Dwayne's right
chrispf007 said:
IThe general rule of thumb is that your optimum crank length should be 21% of your inseam (or as close as possible). For instance, my inseam is 31", which equates to 790mm and 21% of that is 165.9. My crank length is 170.
B/c of my research, I switched from a standard 53/39 172.5 crank to a compact 170 and my knees are already very happy. The shorter crank forces me to spin more, which is improving my form.
I'm afraid you didn't do very good research. Dwayne Barry's summary is spot on. Here is an accurate summary of the research on crank length:
The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. Don't you just hate it when the data doesn't support the theory? One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard. Do you think that we have standardized on this narrow range because of some sort of global conspiracy, or because well over 100 years of experience (and testing the limits) have repeated shown that the 170-180 mm is really what works for human beings?
There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything. Probably the best work done on this VERY difficult to research topic was by Lennard Zinn. He unintentionally showed how our adaptability was more important than our size or riding style. He's sure that the results of his study are wrong, but he just can't seem to find any data to support his pre-conceptions.
You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.
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Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.
So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.
Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing.
All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.
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The problem with these formulas is not for "normal height" people but that they end up recommending 150 mm cranks for short-legged folks and 200 mm cranks for tall people. It's pretty shakey to to assume that you could use a 2 significant figure formula, multiply it by a number that is also probably 2 significant figures (3 at best), and come up with a 4 significant figure answer (172.5 vs 175.0).
