Originally Posted by
AlmostTrick
I'd say the most comfortable/efficient crank length for a particular rider corresponds to their leg length and preferred cadence.
Persons with longer legs will usually feel better running longer cranks. But it's harder to spin a longer crank as fast as one could spin a shorter one, so persons who like spinning higher RPM's will often feel better with a shorter crank. So it's a balance of the two. This is why you really have to try different sizes for yourself and see which you prefer.
I'd also say the popular 170mm length is likely ultimate, or close to ultimate, for a high percentage of riders.
While there is not a lot of research in this are, the little that does exist supports going to less than 160 cranks even for longer legs. The mass component market doesn't support shorter cranks and 165s are about all that's easily available but even 165s are not found in any standard setup except on the smallest road frame. Even so, shorter cranks may be the more efficient setup. For some, as low as 145 may be the most efficient setup and even shorter may be no less efficient than going longer. Learning that is the case if not easy because changing crank size is not as easy as changing gears. Crank lengths in the 170s probably is no more ideal than longer versus shorter stroke gas engines or thinking carbo loading is necessary before a long ride.