What is a bicycle chainline and how to measure it?

What is a bicycle chainline, why is it important, and how to measure it?
CONTENTS:
00:00 Introduction
01:49 Bicycle centre-line
04:30 Bicycle chain-line
06:13 Front and rear chainline
07:40 Offset frames
09:59 Why chainline matters 1
10:40 Measuring the front chainline
18:09 Measuring the rear chainline
27:43 Playing with the chainline
33:06 Catches and caveats
Useful LINKS:
Please use the BikeGremlin forum if you have any questions or comments:
www.bikegremlin.net/
Bicycle chainline article:
bike.bikegremlin.com/1755/bic...
Accurate vs precise, digital vs analogue - Calipers, how to use them - video:
• Accurate vs precise, d...
Rear hubs and their dimensions (and boost hubs):
bike.bikegremlin.com/158/bicy...
32:38 How to properly shift gears on a bicycle - video:
• How to PROPERLY shift ...
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[@BikeGremlinUS]

If you have any questions, please use the BikeGremlin forum (I try to respond to every comment, but UA-cam sucks at notifications, especially when it comes to any follow-up questions):
www.bikegremlin.net/
Relja

[@censorshipBS]

I was always told to use the front small chain ring with the largest gears on the rear cassette and then if i was using the large front to then use the smaller rear cassette rings that way the chain stays straight ❤

[@W_Kolo_Macieju]

Thank you very much for the article and video. He helped me set the correct chain line on my bike.

[@thankmelater1254]

You always have the best explanations.

[@romickva]

Perfect explanation! Thank you!

[@aleksanderiwuc1730]

Many thx for such informative explanation! Grate Video!!!

[@fehehehe]

Care svaka čast za ovo!

[@jurgenmorell5954]

Klasse T-Shirt😂😂😂

[@censorshipBS]

Cant you just use your eyes to look down the chain line to see if its in the correct place?

[@BikeGremlinUS]

If the rear chainstays were parallel to the bike's centre-line, that could work OK(ish).
Since they aren't, you can only reliably tell if the chainline is waaay off or not, but you can't tell if it's "only" 2-4 mm off. Combined with short chainstays on most modern bikes, that's still enough to make the chain rub or drop more easily (and get more component wear) in certain gear combinations.
Now, a steel rod or a ruler that's not flexible - that works better than just eyeballing it, at least when you get the chain and the front derailleur out of the way.
Relja