Crank arm lengths?

[Grant]

Apologies for the billy basic question, but I've just realised that in over 15 years of MTB'ing I've never understood the different crank length reasoning!

My bike has 175mm's for example, now I've seen 170mm's and 180mm's what would the effect of a swap to shorter/longer be?

 

[FluffyChicken]

Apologies for the billy basic question, but I've just realised that in over 15 years of MTB'ing I've never understood the different crank length reasoning!

My bike has 175mm's for example, now I've seen 170mm's and 180mm's what would the effect of a swap to shorter/longer be?

You get a smaller circle, so if your legs are short you need shorter arms else they are moving more than needed. If legs are long then longer arms may be needed to give your leg enough movement.

There's probably a technical way to determine which length you need.
But Small frames, small people so 170mm, Middle to large frames 175mm seems to have been a common spec. I've noticed.

 

[CAS]

I think it's more important for those with short legs! The long legged brigade often seem to manage happily with 170 172.5 or 175 cranks.

Being short legged I even have some 165 crank arms which make a very comfortable ride. You seem to bridge the highest point of the circle more easily if that makes sense!

 

[Steve Kish]

There's also a school of belief that crank length is determined by gearing / rpm, hence a lot of track bikes with 165mm and time trial bikes with 175-177.5mm.

I personally don't subscribe to this and feel that leg length should be the determining factor.

 

[GrahamJohnWallace]

This is an extract from the Highpath Engineering website:

www.highpath.net/

Found under the heading: Cycle information /Cranks and chainsets


It is important to use the correct length of crank to ensure that your legs can work efficiently. You risk damaging your knees if you habitually use cranks which are too long, especially as you get older. 170mm cranks, which most cycle manufacturers fit as standard to most bicycles, are unsuitable for anyone shorter than the average European adult male and are therefore too long for most people in the World!

Normally fit adult riders should use cranks which measure about 20% of their effective leg length, rounded to the nearest 5mm, whereas growing children can safely increase this figure to 22%. This length goes all the way to your hip joint and cannot be measured directly. Ask a friend to measure your height twice - first when you are standing upright against a wall, without shoes on, and second when you are sitting squarely against it. Then subtract one measurement from the other one.


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