hello. is there any relation or rule between crank arm length and height or inseam. I'm considering the XC racing part of the sport because I know downhillers and freeriders use smaller crank arms. thanks.
Yes, but there's a lot of opinions on what that exactly is. Then there's the limited choice of arm length in many brands of crank. I think Sheldon Brown's take is a good read http://www.sheldonbrown.com/cranks.html and you can google for crank arm length calculator and read around a bit...
Leonard Zinn has also written a lot about crank length for taller cyclists and builds large frames with high BBs for custom cranks. I've got an almost 38.5" cycling inseam (book pulled up into crotch) and my road bike spindle to seat height is 34.25". According to some of the calculators, I should be using 210 cranks.
I've been using 180 cranks on both my road and MTBs forever, and I can tell the difference if I get on a bike with 175s, but it's subtle. If I knew that going much longer would significantly improve my performance, I'd have to try it, but from what I've read, I'm not really convinced. Even pro road cyclists of vastly different heights (legs length differences of 20% or more) perform well using cranks that vary by less than 6%.
I've used Zinn's formula successfully when choosing my crank size. Based off of Zinn's formula, I should be running something like 192's. I'm running 190s on my MTB and am quite happy with them.
I'm not sure I agree with slapping 175s on everyone's bike. Why should a guy 5'2" and a guy 6'5" run the same cranks? Frame sizes vary, seat heights vary, stem lengths vary, but the crank length is the same? :skep:
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