I got to wondering recently (uh-oh) The accepted practice, IIRC, is to have a shorter crank for touring, say 165mm, and longer cranks for a strength event, say 175mm. Now "normal" is 170mm. I rode a Rambo a few weeks ago with 165mm and I cannot say I spun any faster or easier on the hills I encountered. And when one considers you are talking a 5mm difference (less than 1/4 inch), why would you? Do any of you really notice a difference in crank length? Or was my experience too short in time from me to notice anything.

When I got a new mountain bike last year, it came with 170mm cranks. My previous mountain bike had 175mm cranks, and I've gotta say it felt like a big difference at first. So much so that I replaced the crankarms on the new bike for a pair of 175s I found on eBay.

Given time, I probably would have got used to the 170s pretty quickly, but I chose to keep mountain biking with 175s.

The weird thing is, I spend more time on my road bike than my mountain bike, and my road bike has (and has always had) 170mm arms. So even though I spent more time overall on 170s, that length felt really "off" whenever I hit the trails.

So in summary... Do I notice a difference in crank lengths? Yes. Do I think it would make a difference in the long run? Not really (for me, anyway).

In Colorado I met a guy touring through from the west coast who thought 190 mm pedals were a good idea. We swapped bikes the day he stayed in Denver and we rode up to Boulder. I felt a sharp difference and geared down a little to ease knee strain. After ten miles he couldn't stand the way my 165s felt and we swapped back. People with really long or short legs might need longer or shorter cranks so their legs and joints would have an optimum range of motion. Maybe?

I think crank legnth depends on you pedaling style. If you are a real spinner, a track racer or criterium racer, then short cranks definitely give you a smoother spin. If you have a lower cadence, then a longer crank gives you some mechanical advantage over a shorter crank.

I would try each and see how it feels and then stick with one.

I test rode a mtn bike with 180 cranks and was blown away by the difference...definitely gave me some extra power on the climbs, but felt odd trying to spin the. In the end I returned to my 175's...oh on the road i ride 172.5 and love it.

I've always matched crank length to rider size. I'm 6'2" so I use 175mm, whereas my wife at 5'4" has 170.

The truth is, 5mm is less than 1/4 inch, so this is hardly critical; it just seems like the right way to go.