RecentlyI was asked about what benefits I thought short crank arm lengths versus long crank arm lengths for single speed use brought, if any. I thought about that for a bit, and shot off an answer, but I thought I would expand on that a bit here for all to read.
First, let's set the stage for what "conventional wisdom" says on this subject....
The old line always was that a "longer lever", (longer crank arm length), gave better power going up steep pitches on a single speed bike. Usually this was meant for 26 inch wheeled single speeds, and the crank lengths recommended were 180mm, or185mm if you could get them. (Today the Surly "Mr. Whirly" gives you those options, plus others are also available).
<===They only come out at night! (180mm crank arms on my single speed Gryphon.
Then I heard about some early 29"er freaks up in Crested Butte. Wes Williams and some others that advocated the use of 170mm cranks for 29"ers. Jeff Jones does this to a degree as well. Why? I tried to find out, but not much talking was going on, only that "it works" and I should just try it for myself.
I have used 170mm cranks on a 29"er single speed of one sort or another for three solid years or more. Before that, I rode a 2003 Karate Monkey with Cooks Bros. 177.5mm cranks. (Yeah....weird, I know!) I have also ridden 180mm cranks right alongside the 170mm experiment on two other single speeds. With all this time on all these different crank lengths on single speed bikes, I have come up with a couple conclusions.
The Disclaimer: As anyone who has delved into the crank arm length debates knows, no real conclusions have been arrived at as to what really works in any given situation for any given individual. In fact, it may not matter at all for many riders, and to some, only one length crank will ever do. Me? Heck.....I can't even do the same dance step twice in a row! I seem to be a pretty adaptable character in regards to this subject, but be that as it may, I have some thoughts to share.
Shorter Arms: In terms of running 170mm cranks on a single speed, I have found that it is at once easier to get on top of a gear and spin like a whirling dervish over any shortish climb. Longer climbs are okay if you can maintain a constant cadence with your gearing choice. Flats are harder, and forget about "mashing", where you simply let your body weight fall into your crank arm till bottom dead center, switch legs and repeat as necessary, slowly walking your way up a hill.
Longer Arms: Besides lending themselves marvelously to "mashing", I find that I like the longer arms on flatter terrain, actually. Bigger gearing, (higher gear inches), flatter terrain, and a "set it and forget it" cadence are awesome with longer arms. Usually, this is a lower cadence than I ride with the 170's. Spinning? Not really all that comfortable with these longer arms.
Conclusions: You can find a way to single speed up and over anything, if you set your mind to it and choose the right gear. That said, I think crank length is a style question perhaps more than anything. If some are pre-disposed to lower cadence, it may be an indication that a longer crank length is necessary. Perhaps those who are spinners should look into shorter cranks. Finally, many 29"er riders may actually find 170's to be an enlightening experience, and I know I did.
Is there an "easy answer"? No. I still would suggest that personal experimentation is required to find ones own crank length that suits them best. Too many variables in the human species to say anything for certain. That said, I think my conclusions are worth considering, and hopefully will be a helpful guide to those looking for crank length solutions on a 29"er single speed.