teetering back and forth and be amazed by it. He explained it but it didn't really get it when i was 4 lmao. (Back and forth though, not side to side.. but same principle)
It would help if you explain it in simpler terms, the key here is the forks being at a specific angle that they turn in the direction the bike is leaning so once the bike is in motion and it starts the lean the front wheel turns into the lean creating a braking force slowing the bike down however, the rest of the bikes weight overcomes this braking force by pushing on the front wheel because it does not want to change direction forcing the bike to stand up because is pushing against the lean... ask any motorcycle racer what happens if you are at a lean and you brake using the front or accelerate, the result is the same the bike stands up because you load the front and because the rest of the bike is heavier than the front wheel forcing the front wheel to alight with the direction of the biggest mass moving forward however if you apply read brake the bike leans more.
Ye not sure I agree, but when I was a kid I tried to do the ghost bike trick with the handlebars removed and it didnt work so in my opinion it has to do with the handle bars on the bike which makes it stable.
you also when at speed you don't turn the bars in the direction you want to turn, you push forward with the hand in the direction you want to turn and physics takes over.
Centre of mass below the axles. Spinning wheels have gyroscopic force, but the steering geometry needs to be such that the front wheel returns to centre.
I have developed balance issues when I walk, so I gave up riding my bike. But after a year, I thought to try it again. Much to my surprise, I discovered my balance is much better on my bike then walking. It actually feels normal. What puzzles me now is that when I was a kid I used to have no trouble riding with no hands. And yet I cannot do that today, or even before my balance was an issue.
Sounds like the position of you holding the steering allows for the bike's self-balancing effect to compensate for your poor balance until you take your hands off. Just my guess though 🤷♂️.
The most surprising part is that you can cancel gyroscopic effect. I can't believe it. I understand why it cancels rotating effect like a double-bladed helicopter without tail blades but canceling gyroscopic effect? Nah, I believe only when I see it.
On helicopters with a standard single primary rotor, the steering mechanism is set up to compensate for the fact that you actually have to provide a swashplate input, that is 90 degrees from what you'd instinctively think the swashplate input should be, so that the controls are intuitive to the pilot.
Hello Neil, my name is Benjamin and I will definitely be more of your patrons someday, my biggest mind boggling question is. Out of all the plants we observe, stars and galaxies, and now having a James Webb telescope that could either see into the past or now how come, we have not found now another planet containing green life like we do where does trees really come from?
Question that popped into my head is if cats change the center of their mass when they fall? As they "always" land on all four. I have read that by pulling the paws in towards the body, the cat causes the front part of the body to rotate faster than the back part. When all four paws are facing the right way, they can absorb the shock of impact. Whats the science behind it?
Apart from gyroscopic forces there is also another self righting force generated by the castor of the front wheels; i.e the angle the handle bars make when connected to front wheel. That's why you can let go of the handle bars in motion and the bike won't tip over and continue to follow a straight line. Perhaps that explains why the bike with the counter rotating wheels to cancel gyroscopic forces was still able to stay upright.
Just thinking about walking a high wire... 100% I’m seeing this through the lens of me doing it. Which is why I it’s thrilling. Because I have a fear of heights. Knowing that physics is applying forces in invisible ways mitigates my fear.
Theres an E-bike (motorcycle) out there, i saw a few years ago, with rotating gyroscopes inside the rims spinning even when bike is not moving. The bike stands up freely using only the gyroscopes as long as the battery is providing power. Its all gyroscopic.
What you are describing is gyroscopes being added in order to make the bike more stable when stationary or going very slow. But once up to a decent speed the gyroscopic effect is not required to make the bike self balance. It works because the steering moves freely. If the bike begins to fall to the right, the steering automatically falls to the right and drives the tire back up under the center of mass. This only works with forward motion. The motorcycle or bicycle has to have a properly designed front steering system in order for this to work. So while you might find ways to use gyroscopes to improve the situation, it is not "all" gyroscopic and gyroscopes are not even necessary. Lots of people have done many experiments to demonstrate this very clearly. People have taken the gyroscopic effect out of the bicycle and it still works. They've also locked the steering and it no longer self balances. You can see all of this for yourself in other videos.
@@Sammasambuddha Interesting. What direction do the gyros in the rims spin? Same axis as the wheels? That shouldn't work because if there's a roll acceleration (lean) the gyros should just cause a yaw motion which isn't self stabilizing in roll (lean). Could there be some other gyros spinning in other directions too perhaps?
Found one that uses gyros at about 4 minutes in: ua-cam.com/video/q1NaXSk1HWQ/v-deo.html&ab_channel=MasterBoyTV Is that the one? That one looks like it's using gyros in multiple axes, not lined up with the wheel spin directions, which is what I'd expect for a gyro system to work on a bike.
Neil, thank you for the excellent explanation of the equilibrium on the rope. But when I was a kid, we used to walk on the rope with a rigid bamboo pole and not the flexible one. Of course the rope on which we walked was not very high and the pole was only about 3 m in length.
The best film version of ghost bike is found in ‘Jour de Fete’ by Jacques Tati. It parks neatly at a cafe in jaunty manner. Two counter-rotating wheels sharing same axle and plane of rotation cancel out? I know very little. A single rotating wheel is more stable than two with one joined to frame and one freely yawing…….
I thought, as the body starts to lean to the side, the center of mass goes DOWN due to both the body shifting to the side and the pole shifting to the side with it. I always figured they use the pole to make the center of mass change and force the person’s body back upright.
Why would anyone think it is gyroscopic effect? For gyroscopic effect to stabilize a rotating mass on it axis, it has to revolve or precess on the third perpendicular axis, that means the bicycle has to go on a curve and make a circle on downhill, but it doesnt, it go downhill straight. BTW, it is not just bicycle, it happens to a wheel, single tire rolling down the hill as well. When a wheel is rolling downhill, there must a force which should act on axis of rotation to brought it down. But during downhill roll, the CG shifts towards front of the tire, so it rolls faster and still no sideway acting force. If anyone who thought it was gyroscopic effect before does not understand gyroscopic effect.
If a counter-rotating wheel on a bicycle cancels gyroscopic action, does the mass of each counter-rotating wheel need to be equal? Also, ust a thought.If the high-wire walker's pole dips on one side, it would tend to raise the other side a nearly equal amount, and the center of mass would not necessaily be lower to the center of the Earth. The walker (himself/herself) is in effect a movable fulcrum in relation to the pole.
Olympic high-jumpers are able to bend their backs in such a way that their center of gravity is outside their body, so when they jump over the pole, their center of gravity never rises above the height of the pole.
That information was fascinating to me. One reason is that women's center of mass is located in their hips. Men's is located in their chest. So if you're a male wire walker, you DEFINITELY need that curved pole to stabilize your connection on that wire. What do you think? I mean if I were a female wire walker I'd still carry that curved pole, but it's interesting to me.
Hey another question also with the pole on the wire wouldn’t lowering the center of mass essentially be to benefit the balance therefore that’s still the reason?
With the floppy balancing pole - if the left side goes down two feet - the right side goes up two feet, yes? So how does that alter the centre of gravity of the bike/rider/pole?
When it comes to bikes and motorcycles it all comes down to the rake (offset angle) of the front axle and the fact that the fork can move turning the wheel left/right, while the rear axis is fixed. Bicycles and motorbikes has two critical speeds. Going forward reaching critical speed the bike is self-balancing. Going backwards reaching the other critical speed the bike becomes completely destabilised. This means that reach enough speed going forward and the bike will lean the front wheel into any turn and self-correct raising itself up due to center of mass, rake and newtons first law. Reaching enough speed going backwards and not even the best computerised system can keep the bicycle upright (it becomes complete unstable). So the video is missing a few key elements to a bike, mainly the effect of the rake and the fact that a bike has two critical speeds, one making it stable, one making it completely unstable.
For the COM to provide absolute stability of a bike, the COM would have to be BELOW the ground level. Think of an inverted V resting on a horizontal rod: completely stable. This is never the case with any bike, no matter how low its COM is. The fact that the bike can stay upright only when rolling strongly suggests the stability is coming from the gyroscopic action of the wheels.
Hey, we actually do that instinctively. Tried to walk over an edge? What's the first thing you do if you're leaning sideways? Spread your arms wide open, just like that pole. And it is not calculated.
I actually have a electric mountain bike with fat tires for stability outdoors, I can 100% say you can ride it for awhile without hands because with pedal assist it’s enough momentum to stay up and depending on the road and how it curves the bike will stay on course. 90° angles are the tricky part.
Bicycles are rear wheel drive so the gyroscopic precession is in the front wheel. The industry term is "e-bike". Which is short for, electric bicycle. The battery pack on e-bikes is typically on the down tube or seat tube. #Bicycle #GCN #Bike
It's all basically about how the bike is steered. Very slight corrections are continuously made as one is moving forward, which is why you can't really stay upright easily moving backward or remaining motionless. Yes, a bike rolling forward without a rider can self correct as it will steer into a lean --- but watch what happens in reverse.
A bike with a higher centre of gravity isn't necessarily less stabile in the same way reversing a long trailer is easier than a short one. The pendulum is longer.
1:57 also if you think about it, even if the bike speed is really low, so that the wheels are just moving, so that the gyroscopic effect is negligible to it's weight..still the bike stays upright no problem
How can the CM be below the wire for a tightrope walker holding a bar horizontally? No part of either the bar (even a sagging one) or the walker is below the wire, let alone half the mass. If the combined walker+bar CM was below the wire, the system would be stable and there'd be no challenge to tight rope walking. I don't think lowering the CM has anything to do with any of this in the tightrope explanation. What you have is a system where angular momentum is perhaps 0 to start, along with a (simplifying a bit) single vertical force at the walker's feet. If there's a little disturbance and his CM shifts left a little bit, the vertical force is now to the right of his CM which will cause a torque to the left. The walker will start rotating in that direction. By applying an opposite torque to the bar directly, he can induce an opposite rotation in his body and momentarily shift his CM back to the opposite side of the wire. If he moves it far enough and holds it there long enough he can then apply an opposite torque to reverse the rotation of the bar, then stop it when it's level again, thereby canceling the angular momentum in the system. His CM is now back to center and with 0 torque and angular momentum in the walker+bar system until it's disturbed again. There's probably more to it like manipulating the force directions laterally with your feet/legs, maybe doing some lateral translation of the bar and so forth. The point is all this stuff about lowering the CM is irrelevant in the tightrope example. If the CM is above the wire at all by any amount, the system is unstable regardless. Lowering it to a height that's still above the wire won't reverse the torque on the system and stabilize it. The Weeble Wobble also doesn't have much to do with the CM lowering. The reason a Weeble Wobble is stable is because a positional disturbance (say rolling it to the left just like the tightrope walker) pushes the contact point further to the left than the CM moves in that same direction. The vertical force is now far to the left of the CM instead of directly under it which produces a torque in the opposite direction canceling and then reversing the roll velocity. That rights it again. How the CM moved vertically is irrelevant.
1st time I don’t agree with you. Its the caster wheel design that keeps the bicycle dive straight and it will steer in to the turn to right itself if the bicycle made a turn keeping it upright. I discovered that when i was young and drove my bicycle into a curb causing the front wheel fork to bend backwards which made the bicycle unstable because the caster angle was reduced.
The angle of the front fork is what helps to keep straight. It's always tilted to the backwards. Even car front suspension is tilted to back. That's why steering wheel automatically comes straight after turning
This is wrong. They balance due to the rake and trail of the front wheel: due to the design of the front forks holding the wheel, when te bike leans left, the front wheel turns left by itself etc
It has little to do with the center of mass being low, more about the fact that centrifugal force does not exist. When the front wheel turn left or you lean the bike left the front wheel turn more left than than the back wheel, this result in the back wheel continuing its course and thus acting like a force pulling the front wheel back to the right. This plus the energy given from the ground hitting the wheel giving it 2 effects permitting is to lift itself back straight.
Can you please explain on one of your episodes how when my key fob is just out of range from my car to unlock it but if I put the fob to my chin pointing up to my head it’ll work 🤷🏽♂️
Dr Tyson, you have to do a video on pumping groundwater and it causing 31inches of tilt to the earths axis, its really interesting!! Need to hear you teach me more about it please!!
For sure, the answer to this is in the wheel gyro and countersteering frame/fork geometry. I don't think the counter-spinning bicycle wheel would cancel out all the main wheel's gyroscopic forces unless it has weight added to it to make them a closer rotating mass. As is, it is missing tube and tire. The weight on motorcycles is not between the feet (foot pegs) and rear wheel. Much of it is right behind the front wheel. this includes both the engine and gas tank (which is fairly high) on most standard style motorcycles. The best way to test this would be to RIDE A MOTORCYCLE! :)
It does. Without a doubt. If you counter-rotated with exactly the same RPM and mass, there would be ZERO gyroscopic effect. (That they didn't match the mass exactly isn't really that important - the result was canceling the vast majority of any gyroscopic effect in action.) Crazy that you're gonna challenge Tyson on this. This is clearly understood. Physics 101. You could design a bike with 500 wheels spinning on one direction and 500 in the other and the net gyro effect would be zero. However, the system would have increased overall mass, so the system has more inertia. But that's it. On other forums, people with zero understanding of physics would talk about "forces acting through the tire" as if they're some mystical phenomenon. Nope.
@@dudeonbike800 Correct but it has to be the same mass right for each counter-rotating wheel right? If you look at the image of the test bicycle at about 2:04, one is clearly missing a tube and tire. This would make a noticable difference in the masses right? And lastly, chlll out. Science is a process of discussion and review. If someone misunderstands or is missing information, we help each other learn. No need for the "how dare you." attitude. Be a better ambassador to the community and art itself.
I believe it's all about how the front wheel is designed to castor. i suspect that even if you raise the center of gravity of a bicycle it'll still tend to right itself. The reason a human riding a bike has to actively maintain a balance - by steering the front wheel into the lean, however slightly, at all times - is because the rider is *not* a rigid mass attached to the bicycle; the rider flops around independently and needs to maintain their balance on the bike. That the bike itself is self-righting is what makes the task easier, nay, possible for the human. Here's another attempt, probably inadequate, to explain the self-righting nature of a bike and how its center of mass affects it... When a bike is upright going straight, its center of mass is directly above the bikes track, the track being the path of it's wheels. If the bike leans, then the CoM is no longer above it's track. The front wheel and forks are designed to castor the front wheel and turn the bike to the side the center of mass is on, curving the path of the bike towards that side. This creates a centrifugal "force" which will push the center of mass back towards the bike's track, righting the bike. Think of it as a negative-feedback loop. When a bike is moving in a straight line, the CoM is above the bike's track. If the bike leans, moving the CoM to either side of the track, the bike is designed to automatically create a lateral force to counter that movement, which will push the CoM back above the track. In order to cause a bike to turn, the rider must then counter the lateral counter force to maintain a stable off-track CoM position for the duration of the turn. How's that? Everytime I think of this it boggles my mind how humans (and maybe some animals?) can learn to do this sort of thing instinctively.
The average mass of the battery is about that of the wheel axles; the mass of the motor is near the pedal center. I find my ebike quite stable. Wheels as gyroscopes help in all sorts of riding, on the ground and through the air.
Have you ever done the "Ghost Bike" experiment before? What happened?
I have. Many times. In fact I've done it when making a video with Derek of Veritasium.
Your explanation of how a bike rights itself, due mostly to its steering geometry, is correct. The low center of mass in that case is a red herring.
In the case of the tightrope walker, I'm 99.3% certain it's not a center of mass issue either. It would be very easy to test this with either a rigid pole, or a pole that's supported with cables as we used to do with the leading edges of hang gliders. In this case I'm nearly certain the tightrope walker is simply utilizing a device with a very high moment of inertia. If you Google tightrope, you'll see that, not only is the C.G. above the tightrope, but no part of the system is lower than the tightrope. Just as it's much easier to balance a long pole vertically on your hand, it's also easier to balance on a tightrope with a long pole (both reduce the natural response of the system down to something easily within human limits).
Also, contrary to intuition, a higher C.G. on a bike or motorcycle will not make it less stable. We have to be careful here to distinguish between static and dynamic stability. In the case of tightrope walkers and bikes, it's always dynamic stability we're considering. Neither will be stable if simply "balanced" at rest and left rigid. But in the case of the bike, the higher C.G. will give the system a greater moment of inertia about its rolling pivot point (where the tires touch the ground) and therefore slow the response down further, making it easier to ride.
Any kid who HASN'T had a terrible childhood!!!
Ha, ha!!!
@@Riccofiori
>> If the rider has a comparatively high center of mass, as the bicycle goes slower, it becomes even more unstable
A higher center of mass does not make the system more unstable.
>> The rider can easily lower the center of mass by standing on the pedals
Standing on the pedals only changes where the rider's weight is applied to the bike. It does not change the center of mass of the system.
@@Rick_Cavallaro The issue isn't mass location, but the fact that when the bicycle leans, steering into the fall allows the CM to stay low. NOT steering in that direction raises the CM, so the natural tendency of the bike is to steer in a way that doesn't raise it. It's like saying "water doesn't flow uphill." Duh!
how can they stand up?.. bikes are are Two Tired..
🥁
I believe the angle of the forks on the bicycle also factor into the 'Ghost Bike' ability to self steer. Kinda like the toe and caster on the steering on a motor vehicle to self straighten the steering wheel (when properly set up). Great topic, thanks for sharing.
Yes. There have been bicycles with incorrectly designed forks and they do not self-adjust to stay upright. Clearly, they did not sell well.
@@lesliefranklin1870 have to check my bicycles because I usually can ride handsfree but I have a single bike which is quite hard to control. Maybe it's because the handlebar isn't centered correctly
The rake (amount of forward bend) on a bike differs for thw use. A cruising bicycle has a long rake and is more stable. A racing bike tends to has a short fork so that you can turn quicker . A bike specially designed for bicycle polo has straight forks - you can turn very fast - and a fixed gear so you can pedal backwards
You are close but no cigar. It is trail that give stability. Angling forks is one way to achieve trail, but you can have vertical forks and still bui,d in trail to the bike geometry.
Yes it's both the Angle of the fork as you mentioned AND in combination to the OFFSET of the wheel center pushed forward
Studying advanced motorcycle handling teaches a combination of how center mass, speed, and friction points interact to keep a bike on track and in control for given conditions. There are all kinds of forces at work, then you add in what changes with applying the brakes or throttle at any given lean angle plus speed. All of this is acting on a contact point between the tire and the road about the size of a credit card. It is really a great exercise in physics and engineering once you realize how complex a system it really is.
So, does being raised by a guy who designed tires... but I'd still wanna take your course before getting on a bike.
The tendency of a bike to balance itself is partly due to the rake of the front wheel that is, the angle at which the fork holds the wheel. I experimented with my bike by turning the front wheel completely around thereby placing the centre of the wheel directly under the handlebars. This made it practically impossible to ride. Try it!
Yeah, that's what causes the wheel to steer in the direction of a roll disturbance which is what he was essentially describing. The main reason it works is because the vertical tire force (wheel load) is behind the steer axis. So if the bike is not perfectly vertical with perfectly centered steering, that vertical force causes a torque to act around that axis which steers the wheel in the direction of turn. There is some gyro effect adding to this in the same direction though which might be negligible at low speeds, but might not be on a racing bike at high speeds. It's not really just one or the other, it's both. Just a question of which one is stronger at which speed and with which tire and which motorcycle...
In your very cool experiment you reversed it and put the load ahead of the steering axis so the torque is reversed. Leaning to the right would cause it to steer left and make you lean to the right even faster. Totally unstable as you know. It'd take some serious brain rewiring to control that! 😁
@@toddwasson3355 Yeah the lowest center of balance occurs when the bike is laying down and really has nothing to do with balancing a bike. That's why they fall over when they're not moving Road racing bikes have the engine higher in the frame to allow the steering geometry to be more responsive and enables the change from a left turn lean to a right turn lean to happen quicker, and to have the bottom of the bike high enough for the extreme lean. One interesting point is the method used to initiate a turn on a racing motorcycle or any bike really, if you just lean the bike, you start to turn that direction. If you want to start a lean quickly, you push the handlebars in the opposite direction. As soon as the front wheel goes that way, the lean is happening real quick and the steering balancing geometry takes over. It sounds crazy, but there you go. You can try it, it works. Especially if you're leaned into a turn real hard already, think about it. To switch up, and turn the other way, you've gotta get the wheels on the other side of the bike so you turn them that way, harder into the turn, the bike stands up and leans the other way. I had to try, I didn't believe it either. It changes things really quick. Wheels and tires are lighter than engines, they can switch sides faster. If the center of gravity is too low, it's too stable, and you turn slower, and can't lean far enough. All that being said, steering geometry does lower the center of gravity when it's upright and straight until it leans, whereupon it turns to balance.
Thanks, both of you. You both gave better explanations of my experiment than I could have!
@@rogertulk8607did you try to run it backwards?
@@Gurmukkh no.
Its less about center of mass and more due to a few aspects of bike geometry, specifically head tube angle and the mechanical trail measurement. These can vary greatly between bikes but they have the most to do with a bike’s tendency to right itself.
When the bike is *moving*. Neither of these were discussed in the video. The question was never answered. I'm learning quite a bit from the comments though; never occurred to me that frame geometry had to do with stability.
Neil on your example of the bike with the wheels that spin backwards it doesn't cancel out the gyroscope factor. Because that video with the wheel spinning and you can levitate it from one side. That wheel doesn't care which direction it spins the angular momentum still holds it upright. So the bike no matter which direction the wheels spin it would still hold it up. You were onto something with the front wheel turning into the lean though. Because fyi if you try to do that with the bike backwards down a hill it won't work the bike will tweet and fall over every time. The reason the bike steers into the turn going forward is the caster angle of the front forks. That's the reason a car wants to go straight if it's aligned right. But in reverse if you don't touch the wheel the wheel will try to turn full lock one way or the other. It's the caster angle. If that bike goes backwards down the hill the caster is backwards and it will fall. If the forks were perfectly vertical the bike would not be as stable. If the forks went the other way it would want to crash all the time. The caster is what keeps the bike going down the hill.
Lots of really good comments on this one! I want to try to clarify the correct answer to the original question as best as possible.
"HOW" does a bike self-balance? Well,
"Trail" is the distance from the tire's contact point with the ground to an invisible line through the front wheel's steering axis where it intersects the ground. Tilted steering axis or not, if the tire's contact with Earth is behind the steering axis, you have trail. If the bike is pointing straight ahead, and both tires are directly along the bike's centerline beneath the center of mass (COM), then there is no effect on steering from the mass of the bike or how it relates to the COM of the bike/rider system. But if the bike leans, the contact point of the tire is no longer directly under the COM, and TRAIL becomes a lever arm that turns the bike's steering in the direction of the lean. Here's how you can visualize that.
Let's go the the extreme: If you lay a bike horizontally on its side and push up on the front tire where it would make contact with Earth, it easily turns the steering towards the ground, towards the side you laid it down on. Another thing you can do with your bicycle (or motorbike if you're really strong). Hold it upright, not moving, steering straight ahead. Then without touching the handlebars at all, lean it to one side. Because of friction it may not steer right away, but pretty soon, that steering will turn toward the direction of the lean, and that's because of the lever arm of this magic thing called "trail." And you're not even going anywhere!
So if you're riding, at most realistic lean angles the force from that invisible lever arm causes the wheel to turn toward the direction of lean which moves the tire contact points back underneath the COM hence balancing the bike.
But COM does matter. A high center of mass will require different steering geometry (angles) to automatically balance the bike, and as some here have said, you may want more or less balancing depending on what the purpose of the bike is. Fast and nimble - maybe less trail (a shorter lever arm), or more for comfort and easy riding - go for more trail.
Finally, if you have an old bike where you can spin the steering around backward, this gives you negative trail, and all the balancing effects are reversed and good luck giving that a go!
This. And why did i spend time writing down pretty much exactly the same thing even with the front wheel backwards. :)
Neil, I'm with you on most things but you're wrong on much of this. You're right that gyroscopic force isn't a big factor but a motorcycle can still be ridden if it's 10 feet tall with the center of mass several feet above the ground. You're right that the front wheel wants to turn as the bike leans and that has to do with the geometry of the front forks, "trail" specifically.
Yup, he's giving a terrible response to this question. A bike needs forward momentum
I don't get where he says wheels rotating in opposite directions cancels out gyroscopic effect. I'd like to see that in action. I don't think it'd have any effect.
@@alanbiker5838yeah I'm sure a pair of counter rotating wheels would still have gyroscopic properties ie resisting axial movement perpendicular to wheel rotation because they don't occupy the same space but are a couplet
When I was a young man, I had a go on a 'high wire' training bicycle. It didn't have the big floppy pole (pfffft!) but it had a framework attached to the rear wheel hub that hung 4 ft below the wire at an angle which put it directly below the saddle. There was a 28lb weight in it and that made it almost impossible to fall off! A perfect example of stability through the lowering of the centre of mass.
(The wire itself was a nosebleed -inducing SIX FEET off of the ground!)😅
💚🐇🐴💚
What an exciting experience. Was it?
It only works if the CM is below the tightrope which is why the framework is there. If that were the case in the typical tightrope walker case without the framework, tightrope walking would be about as easy without the framework as it is with it. In other words, they wouldn't bother with the extra training gear.
@@alexiusgottsgamechannel5243
Yes.
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@@toddwasson3355 For the walking, yes. But for a clumsy teenager riding a bike 30ft along a wire in the mid 1980s - that framework was very much required!
💚🐇🐴💚
Impressive!
I saw Veritasim's video on the same topic couple of months ago. He experimented riding bicycle with locked steering wheel and it was impossible to balance. Nice to see further explanation and example of highwire.
Someone did a video where they made the steering work the opposite way (turn left and the wheel went right). It took him a very long time to re-learn how to ride (like months iirc). Once he finally got it, he then could not ride a regular bike. Crazy stuff.
@@lemongavine Same video.
@@lemongavine No need for a different bike. Just cross your arms and grab the handlebars carefully (of course much nearer to the center) Be careful you might fall, but after some practice you will get the hang of it. In about ten minutes, not months.
@@bmcwxyz That was Destin of Smarter Every Day. He has been a guest on Veritasium's channel several times.
@@lemongavine Yes! Destin Sandlin from "Smarter Every Day" UA-cam channel.
Not often is Neil off the answer, but he completely is this time.
Of course wobbling brings the center of mass CLOSER to the ground, which is opposite to what Neil is saying.
The reason the system is stable is that the caster angle makes the steering wheel a self-balancing system that compensates instability with a turn to the proper direction due to the gravity pull.
Agreed.
I've watched a lot of his videos. He was also wrong about an airplane in one of his videos. As an astrophysicist, I absolutely love his way of explaining things. Bicycle (or airplane) builder, he is not. His overconfidence in his incorrect understanding of some of the tips he speaks about is a little bothersome for me.
Classic “im smarter than you” overthink. Now ask him to explain why your car steering wheel self centres. He’s clueless.
I'm a vehicle dynamics model programmer and agree with you. If the bike rolls (leans), you'll get some camber force immediately. Since the tire vertical force is behind the steer axis (mechanical trail), the tire will naturally steer in the direction of lean. That causes a non-zero slip angle and lateral force to develop in the direction of steer and lean which rights the bike again. There's some gyroscopic effect there too (the bike having a roll (leaning) velocity will tend to steer the wheel in that direction too because there's a reaction torque at 90 degrees to the added angular momentum vector) but it might be minor in comparison. So strictly speaking, it's some of both, but the primary stabilizer is most likely the steering as he described. On a racing bike at high speeds this might not be such a minor effect though.
I don't like Neil's description of the bike other than the part about the wheel steering and trying to right itself. That part is good enough for the layman, but all this stuff about a low CM is irrelevant, including what he described with race cars which has nothing whatsoever to do with roll stability and everything to do with lateral load transfer and tire load-mu sensitivity to increase maximum lateral acceleration.
I wish Neil would talk to some vehicle dynamics engineers before going on about these subjects because they often don't work quite the way he imagines. He's an astrophysicist, not a vehicle dynamics engineer with any expertise in that area.
I don’t doubt that low CG and the so called weeble effect can help prevent tipping, but as a long time gyro guy it’s the spinning motion of tires( whether forward or backward) that resists ANY change. It wants to remain where it is.
Ask yourself if the bike isn’t moving will it stay up? No. So the forward motion , and thus wheel spin, is the key.
Used to have a bicycle tire in my office and a lazy Susan. Would have customers spin the wheel holding in both hands on axle while standing on lazy Susan. Try turning tire in any direction and see what happens.
@@ericbailey872 the centrifugal force, while it's tangible, is not the key factor. You can replace the wheels with as something as light as possible with no significant difference for the rider's ability to balance the moving bycicle as long as other properties of the wheel are not impacted.
The opposite is also true: it's impossible to ride a bycicle with its wheels as heavy as one can imagine, even along a significantly long straight line, if the system doesn't have the key element: a steering wheel with the caster angle which allows for a self-balancing ride.
As a long time bike person I can also comment on that the amazing thing about a bike is that when you make a turn, you actually first slightly turn the handlebars in the opposite direction of your turn (so called "counter steering"). Many bike riders deny this because it is so counter-intuitive, but it is actually for real.
I'm also a many decades long rider of bicycles and motorcycles still, and I agree. You are correct, weird as it may be to the inexperienced. Only a nuance I'd add is (of course) it applies at a decent rolling speed once it 'kicks in as gyroscope effect. In a parking lot, one actually turns the bars from the get go in the direction desired, not so at at speed (10mph say) initially counter steer for sure to start the lean, which is how the bike turns at speed. Once started, of course the hands/eyes/brain combo finesses to smoothly, almost imperceptibly, align the bars again in the travel direction a bit, the leaning doing most of the work. (And providing fun... .)
Ride safe... .
@@darcyoneill9377
>> In a parking lot, one actually turns the bars from the get go in the direction desired,\
Most riders believe this to be the case, but I assure you it's not. I made a video with Derek of Veritasium that demonstrates this.
@@darcyoneill9377 While riding in a peloton and someone is about to cross your front wheel you may find a reflex front wheel turn away from their wheel while leaning/ throwing opposite direction then flicking your wheel back will save a spill. You watch yourself do it by reflex.
@@olmalone I'm sure, although my experiences don't include riding in a peloton, I relate from single track dirt trail, close following/followed sometimes, trail rocks/roots to deal with 'on the fly', sharp turns, along, up, and down hills, and, when not that a big city bicycle commuter to work... !
That science UA-camr guy build a bike that didn't allow you to do that and it turned out you could not rider the bike 😂
Came as soon as I got the notification popped up and just as a convenience i was just thinking about this very topic. Amazing work startalk team keep up the good work.
In other words bikes have a very cool mechanical auto-steering feedback system for maintaining balance. They self correct their balance with clever mass distribution and head tube angulation.
As a bicycle enthusiast, this is a great description.
If there is no external force to cause acceleration of one half of the tyre and an equal but opposite deacceleration of the other half then the front tyre will remain straight , i.e stability while in motion.
The balance pole seems to be more of an inertia thing, if you start leaning to the left, you push the left down which will push you up, you can tightrope walk with no pole by swinging your arms above your head, much like in Tom Scotts tightrope challenge
A centre of gravity demo would be the balancing toys where a weight is physically below the balance point
Also wouldn't the pole be used as a weight as well to help contribute to the lowering of the center of mass?
@sparkyprojects Yep, exactly. As I wrote elsewhere:
What you have is a system where angular momentum is perhaps 0 to start, along with a (simplifying a bit) single vertical force at the walker's feet. If there's a little disturbance and his CM shifts left a little bit, the vertical force is now to the right of his CM which will cause a torque to the left. The walker will start rotating in that direction.
By applying an opposite torque to the bar directly, he can induce an opposite rotation in his body and momentarily shift his CM back to the opposite side of the wire. If he moves it far enough and holds it there long enough he can then apply an opposite torque to reverse the rotation of the bar, then stop it when it's level again, thereby canceling the angular momentum in the system. His CM is now back to center and with 0 torque and angular momentum in the walker+bar system until it's disturbed again.
All this stuff about lowering the CM is irrelevant in the tightrope example. If the CM is above the wire at all by any amount, the system is unstable regardless.
@@dre3k78 Lowering the CM is irrelevant if the CM stays above the wire the whole time, which it does in this case. Neil is talking out of his lane on this one. Seems to have been doing that a lot lately unfortunately.
@@toddwasson3355 Right! It just gives something to momentarily rotate and push against to get centered again. Unless the pole is heavier than the walker, and totally below the rope, the center of mass won't balance you. I'll bet the hardest thing would be keeping the rope directly beneath you. That's gotta take a lot of control and core strength.
@@edbruder9975 Yep. I'm sure there's more to it than what I suggested, like altering the force direction on the wire away from vertical to reverse the torque and translations, but yeah, nothing to do with the CM being below the wire. I'm a little surprised Neil thinks the CM is below it given no part of the bar or walker is below the wire when the bar is level....
There was another commenter here somewhere who said as a kid he did a training class on tightwire rope walking. They had a cradle attached with a 28 pound weight hanging way under the wire somewhere that'd move. He said it was basically impossible to fall off.
If the CM was really below the wire like Neil thinks, it'd be a stable system and require no skill whatsoever to do.
Fun Fact: What a lot of people do not know is that high wire bicycles are what we bicycle people call a “fixie”. They have the rear wheel mounted in reverse so the caliper and or disc brakes can be removed and the way you break is by back pedaling. Which is why they are able to go forward and backwards. Fixies are bikes that not only have a single gear, but that single gear is fixed at all times. So no matter if you go forward or backwards, up hill or down hill you are constantly pedaling and your speed is determined by how fast your legs can move. You can control your speed by controlling your cadence. There is no coasting. Also a ‘secret weapon’ that fixies have is bringing your feet up off the pedals and resting them on the handlebars so the pedals can freely spin when going down hill. They pick up incredible amounts of speed this way but the only way they have to break is to put their feet down, back pedal or skid the rear tire left and right. Fixies rarely get stolen because you can tell they have no brakes. My ultimate goal on a bicycle is to be skilled with my own fixie one day.
Still funny seeing Gary O’Reilly with these two, that semi and final were terrific in '90, good times.
Always good to hear from any of the CPFC faithful. 90' was a vintage that will forever be a year of years. So glad you love our show. From Selhurst to Startalk That's Life !
Neil didn't quite hit the nail on its head here in my opinion.
- Trail (contact patch behind axis of rotation of the wheel) is the mechanism that auto-centers the wheel at speed, though it doesn't have any effect at a standstill. Think of caster wheels on a shopping cart.
- Rake (inclination of the head stock) inevitably produces trail on a bike with straight forks but it also creates another effect: if you lean the bike a bit and then rotate the wheel slowly, the whole bike will shift side to side slightly but also there will be an angle where the bike sits the lowest on the ground. Now whenever you leave that angle, it raises the bike up very slightly. If we eliminate friction, gravity will always keep the wheel angled in that special angle, which seems to be the correct steering angle for a given lean of the bike, yet I wouldn't be too sure about that. This effect is similar to Neil's balancing stick example, but it's probably only relevant at a standstill, when friction of the tire on the ground is minimal.
- Gyroscopic effect is an apparent force that pushes the bike up when it's in motion and leaning to one side. But it can't be the only relevant effect here, because then the bike would just come to rest at a certain lean angle but roll straight on.
- When the bike leans, the contact patch shifts to the inside of the curve. This might put leverage on the front wheel to steer it into the turn. The rolling circumference also gets smaller as the bike leans, prompting the wheel to accelerate its rotation, but because of inertia the wheel will instead react by putting leverage on the steering axis, steering the wheel into the turn. If this effect is significant or not, I don't know.
I'd be curious to see a computer simulation which can eliminate various factors to see which ones truly contribute to the self-stabilizing of a bike.
Balancing a bike without a rider has everything to do with the caster of the fork, not the center of mass.
There is no doubt, Neil Degrasse Tyson is my favorite explainer.
Yeah it's called counter streeting, that's how all bikes are steered by riders. Much more obvious on a motorbike at speed. So you initiate a fall to the side in the direction you want to steer in then you steer the opposite direction so that the front tyre tracts the desired radius.
An interesting fact about motorcycles is that a bike set up for Trials, which involves a lot of low speed maneuvering, has a steeper rake then a bike set up for road racing which has a gentler rake angle. Motorcycles set up for drag racing which aren't intended to turn, have their front wheel extended out a few feet to keep them going straight. To get a motorcycle to turn left you actually press down on the left handlebar and to turn right you press down on the right handlebar. Having a single disk brake on the front wheel doesn't make the motorcycle veer to what ever side of the wheel it's mounted on. And you don't lean on a motorcycle in curves, you keep your body straight to maintain stability! Understanding the physics of a motorcycle is important!
Another fun counterintuitive thing about bikes: the weight of the rider is not actually supported by the bottom of the wheels; instead the rider is dangling from the top of the wheels. The spokes are very thin and have very little strength under compression, but they have great strength under tension.
Neil, would love to hear about the physics of a motorcycle jump. Especially what looks like rotational control in mid air. Is the rider, for example, applying the brakes a bit to rotate the nose downwards?
You're really smart just because you guessed that! Yeah, when you accelerate the wheels, nose goes slightly up, when apply brakes - slightly down. I certainly wouldn't have guessed that, I wouldn't have thought that's something even possible
@@ldmtagSounds exactly like what happens on the ground 🧐. It's almost as if angular momentum is the reason why it behaves that way in both cases 🤷♂️. Idk if that's the (only) reason or if it's a pure coincidence 🤔.
@@louisrobitaille5810 no, it's not the only reason, and no, it's not a coincidence. On the ground the main force that makes you do a wheelie is tire friction. It's applied not directly to the center of mass, hence it generates rotational torque. Normally it just slightly squats the bike during acceleration making the center of the rear wheel the center of bike's rotation, around which now the force of gravity is off-centered and too makes the torque, and this torque is what keeps the bike from doing a wheelie.
I still got the scars of me standing up on a bike. 😂😂😂
We like to superman while riding
@@Indypacecar82 exactly what I was doing. On the back pegs
We also do the UCI banned supertuck. It gets a little scary on descents while approaching 50mph
ME too lol
i have scars of my attempting to stand on the seat 🤣
Derek has an amazing video on this topic on his channel Veretasium. It explains the balancing of bicycles so well.
Interesting... I was convinced by the gyroscopic effect explanation - because this is what I learned while training for my motorcycle license...
But if that is not the proper explanation, then there is something else my instructor said that does not make sense anymore: he told me that past a certain speed (around 30 km/h so about 18mph) the gyroscopic effect makes it so the bike simply cannot fall - unless you lose traction or hit something of course.
And it does indeed require a conscious effort from the pilot to keep the bike up at low speed - some bikes require more effort than others, due to their center of mass of course...
So here's my question (to which I'd bet you have the answer). If the gyroscopic effect is not responsible for this... Why does the bike have to reach a certain speed to be automatically stabilized?
Thanks for everything you teach us through your videos btw!
The gyro effect acts in a similar way to the effect Neil talked about: When the bike has some roll velocity (meaning it starts leaning left/right) the gyro effect also acts to steer the front wheel in the direction of the turn. So it's not one or the other, it's both. As you go faster, the gyro effect becomes stronger. I'm not sure if there's a point where the gyro effect is stronger or what, it'd depend on the particular bike, I suppose. Maybe it's always less, maybe it's sometimes more. Not sure, but it should get stronger as bike speed increases anyway.
A UC Berkeley professor hosted a "Bicycling Science" lecture at open house. He had a bicycle made with geometry that caused it to steer left when leaning right, and steer right when leaning left. (But not a reverse-steering bike via the handlebars like "Smarter Every Day.") He let me ride it and was impressed that I had no problem first try. But it was horrible how you had to fight the natural geometry to get it to "steer into the fall."
We gotta "know" the 'critical' center of mass in the system we desire - like on our bikes or up on the wire. 'Critical mass' like the moment between not boiling and boiling - there's a point of no return - the center of mass can bring you down to the ground because it is so heavy- it ain't easy till it's easy, then it balances. You don't forget how to ride a bike, so they say.
As an off-road rider I grok this, the bike will just stay up if you keep yourself flexible, strong and ready to go with the bike. Use the throttle and clutch to keep momentum and brakes to control speed. You put the body's momentum toward the line you want while looking fairly far ahead. Hitting huge rocks will fling you one way or the other and the bike will just stay up if you can hang on and stay ahead maintaining correction momentum for the body to match the center of mass toward the lowest point in the system. It's an amazing fun ride.. hang on!
Thank you, Mr. Tyson for the explanation. As a motorbike messenger for 6 years straight, I've always thought about the Center of Mass and Gravity's effect on my motorbike. And I've taken a pro riding course that involves lowering the CMG during cornering too few years ago, which makes me believe on it even more. Thank you, Sir. Once again.
Question: In which direction do you turn the handlebar on your bike if you want to go right? Clockwise or counterclockwise?
@@thanksyutbeisacn7fu6r50 I clarified my question since pushing right or left doesn't make much sense. :'D
Initially, you counter steer
I am no physicist but I think that for a bike (or any comparable vehicle) to stay upright, the center of mass must actually be ABOVE the center of the wheels. And if you think it through that kind of makes sense: A system of any kind is only stable if it is able to counteract any disturbances from its equilibrium (like those wiggle-dolls you mentioned). For a bike that means that if it is going in a straight line and upright, it must counteract any motion that drives it away from that configuration. Imagine the bike (going at some speed) tilting slightly to the right (or left). What happens initially is that the front wheel will - due to its gyroscopic precession - turn inwards to the right, ultimately forcing the bike to turn. Now, because for most bikes the center of mass is located pretty high up relative to the center of wheels (plus rider even more so) that turn will induce, loosely speaking, a centripetal force on that center of mass to the opposite side, resulting in the bike leaning back to the left. This will go back and forth until the front wheel is aligned again. If that doesn’t appears convincing, picture a bike where the upper half of the frame is made of light plastic and the lower half is made of heavy lead. If that bike, rider excluded, were to go downhill starting in a stable configuration and started to tilt to the right, I would not expect the bike to realign itself. But that’s just my theory and I could be totally wrong haha. Cool video anyway✌️
I have a different explanation: centrifugal force.
Every time a rolling wheel begins to fall over, it's path to motion curves to the direction of the fall.
At that moment, centrifugal force kicks in and puts the wheel back upright.
Watch a rolling coin carefully and you will notice that it wiggles as it slows down before losing balance and falling over.
As it slows down, the centrifugal force weakens hence the wriggling. Eventually got, the centrifugal force is to weak to counter the fall and the coin falls.
It's the same effect with a bike.
Is there any way an unsteady walker could use any of the principles to make it harder to fall? Or does a walker do that, even though the idea seems simpler? I'm not there yet, but I wanted to know if there's any way to improve walking around for those who have trouble.
Neil, I would suggest you buy a book called The Racing Motorcycle by JohnBradley. In it, you will find all the correct information and science as to the dynamics of a bike and how they steer and stay up. It is not how you have been misled and put in this video. This info has been known by bike desugners for decades.
Having your center of mass on a 2-wheeler low is not actually helping with stability, it might be doing the opposite. I have riden very low lowracer recumbents with center of mass of the rider just about 40cm above ground and tall 29" upright bikes with their center of mass maybe 110cm above ground. The low bike requires faster reactions to anything that destabilizes you. It like balancing a short stick on the palm of your hand versus a long stick. Other things being equal lower bike will require faster reflexes and therefore feel less stable. But you need a big heigth difference to notice this. Among just upright bikes for example other differences will be so much more noticable they will hide this one.
To make the bike balance without rider you need the steering assembly (front wheel, fork, stem, handlebars) to lean to the side the bike wants to fall to faster then the rest of the bike. Then any disturbance of the initial upright position will result in a turn that corrects it back to upright. It does not matter if your design uses the gyroscopic effect to help this or not, or how much exactly the front wheel trails behind the steering axis. As long as the steering falls into a turn fast enough (but not crazy fast, that could induce fast oscilations) it will work and you can send that bike down the hill and see it keep going.
However once you put a human on it it becomes more complicated. The weight of the arms on the handlebars by itself already changes the behaviour of the bike. The overall mass is now much greater and center of mass is in different place. And the mass is no longer rigid, most of it is somewhat flexible and actively moves too. And the person steers actively to keep balance but with the limitations of human reaction times. So now describing the stability of this bike-human system becomes crazy complicated. To my knowledge nobody ever built a good mathematical model for this that would let you predict what bike geometry would be nice to ride. The only way is to build the bike and test it.
As a result I have seen bicycles that were not only not stable to ride themselves down a hill but you could not even push the bike just by the saddle, you had to have a hand on the handlebars when pushing it. And yet it was perfectly nice bike once you sit on it and ride it. Just the change in mass and its distribution did the trick. And I have seen a bike that is nicely stable when you push it by saddle alone and yet it is unpleasant to ride - steering feels rubbery and like it is fighting you.
I remember when I was a kid, I did this experiment (I heard or read about it somewhere, but I don't remember where) where you put a fork and spoon together with a toothpick pushed between them, balanced on the edge of a glass, then you light the inward-facing end of the toothpick on fire. It burns to the edge of the glass and stops, and you're left with this thing standing on the edge of a glass, barely touching it, yet staying put. Pretty simple physics, since it's just keeping the center of gravity so low, it can't tip in either direction, but it looks completely counterintuitive.
Here's my counter question: If the pole is for lowering the center of mass, the highwire walkers who do this without a pole usually use their outstretched arms to balance...If lowering the center of mass is the way to go, why lift your arms up to shoulder height, instead just keep them low by your waist?
Its a weird answer alltogether. If you even tried balancing a broom or something on your hand, you will notice it gets easier with the heavy side up instead of the other way around.
A balance pole need not be "floppy" or hang down to be effective. What the pole does is give the arealist a source of torque. Example: From the arealist's viewpoint, if they should lean to the left they can impart a quick counterclockwise torque on the balace pole. The counteracting torque enables them to shift the center of gravity to the right, to a point directly over the wire. Of course, you could have a pole whose ends were below the wire and were heavy enough to make balancing automatic, but you won't see that in a circus act.
The length of the pole also increases the polar moment inertia, making the arealist less likely to rotate off center.
Regarding Gary's mention of "food delivey drivers", In urban UK cities, those services are very often bicyclists, not motorists in automobiles.
It's the curve in the front forks that makes the bike go straight. if you analyze the geometry you can see that when the handle bars are turned that lifts the front of the bike slightly. therefore, in order to turn you have to put energy into the system. i.e. the bike goes straight unless some force acts on it.
ohhh that's interesting. I've mentioned in another comment that I have a single bike which is quite hard to ride handsfree. I have to shift my weight and lean to one side to drive straight. The handlebar is not exactly straight adjusted. Maybe that's the reason. I got this bike for free but wasn't able to center the handlebar due to corrosion
Nope. That helps you come out of corners without applying force to the handlebars to go straight. What keeps the bike up is the self aligning torque of the wheels, the natural tendency of them to roll straight and true on their own.
I ride motorcycles, and used to do it competitively, and that takes a LOT of science.
@@macgoryeo Sometimes if a bike is crashed the forks are bent slightly to one side, so the front wheel isn't centered properly. That throws off the alignment. Handlebar setting is more cosmetic, particularly when no hands are involved.
When I sold bicycles, someone came in and paid cash ($600 on 1973) for a Lejune Pro. I had it put together and rode it before the customer picked it up. All you had to do to turn that bike (a really good one at that time) was to lean one direction or another.
4:58 I was thinking that this is how i definitely do it, though it’s a conscious effort for me
This is the first time I have seen Tyson get something wrong.
For a Motorbike it is all about Castor not Centre of Mass. The contact point on the ground is dragging behind the pivot point of the wheel. This is why it only works when moving, Tyson’s description would also work when standing still. Other than that his description of it turning into the lean was correct.
Really great and fun explainer 😁
I remember Veritasium doing a long deep dive video into this subject.
Question, if you drive a bicycle without your hands, what is the difference between hit a bump and hit a hole of same size and same shape knowing that the back wheel, will push forward? Do you have more chance to fall because of a hole of a bump?
On my Dad's desk was a metal highwire figurine that stood on a metal cylinder. I would watch that thing teetering back and forth and be amazed by it. He explained it but it didn't really get it when i was 4 lmao. (Back and forth though, not side to side.. but same principle)
It would help if you explain it in simpler terms, the key here is the forks being at a specific angle that they turn in the direction the bike is leaning so once the bike is in motion and it starts the lean the front wheel turns into the lean creating a braking force slowing the bike down however, the rest of the bikes weight overcomes this braking force by pushing on the front wheel because it does not want to change direction forcing the bike to stand up because is pushing against the lean... ask any motorcycle racer what happens if you are at a lean and you brake using the front or accelerate, the result is the same the bike stands up because you load the front and because the rest of the bike is heavier than the front wheel forcing the front wheel to alight with the direction of the biggest mass moving forward however if you apply read brake the bike leans more.
Ye not sure I agree, but when I was a kid I tried to do the ghost bike trick with the handlebars removed and it didnt work so in my opinion it has to do with the handle bars on the bike which makes it stable.
you also when at speed you don't turn the bars in the direction you want to turn, you push forward with the hand in the direction you want to turn and physics takes over.
Centre of mass below the axles. Spinning wheels have gyroscopic force, but the steering geometry needs to be such that the front wheel returns to centre.
I have developed balance issues when I walk, so I gave up riding my bike. But after a year, I thought to try it again. Much to my surprise, I discovered my balance is much better on my bike then walking. It actually feels normal. What puzzles me now is that when I was a kid I used to have no trouble riding with no hands. And yet I cannot do that today, or even before my balance was an issue.
Sounds like the position of you holding the steering allows for the bike's self-balancing effect to compensate for your poor balance until you take your hands off. Just my guess though 🤷♂️.
Cool. Next, do one about how cars stand up.
The most surprising part is that you can cancel gyroscopic effect. I can't believe it. I understand why it cancels rotating effect like a double-bladed helicopter without tail blades but canceling gyroscopic effect? Nah, I believe only when I see it.
On helicopters with a standard single primary rotor, the steering mechanism is set up to compensate for the fact that you actually have to provide a swashplate input, that is 90 degrees from what you'd instinctively think the swashplate input should be, so that the controls are intuitive to the pilot.
Neil can you do a follow up about motorcycles and the notorious death wobble and how to correct for it?
Hello Neil, my name is Benjamin and I will definitely be more of your patrons someday, my biggest mind boggling question is. Out of all the plants we observe, stars and galaxies, and now having a James Webb telescope that could either see into the past or now how come, we have not found now another planet containing green life like we do where does trees really come from?
Question that popped into my head is if cats change the center of their mass when they fall? As they "always" land on all four. I have read that by pulling the paws in towards the body, the cat causes the front part of the body to rotate faster than the back part. When all four paws are facing the right way, they can absorb the shock of impact. Whats the science behind it?
Apart from gyroscopic forces there is also another self righting force generated by the castor of the front wheels; i.e the angle the handle bars make when connected to front wheel. That's why you can let go of the handle bars in motion and the bike won't tip over and continue to follow a straight line. Perhaps that explains why the bike with the counter rotating wheels to cancel gyroscopic forces was still able to stay upright.
Wait, how'd I miss THIS?????
Thanks!!!
Just thinking about walking a high wire... 100% I’m seeing this through the lens of me doing it. Which is why I it’s thrilling. Because I have a fear of heights. Knowing that physics is applying forces in invisible ways mitigates my fear.
Theres an E-bike (motorcycle) out there, i saw a few years ago, with rotating gyroscopes inside the rims spinning even when bike is not moving. The bike stands up freely using only the gyroscopes as long as the battery is providing power.
Its all gyroscopic.
What you are describing is gyroscopes being added in order to make the bike more stable when stationary or going very slow. But once up to a decent speed the gyroscopic effect is not required to make the bike self balance. It works because the steering moves freely. If the bike begins to fall to the right, the steering automatically falls to the right and drives the tire back up under the center of mass. This only works with forward motion. The motorcycle or bicycle has to have a properly designed front steering system in order for this to work. So while you might find ways to use gyroscopes to improve the situation, it is not "all" gyroscopic and gyroscopes are not even necessary. Lots of people have done many experiments to demonstrate this very clearly. People have taken the gyroscopic effect out of the bicycle and it still works. They've also locked the steering and it no longer self balances. You can see all of this for yourself in other videos.
@@adamharnois7072
I never mentioned going at speed.
You've wasted our time.
@@Sammasambuddha Interesting. What direction do the gyros in the rims spin? Same axis as the wheels? That shouldn't work because if there's a roll acceleration (lean) the gyros should just cause a yaw motion which isn't self stabilizing in roll (lean). Could there be some other gyros spinning in other directions too perhaps?
Is it this? ua-cam.com/video/Okf283Ct-NY/v-deo.html&ab_channel=TomoNewsUS
That's using steering to do it. Maybe you're talking about a different bike?
Found one that uses gyros at about 4 minutes in:
ua-cam.com/video/q1NaXSk1HWQ/v-deo.html&ab_channel=MasterBoyTV
Is that the one? That one looks like it's using gyros in multiple axes, not lined up with the wheel spin directions, which is what I'd expect for a gyro system to work on a bike.
Neil, thank you for the excellent explanation of the equilibrium on the rope. But when I was a kid, we used to walk on the rope with a rigid bamboo pole and not the flexible one. Of course the rope on which we walked was not very high and the pole was only about 3 m in length.
The best film version of ghost bike is found in ‘Jour de Fete’ by Jacques Tati. It parks neatly at a cafe in jaunty manner.
Two counter-rotating wheels sharing same axle and plane of rotation cancel out? I know very little.
A single rotating wheel is more stable than two with one joined to frame and one freely yawing…….
I thought, as the body starts to lean to the side, the center of mass goes DOWN due to both the body shifting to the side and the pole shifting to the side with it. I always figured they use the pole to make the center of mass change and force the person’s body back upright.
Why would anyone think it is gyroscopic effect? For gyroscopic effect to stabilize a rotating mass on it axis, it has to revolve or precess on the third perpendicular axis, that means the bicycle has to go on a curve and make a circle on downhill, but it doesnt, it go downhill straight.
BTW, it is not just bicycle, it happens to a wheel, single tire rolling down the hill as well.
When a wheel is rolling downhill, there must a force which should act on axis of rotation to brought it down. But during downhill roll, the CG shifts towards front of the tire, so it rolls faster and still no sideway acting force.
If anyone who thought it was gyroscopic effect before does not understand gyroscopic effect.
If a counter-rotating wheel on a bicycle cancels gyroscopic action, does the mass of each counter-rotating wheel need to be equal?
Also, ust a thought.If the high-wire walker's pole dips on one side, it would tend to raise the other side a nearly equal amount, and the center of mass would not necessaily be lower to the center of the Earth. The walker (himself/herself) is in effect a movable fulcrum in relation to the pole.
Olympic high-jumpers are able to bend their backs in such a way that their center of gravity is outside their body, so when they jump over the pole, their center of gravity never rises above the height of the pole.
That information was fascinating to me. One reason is that women's center of mass is located in their hips. Men's is located in their chest. So if you're a male wire walker, you DEFINITELY need that curved pole to stabilize your connection on that wire. What do you think? I mean if I were a female wire walker I'd still carry that curved pole, but it's interesting to me.
Hey another question also with the pole on the wire wouldn’t lowering the center of mass essentially be to benefit the balance therefore that’s still the reason?
With the floppy balancing pole - if the left side goes down two feet - the right side goes up two feet, yes? So how does that alter the centre of gravity of the bike/rider/pole?
Me and my friends used to do that all the time!!! 1:45 😂
Nice 👏
When it comes to bikes and motorcycles it all comes down to the rake (offset angle) of the front axle and the fact that the fork can move turning the wheel left/right, while the rear axis is fixed. Bicycles and motorbikes has two critical speeds. Going forward reaching critical speed the bike is self-balancing. Going backwards reaching the other critical speed the bike becomes completely destabilised. This means that reach enough speed going forward and the bike will lean the front wheel into any turn and self-correct raising itself up due to center of mass, rake and newtons first law. Reaching enough speed going backwards and not even the best computerised system can keep the bicycle upright (it becomes complete unstable). So the video is missing a few key elements to a bike, mainly the effect of the rake and the fact that a bike has two critical speeds, one making it stable, one making it completely unstable.
For the COM to provide absolute stability of a bike, the COM would have to be BELOW the ground level. Think of an inverted V resting on a horizontal rod: completely stable. This is never the case with any bike, no matter how low its COM is. The fact that the bike can stay upright only when rolling strongly suggests the stability is coming from the gyroscopic action of the wheels.
Hey, we actually do that instinctively. Tried to walk over an edge? What's the first thing you do if you're leaning sideways? Spread your arms wide open, just like that pole. And it is not calculated.
I actually have a electric mountain bike with fat tires for stability outdoors, I can 100% say you can ride it for awhile without hands because with pedal assist it’s enough momentum to stay up and depending on the road and how it curves the bike will stay on course. 90° angles are the tricky part.
Bicycles are rear wheel drive so the gyroscopic precession is in the front wheel.
The industry term is "e-bike". Which is short for, electric bicycle. The battery pack on e-bikes is typically on the down tube or seat tube.
#Bicycle #GCN #Bike
Neil deGrasse Tyson is educating America through UA-cam. Amazing.
And he is wrong in this one unfortunately.
It seems to me that the most glaring omission in this explanation involves the effect of momentum.
It's all basically about how the bike is steered. Very slight corrections are continuously made as one is moving forward, which is why you can't really stay upright easily moving backward or remaining motionless. Yes, a bike rolling forward without a rider can self correct as it will steer into a lean --- but watch what happens in reverse.
How can you ask a question like that ? Gyroscopic action of the wheels and a person's own balance
A bike with a higher centre of gravity isn't necessarily less stabile in the same way reversing a long trailer is easier than a short one. The pendulum is longer.
Any explanation of the physics of keeping a bicycle upright has to account for the fact that it only works when rolling forward, not backward.
Does anyone know where the full episode is?
1:57 also if you think about it, even if the bike speed is really low, so that the wheels are just moving, so that the gyroscopic effect is negligible to it's weight..still the bike stays upright no problem
How can the CM be below the wire for a tightrope walker holding a bar horizontally? No part of either the bar (even a sagging one) or the walker is below the wire, let alone half the mass. If the combined walker+bar CM was below the wire, the system would be stable and there'd be no challenge to tight rope walking.
I don't think lowering the CM has anything to do with any of this in the tightrope explanation. What you have is a system where angular momentum is perhaps 0 to start, along with a (simplifying a bit) single vertical force at the walker's feet. If there's a little disturbance and his CM shifts left a little bit, the vertical force is now to the right of his CM which will cause a torque to the left. The walker will start rotating in that direction.
By applying an opposite torque to the bar directly, he can induce an opposite rotation in his body and momentarily shift his CM back to the opposite side of the wire. If he moves it far enough and holds it there long enough he can then apply an opposite torque to reverse the rotation of the bar, then stop it when it's level again, thereby canceling the angular momentum in the system. His CM is now back to center and with 0 torque and angular momentum in the walker+bar system until it's disturbed again.
There's probably more to it like manipulating the force directions laterally with your feet/legs, maybe doing some lateral translation of the bar and so forth. The point is all this stuff about lowering the CM is irrelevant in the tightrope example. If the CM is above the wire at all by any amount, the system is unstable regardless. Lowering it to a height that's still above the wire won't reverse the torque on the system and stabilize it.
The Weeble Wobble also doesn't have much to do with the CM lowering. The reason a Weeble Wobble is stable is because a positional disturbance (say rolling it to the left just like the tightrope walker) pushes the contact point further to the left than the CM moves in that same direction. The vertical force is now far to the left of the CM instead of directly under it which produces a torque in the opposite direction canceling and then reversing the roll velocity. That rights it again. How the CM moved vertically is irrelevant.
In “The Whole Earth Catalog” there is a listing called “Intelligent Motorcycling” and the review said, A Contradiction in Terms”.
1st time I don’t agree with you. Its the caster wheel design that keeps the bicycle dive straight and it will steer in to the turn to right itself if the bicycle made a turn keeping it upright. I discovered that when i was young and drove my bicycle into a curb causing the front wheel fork to bend backwards which made the bicycle unstable because the caster angle was reduced.
The angle of the front fork is what helps to keep straight. It's always tilted to the backwards. Even car front suspension is tilted to back. That's why steering wheel automatically comes straight after turning
This is wrong. They balance due to the rake and trail of the front wheel: due to the design of the front forks holding the wheel, when te bike leans left, the front wheel turns left by itself etc
It has little to do with the center of mass being low, more about the fact that centrifugal force does not exist. When the front wheel turn left or you lean the bike left the front wheel turn more left than than the back wheel, this result in the back wheel continuing its course and thus acting like a force pulling the front wheel back to the right. This plus the energy given from the ground hitting the wheel giving it 2 effects permitting is to lift itself back straight.
Can you please explain on one of your episodes how when my key fob is just out of range from my car to unlock it but if I put the fob to my chin pointing up to my head it’ll work 🤷🏽♂️
The stick also increases the moment of inertia, making it more difficult to rotate/tip while progressing along the wire..
Dr Tyson, you have to do a video on pumping groundwater and it causing 31inches of tilt to the earths axis, its really interesting!! Need to hear you teach me more about it please!!
Momentum under a locomotive force, down is forward
force.
This explainer reminds me of the balancing bird (“balancing eagle”) that can balance on your fingertip with only its beak.
For sure, the answer to this is in the wheel gyro and countersteering frame/fork geometry. I don't think the counter-spinning bicycle wheel would cancel out all the main wheel's gyroscopic forces unless it has weight added to it to make them a closer rotating mass. As is, it is missing tube and tire. The weight on motorcycles is not between the feet (foot pegs) and rear wheel. Much of it is right behind the front wheel. this includes both the engine and gas tank (which is fairly high) on most standard style motorcycles. The best way to test this would be to RIDE A MOTORCYCLE! :)
It does. Without a doubt. If you counter-rotated with exactly the same RPM and mass, there would be ZERO gyroscopic effect. (That they didn't match the mass exactly isn't really that important - the result was canceling the vast majority of any gyroscopic effect in action.) Crazy that you're gonna challenge Tyson on this. This is clearly understood. Physics 101. You could design a bike with 500 wheels spinning on one direction and 500 in the other and the net gyro effect would be zero. However, the system would have increased overall mass, so the system has more inertia. But that's it.
On other forums, people with zero understanding of physics would talk about "forces acting through the tire" as if they're some mystical phenomenon. Nope.
@@dudeonbike800 Correct but it has to be the same mass right for each counter-rotating wheel right? If you look at the image of the test bicycle at about 2:04, one is clearly missing a tube and tire. This would make a noticable difference in the masses right? And lastly, chlll out. Science is a process of discussion and review. If someone misunderstands or is missing information, we help each other learn. No need for the "how dare you." attitude. Be a better ambassador to the community and art itself.
I believe it's all about how the front wheel is designed to castor. i suspect that even if you raise the center of gravity of a bicycle it'll still tend to right itself.
The reason a human riding a bike has to actively maintain a balance - by steering the front wheel into the lean, however slightly, at all times - is because the rider is *not* a rigid mass attached to the bicycle; the rider flops around independently and needs to maintain their balance on the bike. That the bike itself is self-righting is what makes the task easier, nay, possible for the human.
Here's another attempt, probably inadequate, to explain the self-righting nature of a bike and how its center of mass affects it...
When a bike is upright going straight, its center of mass is directly above the bikes track, the track being the path of it's wheels. If the bike leans, then the CoM is no longer above it's track.
The front wheel and forks are designed to castor the front wheel and turn the bike to the side the center of mass is on, curving the path of the bike towards that side. This creates a centrifugal "force" which will push the center of mass back towards the bike's track, righting the bike.
Think of it as a negative-feedback loop. When a bike is moving in a straight line, the CoM is above the bike's track. If the bike leans, moving the CoM to either side of the track, the bike is designed to automatically create a lateral force to counter that movement, which will push the CoM back above the track.
In order to cause a bike to turn, the rider must then counter the lateral counter force to maintain a stable off-track CoM position for the duration of the turn.
How's that?
Everytime I think of this it boggles my mind how humans (and maybe some animals?) can learn to do this sort of thing instinctively.
The average mass of the battery is about that of the wheel axles; the mass of the motor is near the pedal center. I find my ebike quite stable. Wheels as gyroscopes help in all sorts of riding, on the ground and through the air.
The Geometry of Caster plus COG makes for a better argument.