what crank size

[stewie911]

How do I know which size to get? 165mm, 170mm etc.

 

I have a 18" mtb frame and I am 1.69m, so about a 17" is my perfect size.

 

[no calves]

170mm

[Sterad]

Go with that. 170mm

[JPW#]

170s are for the track. I wish 180s were easier to come by.

[stewie911]

Why different sizes?

 

Is it for better rider fit, or is it a torque thing.

 

[GoLefty!!]

your height does not help with a recommendation for a crank length. Measure the length of you femur please and repost.

 

Eg, I am 171cm, 39.7cm Length femur translates into a 175mm crank road and MTB.

 

You could be either 170mm or 175mm depending on your femur length.

 

[Ha Ha Kieran]

What is the relationship between femur length and crank length Lefty? Is there a ratio or something?

[GoLefty!!]

there is no real ratio althought here is a good article on the various models in use today. all are flawed.

 

http://www.cyclefitcentre.com/pdf%20final%20docs/CRANK%20LENGTH%20ARTICLE%206_final.pdf

 

Cranke length is dependant on the femur length of the rider, the seat tube angle, the riders foot size and the saddle height.

With those dimentions it is possible to achieve a suitable crank length from first principals.

 

what i would say is that unless a rider is less than 160cm tall, or has unusually short legs a 170mm or 175mm is probably going to fullfill the requirement.