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I’ve got a couple of bikes getting ready for some drivetrain TLC / upgrades, and increasingly read about different chain lines lately. Please can someone explain what chain lines relate to what hub standards etc or is it more complex than that (if so, please elaborate!)? Thanks!
Chainline is the distance from the centre of the bottom bracket to the chainring, to the middle chainring in a triple or to the distance between the two chainrings in a double. 142 & 135 hubs will normally have a ~49mm chainline. 148 Boost hubs can have either a 52mm or 55mm chainline. The 55mm chainline will move your chainring 3mm further away from the frame compared to the 52mm so that for the higher gears you have a better chainline but for the lower gears the chain will have more of an angle.
You cannot have a chainline that's in the middle of the cassette as shown above because the cranks will not clear the chainstay so that's why we have 49mm, 52mm 55mm chainlines. Generally you want a chainline as close to your frame as is possible because that will the position the chain closest to the centre of the cassette. If you do a lot of riding in the higher gears then a chainline further away from the frame will be better for you.
Brilliant, thank you. One bike is 135mm hub and the other is 148mm boost so that’s really helpful - cheers!
This will probably get me lynched on here, but in 30 years of building and maintaining bikes of all sorts I've never once measured or worried about chainline. I haven't died and I'm pretty sure my shifting and drivetrain lifespan is no worse than anyone I ride with, so it's not something I intend to worry about in the future either.
I just changed to AXS Transmission. In theory that needs a 55mm chainline. The previous stuff needed 52mm.
The new cassette is spaced 2.5mm further outboard.
When I had a chat with Wolftooth about a new chainring, they said that they had colleagues running 52mm and 55mm quite successfully. Given that they make chainrings in 0mm, 3mm and 6mm offsets they don’t really have any axe to grind in what they recommend. Given the distance between the cranks and cassette combined with the flexibility in the chain and the length of modern chainstays I can’t really see 3mm making a difference.
Back in the day when tighter rear ends and smaller wheels were commonplace I could understand how important it was to get the chainline just right, and traditional adjustable bottom brackets and swappable bb axles made it possible to adjust quite easily. Now that axle length is determined by the cranks and bb width is pre determined I guess it is more difficult.
Having said all of that, SRAM do recommend a 55mm chainline so that’s what I went for and it all works smoothly. I have no experience of trying it with a 52mm chainline to know if it really matters.
With chainline too far inboard the chain can graunch on your n-1 gear when in your smallest gear, which I have read as an explanation for chainlines outside of the middle of the cassette. Even in pre- boost days when frames were slim enough to achieve this.
To really delve deep there's other considerations of chainstay length and chainring size, Garbaruk chainrings for DM Shimano actually alter the offset depending on what size you pick to find the optimal chainline.
An inch longer chainstay, a chainring with smaller or larger diameter, the size of your cassette's biggest cog can all alter the chains angle in extreme gears, so it not as simple as saying 52 or 55 is the best.