What size of chainring causes chain rub on big ring (in small-small cross-chainining)

ajmitchell

A quick question but does anyone know how to predict chain rub when installing a new chainset. that is, when cross-chaining in small chainring (eg 39) and small cassette (eg 11) I want to know in advance what size of big chainring I can install before the chain rubs on the inside (maybe the rivet) of the big ring. I am assuming 53 is total fine as its stock, and I guess chainstay length has a role but in general what size of chainring will cause a problem? thanks

imposter2.0

Most will rub on big/big or small/small. Best way to avoid it is by not using those gear combos..

StillGoing

Simple answer is don't do it. Aside from the stresses you'll put on the chain, the added friction for the drive chain zapping watts and looking like a newb, mall/small is a combination I can't ever imagine when you'd need it. The opposite of big/big when you may have short ramp to ascend and don't want to change down on the front and which most front derailleurs cater for with trimming.

ajmitchell

thanks; whilst I appreciate the feedback I am looking for an exact number (chainring size) which is likely to cause rub (I am already aware of cross chaining losses)

cooldad

There is no answer to your question.

arlowood

ajmitchell wrote:

. I am assuming 53 is total fine as its stock

You're not Tony Martin by any chance. Can't see why any amateur cyclist would want or need anything larger than a 53 on the front.

So your question is somewhat hypothetical and I'm not sure anyone will be able to give you a definitive answer other than a Pro Team mechanic who has worked on chainsets where the rider demanded more than a 53 as his largest chainring

imposter2.0

ajmitchell wrote:

thanks; whilst I appreciate the feedback I am looking for an exact number (chainring size) which is likely to cause rub (I am already aware of cross chaining losses)

It's a moot point, as people have already said - because cross chaining is not good practice and it is simply not necessary. A ratio you might need by cross chaining - 39x12, for example - will also be available by running 52x16 - one is cross chaining, one isn't. Simply use the one that isn't.

Nick Payne

Impossible to give an exact answer as it depends on the length of the chainstays. The longer the chainstays the less the chain angle will be when cross-chaining.

svetty

Nick Payne wrote:

Impossible to give an exact answer as it depends on the length of the chainstays. The longer the chainstays the less the chain angle will be when cross-chaining.

This - it varies enormously from bike to bike depending on Q-factor/BB width, chain-line, chainstay length, make and model of chainrings etc etc

You cannot assume that 53/39 will avoid this in all cases.