Originally Posted by
Moisture
there is a specific crank arm length according to inseam. What the exact length is in proportion to inseam i won't argue as if I know exactly.
that's a general rule that many use as a shortcut but it is not a law of physics. if you ask five bike fitters with adequate experience, they'll give you eight answers to that question. inseam is only one factor in that equation and the only way to optimize it is to put the rider in a trainer with an experienced fitter observing.
Steve Hogg has some interesting thoughts on the matter.
John Weirath has some thoughts on it as well. I suggest you read up on those nuances and spend a few years fitting bikes for hundreds of riders before declaring such generalizations to be absolute truths.