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Long cranks vs. Short cranks

I've read with great interest, many posts on Bentrider Online about this subject. I'm curious what the experience has been with those of us who specifically ride the P38. I'm 6'4" and ride an XL P38 with 175mm crank arms. I have no problems with the 175mm arms but want to make sure that I'm not setting myself up for future knee problems associated with the longer arms which is the general opinion according to the posts on BRO. Thanks for your thoughts on this subject.

I have short cranks on both my P-38 Voyager, Phantom, and 160mm cranks on a R-84 (that I'm planning to sell soon). I've never had any knee problems on any length crank, but the short cranks are great for me in terms of fairing clearance, cadence, and I've had no problems on hills.

1) I vaguely remember reading about scientific studies trying to measure power output in relation to crank arm length. Some studies indicated that unusually short cranks allowed cyclists to generate more power. Other studies showed that crank arm length did not matter, and that over a very wide range of length power output remained much the same. So I would say the jury is still out with respect to the power issue.

(Though I have never heard of a study indicating that longer-than-usual crank arms were better.)

2) Different riders report different personal experiences when trying short cranks. Some people clearly prefer them. But if your present cranks are not giving your knees any trouble, why switch? If it ain't broke...

3) I have a front fairing on one of my Lightning bikes. It was originally fitted to the bike when the seat was more or less normal. When I switched to a different seat, the old fairing position no longer worked, and I was forced to try shorter cranks in order to squeeze into the front fairing without having my big feet strike the inside. First I tried 140mm BMX arms, but I was never able to really get used to them. They solved the fit problem inside the fairing, but always felt weird.

I then tried 155mm cranks. They felt a little strange at first, but after a few weeks of riding I got used to them. Now they feel fine. I do not think they make me any faster - or slower - but they solved the fairing fit problem.

4) I am an empiricist. There is no substitute for personal experimentation, and one size definitely DOES NOT fit all. You can find cheap BMX cranks used on eBay, and see how they feel to you. You will need to give them a chance over some extended period of usage. They will not feel right at first.

Also: You will need to move the crankset boom on your Lightning bike out a little bit to maintain similar leg extension. Exactly how much? Experiment and see what feels good to you. Pushing somewhat smaller gears may be a good idea too.

If you try them and like them, you could get something nicer, like the cut-down cranksets sold by Mark Stonich of Bikesmith Design. Mark's website can be found at:

If they do not work for you, you can probably sell them for the same price you paid.

Safe riding,

I've ridden 175 and 180's for the past 30 years. I'm 6"1". I have to admit I always liked the feel of some resistance when I pedaled, not much of a spinner, until now. I know there's tons of theories out there but I think it's all in the spin and the correct leg extension that saves your knees. I've gone the opposite way you have and now ride a 165 and I'm a spinning fool. It's taking some time to get used to but it feels fine. Now I did try a 185 once and the arc of the circle just felt way too weird. If it feels good, do it!


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