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Some will tell you there is a formula
http://www.zinncycles.com/cranks.aspx

The formula tells me I need 180mm cranks. The best performances I've ever had have been on 170's. 170's feel best to me. I've tried 167.5, 172.2, 175, and 180. I like 170 and that what I use?

I wanted to believe there was a magic formula for crank size to reveal some unforseen wattage output, but it doesn't exist. The Pro's and Con's cancel each other out. Ride what is comforatble.
 

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No choice

nismosr said:
whats the difference besides the obvious number. does the length of the arms should correspond to the length of the riders legs as well ?
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. 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 tend to 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 165-180 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.

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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nismosr said:
lets see i have a 30.5 inseam - I probably would try 170mm this time. thanks
Don't use your pants inseam as a measurement. You don't wear pants down to your feet, or buried deep in your crotch (hopefully...disco's dead).

Take a 1.5" binder, stand with your feet 6-8" apart against a wall, press your butt and shoulders against that wall, and have someone push that binder into your crotch with similar pressure that a bike saddle would give.

Mark the wall and measure.
 

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2.5 mm is the size of the number two at the beginning of this sentence. Think your really going to feel that? I can ride anything from 165 to 175, but my legs are short and I prefer the 165's. If you are going to try a different size, make a jump you will feel....
 
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