Why Do Big Trucks Use Air Brakes? How Does It work?
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- Опубліковано 8 лют 2025
- In this video, we explore why big trucks use air brakes instead of the hydraulic brakes commonly found in cars. We dive into the mechanics of how air brakes work, the safety features that make them essential for heavy vehicles, and the importance of regular maintenance. From understanding the role of the air compressor to the critical function of the dual-circuit system, this video covers everything you need to know about the air brakes that keep these giants of the road safe.
Timestamps:
0:16 - Introduction
1:06 - The Need for Air Brakes
2:47 - How Air Brakes Work
5:02 - The Role of the Air Dryer and Safety Valves
6:13 - The Importance of Regular Maintenance
7:13 - Why Air Brakes Are Standard for Big Trucks
8:41 - Conclusion
#AirBrakes #TruckBrakes #HowAirBrakesWork #HeavyTrucks #TruckSafety #BrakingSystems #TruckMaintenance #SemiTruckBrakes #CommercialVehicles #TruckEngineering
I feel one of the key reasons air brakes are used was missed. It's the ability to readily attach the tractor's braking system to the trailer's braking system, simply by connecting air hoses together. Just like on a train, where the locomotive's braking system is easily attached to all the cars behind it. Imagine trying to that with hydraulics. Messy, inefficient, and not fail-safe.
There's hydraulic quick couplers to get around this
Short answer....
The moment air brakes stop working is when they work best.
One of my vivid memories of my teenage years was lying under my dad's semi at 2:00am in 20° weather trying to find a air leak.
As a 40 year trucker, I see a couple things not covered -
First, while the system can automatically apply the brakes, it depends on the condition of the shoes/pads - if they're hot, they're less effective; if they're smoking, the brakes are wholly ineffective. The brakes on ALL vehicles rely on the ability of the shoes/pads absorbing heat and re-radiating it. Heat is created by friction - basically the brakes exchange motion energy into heat energy; if the brakes heat up enough, they aren't transferring heat any more, and continued friction will eventually set the brakes on fire.
Second, and related, hydraulic fluid can boil. When it does, it creates bubbles, which cause failure in the system, as the system relies on the closed liquid system.
As a truck driver, and mechanic. without even watching the video. I can tell you the reason why. It's because they're failsafe. Anything at all goes wrong with the brake system, and they lock on instead of just failing. Well then, you might ask why every car doesn't use them. They're marginally more complicated, require much more maintenance. Daily checks for leaks, and possible problems. As well as monitoring of various gages (and knowing what those gages mean, and the problems they're indicating). Asking the average person who doesn't even know how to check their own oil to do that? Yeah not going to happen.
Another thing about air brakes is you can work on and replace any part of it, start the vehicle, wait for it to build air pressure, and then test the system. With hydraulic brakes, it is common to have to get someone to help you bleed the brakes. I find brake bleeding to be a time consuming process that is difficult with the configuration of the brake lines on some vehicles.
All of the smaller vehicles built after the sixties have duel circuit hydraulic brakes. Light and medium rigid tray trucks sometimes use an air over hydraulic system. The main reason large trucks use air brakes is for connecting the trailer brakes simply and easily, something that cannot be done with hydralics.
Hello from Finland. Nice tutorial, thanks. Here our own trucks are in great condition due to mandatory yearly inspections (MOT) which are very strict. But Russian trucks are very often in really bad condition. Our Customs check Russian trucks when they arrive here. They must pass inspection before they are allowed to drive here. Often the driver loans or hires tools and fixes the problems themselves as they are used to maintain their cars even if it isn’t their own. And often they are much overweighted. Many trucks are turned back to Russia.
I never noticed the massive trucks sharing the road with me 0:19 🤣😭
Every single information in this video is new to me.
Thank you for the education.
Now that I think about it, using a hydraulic system would be problematic when changing out a trailer. If you introduce air bubbles into a hydraulic system, it would create a dangerously spongy pedal. When attaching the air connection through glad hands to link trailer with truck, a small amount of introduced air would cause no problem at all.
Holy crap! I’m about to start my fourth year of mechanical engineering undergrad. This channel is awesome!! It has the informativeness of those old discovery channel documentaries but with a modern flavor and quality. I’m glad channels like you exist :)
2:27 It should really be called air-released-spring-actuated-brakes. The air pressure is actually holding back the spring that when released would mechanically clamp the brake pads or press on the wheel drums to stop the spinning wheels.
When you press a hydraulic brake pedal you apply hydraulic pressure to the brakes. When you press an air brake pedal you release air pressure allowing the brake springs to activate the brakes.
Incorrect mate. When you push the brake pedal it sends a charge of air to a proportioning valve on the air tank that sends the application air to the brake booster. The brake booster has 2 chambers. ! with a spring being held back but the release of the park brake and 1 that applies the brake with air. Thischamber has a soft spring to aid with releaseing the brake when you lift off the brake pedal.
Something that should be mentioned and is often overlooked is that even though trucks have very good braking systems, their increased weight and size means that they still take more distance to stop - kinetic energy is a thing and ALL drivers of ALL vehicles on the road need to be aware of this. Truck drivers often get the rap when there are large mishaps, but they haven't been given the room they need to execute their job safely.
Thank you for the knowledge. I'm from the Philippines. 🇵🇭 🇵🇭
To be fair, modern hydraulic system on cars and light trucks in a seen have two separate systems as well. Meaning if you lose fluid to the front brakes there is still a reservoir with fluid to control the rear’s and vise versa in case a failure. It will allow multiple petal applications to an extent on a dual master cylinder provided it doesn’t allow air to enter the piston area and back into the system that isn’t leaking.
Great job !
In theory you equip air brakes even to smaller cars, the only thing stopping them are weight, design and very expensive
Interesting parallels with aviation here. Airliners use hydraulics (obviously), but like air brake systems, they use redundancies in a similar manner. Usually there are three systems that overlap, making failures of any given flight control or aircraft system very rare, and even if all three hydraulic systems fail, other methods provide pressure sufficient to safely operate the aircraft. In some smaller aircraft, the hydraulic pressure provides a fail-safe because landing gear is held up with the pressure, so any loss will result in the gear deploying to allow a safe landing. (In systems where this is not the case, there are numerous backup ways to drop the gear.)
As a truck driver for 35 years the reason air brakes are effective is due to their failure safe setup. Air is sent from the Air Compressor to the Storage Air Tank, which also sends Air to Two Tanks known as Primary and Secondary, after which the Air is sent to the Front (coming from Secondary Tank) Air to rear (coming from Primary Tank) so even if the Storage Tank for whatever reasons fails, the other two tanks will still have air.
Air brakes main advantage is that it is not the driver’s muscles stopping the truck. An air brake pedal opens a valve and then air does all the work. You are not forcing fluid down a pipe with a plunger limited by your own strength. If you’ve ever had a trailer push you down a road you’d know that you may have to get up off the seat and stand on the pedal.
I remember as a kid that trucks used to have signs at rear stating "Caution, Air Brakes".
Big trucks and their trailers NOW have AIR DISK BRAKES ON ALL WHEELS, making the vehicle much easier to stop, and ABS to control skidding even on the trailers. This is the best safety system being installed on all trucks and trailers is long in coming.
Same system on trains by the way 🎉
LESSON LEARNED:
For an extra added safety feature, from now on, I’m definitely getting air brakes installed in my old 1975 Fiat 500 first thing tomorrow morning!☝️
It’s the way of the road boys way of the road.
To clarify: spring pressure APPLIES the brakes. Air pressure is used to overcome the springs and thus RELEASE the brakes. Do I have that correct?
Do a video about things Truck Driver wish private car drivers knew.
Car drivers don't understand why we do rolling stops, why we maintain an increased following distance, amongst other things.
I love learning.... subbed
Interesting, diagram labelled a spring brake. But wasn't a spring brake chamber. Stated air brakes are used because of the safety of a spring brake chamber. Air brakes were in use prior to the introduction of the spring brake chamber.
As mentioned by someone else. Input (brake pedal) application pressure to actual available output pressure is much higher in an air brake system, as opposed to a hydralic system.
Yeah, that was a simple service brake with no emergency spring. Good catch!
When I was an over the road driver, I drained the air tanks every morning while the truck was idling up or I was fueling. I rarely had enough water in the tanks to drown a mouse, lol
congrats for the explanation
When hydraulic brakes fail, they stop stopping. When air brakes fail, they stop not stopping. So air brakes are safer.
Good video - very informative.
Large trucks and trailers now can be fitted with disc air brakes instead of shoes and drums. They work amazing well. Very responsive and more reliable.
Air brakes can still function perfectly with a minor leak. The air brake system will never run out of air but a hydraulic system will run out of brake fluid if there was a leak even a minor one.
It was a crossover from railways, as airbrakes were well established
Pretty interesting I never really knew this is how the brake functioned on these type of vehicles lol
You should do a video on michigan gravel trains and the weird axle limits we have here
theoretically they are great. but in reality most neglects to maintain the failsafe system. ive seen too many cases where trucks keep moving at insane speeds when the brake fails, instead of the failsafes (automatic engaging of brakes) deploying
You can get brake bands in widths from 7 inches (normal) to about 10 inches ( for extreme duty, like coming off a mountain with a huge load of logs) making air brakes easily adaptable to the expected weight and other stresses that come with the variety of job commercial trucks are made for. Such modifications would be difficult with a hydraulic brake system without significant alteration of components, like a double or triple size fluid reservoir to power the larger brakes, and other mods that might not fit the space for the tires and wheels.
Great stuff!
7:52 Hydraulic brakes also have two independent circuits.
you could design a hydraulic system with the same fail safe spring system integrated. the connect / disconnect from the trailer is the biggest challange to resolve here. hydrualic connections will leak, just look at the back of a farm tractor, they are ussually covered in oil. the air system is clean and easy to connect and disconnect from the trailers. nice video.
The biggest problem with hydraulic brakes is the amount of pedal pressure required to pull the vehicle up. Your car has a vacuum booster to boost the system pressure, that would never work on a heavy vehicle. The alternative is to use compressed air, at which stage you are better off just to go full air anyway
I feel like I just watched a work related training course… oh well I stayed to the end, even with the long pauses and multiple summaries.
In effect, each brake "can" has a normal mode, where the driver controls how much, if any braking is applied, and an emergency side so that if air pressure to the axle end (can) is lost, the brakes apply with no human intervention.
A couple of reasons for not having an air brake system on a passenger car is the increased size, weight and power requirements.
I work on and drive school buses with air brakes. It feels much safer and easier to stop a bigger vehicle. I drove a 36 ft class A rv with hydraulic brakes and hated it. Hydraulic just doesn’t feel the same as air.
Nice Explanation. Need to know how hydraulic power steering works in a next video.
Incredible .
I see a channel called history of simple things, I subscribed.
My question I’ve given a few guys is why not electric brakes like in most light trailers. Anybody who has used air brakes in the north is well aware of frozen brakes (usually on the trailer) that lock up and leave the trucker down until such time they can be thawed. All the methods of thawing them introduce more moisture that refreezes shortly after the heating source is removed. That herein the Canadian north. I’ve been working in heavy industry for decades and we’ve had multiple trucks every winter with frozen brake lines on their trailers.
There was a movie I watch where the enemy cut off the air break hose, and truck failed to stop even if the driver press on the break pedal. After this video, I'm confident that the scrne was inaccurate.
do you have a video on "Jake Brakes" for big trucks?
Dual-circuit brakes have been standard on passenger cars since the 1970's for the same reason.
Why not do a video on : "How does the gears work on a large truck ?"
Hydraulic systems can also be designed to be fail safe by using spring applied hydraulic release braking systems. I am guessing the main reason air brakes are used is because of cost, ease of maintenance, and mainly because it’s much easier to couple air brakes to a trailer than using hydraulics
Training of new drivers who are not familiar with air brake application versus hydraulic brakes would have been useful.
Can airbrake applied to small vehicles? Is it possible?
Oh damn, for a moment I thought that he meant those flaps, like on planes, which are used for increasing the drag 😂
The thing is on some trucks only one drive axle has the spring brakes so the other drive axle and the front brakes don't do anything if you loose pressure. It's more common for both drive axles to have springs though, but you're still not getting any help from the front. The front brakes really don't do much, but in an emergency the little bit they do will help a lot!! Normally all trailer brakes have springs so they will do some good. Springs can get worn and break over time and will never provide the same force as the air pressure operated part of the brake system even when new.
It stands to reason. Most of the braking effort is always on the wheels carrying the most weight. In cars, 70% of the braking effort is done by the front wheels because the front wheels carry 70% of the weight in most cases. In a truck the rear and trailer wheels carry the weight, so that is where the effort is generated. In Australia, where a majority of long-haul trucks have more than one trailer, braking systems are even more complex. Road trains in NSW and VIC include either two 40-foot trailers or a 40 footer and two 20 footers in a C-triple combination and road trains in other states and territories are often twice as long and can weigh over 200 tonnes (440,000lbs), depending on the load being carried.
All trucks built since at least the 80's have spring brakes on all drive axles, and all trailer brakes are spring brakes. Steer axles don't have the spring brakes because if the pressure drops too low, the spring brakes will activate on all axles. That's not good because locked-up steer wheels go in the direction they were last pointing - despite where the steering wheel is turned.
I think they also use air brakes because brake fluid can heat up and introduce air bubbles that can cause failure, on a typical car it doesn't really happen but on heavy truck with load can easily heat up and fail
0:25 I used to be a Truck Driver...
The truck/Trailer combination shown weighs a MINIMUM of 36,400 Lbs. with an empty trailer...maximum Weight for an 18 wheel, 5 axle truck & trailer (like shown) is 80,000 Lbs. (USA) and 88,000 Lbs. (Canada)...more axles increase that limit, fewer reduce it...this is a general rule because the spacing between axles also affects the maximum Weight Limit as well...
The last semi I drove was in the '70s. The tractor had two air lines to the trailer, the one that followed the brake pedal, and the red one that charged a trailer air tank up, which would put on the trailer brakes if the pressure in the charging line from the tractor dropped too low.
I'm surprised nothing was said about this. Doesn't this still exist?
My son wants a video on how a circuit breaker works please.
Very complete video!
I much prefer rigid forks. If you have wide tires, like 2.5 and up, you can run the pressure a little low that will absorb small bumps. It gives much better handling, and you can handle larger bumps with a little technique.
Unless you have actual high quality suspension, which you only see on $4k+ e-bikes, then they actually feel good. Rigid forks are one reason i like radio flyer e-bikes
They also use air brakes to scare automobile drivers next to them when stopped at a red light.
I saw an air brake fail open. It happens. Nobody was seriously hurt, but the driver had to crash land in the offramp going into Fremont St in Las Vegas.
So thats the loud shwooshing sound when I'm near them. I thought they were suspension sounds.
Is it because of the brakes or compressor that you hear this loud rush of air when trucks are in motion or are stopping? I've always wondered what that is.
I call Brave Serria on the spring brake reasoning as to why air brakes are used on big trucks. When I began driving semi trucks, in the early ‘70s, air brakes were widely used and spring brakes were just coming onto the market. Spring brakes soon became a DOT requirement as they do greatly increase the safety of the braking system but they didn’t exist when air brakes took the lead in large vehicle braking. Get your facts right if you really want to explain things.
I worked on psv,and periodically we had to drain air tanks.
The most important part of the air system is it makes my seat bouncy 😂😂😂
Busses use the same air brake system, that's why trucks and busses don't move till the air pressure hits a safe release threshold.
Cracks me up in Pete's dragon went they cut the air line so the truck cant stop 😂
How comes I never came across this channel?
Air brakes were'nt always failsafe. In the early days of air brakes air pressure was used to apply the brakes. In these systems if air pressure was lost there were no brakes. I think they were better than hydraulic brakes for larger vehicles but were a bit more prone to failuer. I'm sure today's air brake systems have prevented many accidents and saved many lives.
Nice
It is a relief to hear this described correctly
Even a standard truck and semi trailer combination has at least 12 brakes these days, imagine the pedal pressure needed to activate all those babies hydraulically. The dual circuit argument is a bit irrelevant though - cars have had dual circuit braking since the 1960's
If you have a leak in your air brakes, it will still work, indefinitely if its not bad, as the source of the pressure can be restored. If its hydrolic, then it will fail eventually, even with a small leak.
I was always wondering what that tssssssssss sound was when a big bus or truck was braking
Thanks for the clear explanation & nice graphics too. I suggest topic on solar energy & its important applications in today's world. 💯✌️👍
Hi! We'll try to cover that on our next video, but for now you can check this out :) ua-cam.com/video/jneaauRjI8I/v-deo.htmlsi=MRXpn68seZJf3aA3
Big trucks use air brakes because the heat induced into the system by the massive friction it takes to slow a tractor hauling 80,000 lbs of goods would boil brake fluid in the lines and you would lose braking after the first few stops.
It’s also important to note that although we are watching this through the Internet, the big trucks that use air brakes are not the Internet. In fact, the Internet is a system of tubes.
Kindly if possible, make a video on drainage system and drainage water purification in each country ,i really need that information and presently not available on UA-cam
one huge advantage is under hard braking you wont boil the brake fluid, and the air system and the valving allows for more pressure exerted to the brakes with minimal effort from the drivers foot and leg muscles.
Im a lucky boy my mercedes vario has air over hydraulic, twice the fun!!
Tell us about the Jig brake please.
Break failures is serious issue in cars with hydraulic break systems. Why is the air break system not used in cars and lighter vehicles?
so that is why big truck making that psssshhh sound when they are slowing down or stopping
Trucks use air brakes because long ago George Westinghouse designed and then proved they worked on railroad trains, significantly reducing train accidents.
When reversing if it's rolling too quick to slow it down doe one use clutch pedal or just tap the brake pedal?
The air is used to disengage the brake pads..
Same can be used by hydrolic flued system
Interesting
It's not just trucks that have air brakes they also have them on trains as well as trams and buses have them
1:15 Its better to use metric system values also in your videos. This will make everyone understand the content.❤
Thank you! We'll note this in our next videos :)
I saw a truck stopping by a red light and I heard that air braking sound