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ok my measurement are
im 5' 8" about 30 inch in seem and weigh 220

i guessing i need a size 54 cm bike for my size
what i wounder is why are most the companies have a 172.5mm crank
is that the size i need

i race bmx and it know it 2 diff sport and i run a shamino DXR crank in a 175mm
and some of the MTB i seen at like wallie world and LBS run the 175 cranks
so why would i used a 172.5 crank
i just wounder thanks
 

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Crank sizing

jimlmackjr said:
ok my measurement are
im 5' 8" about 30 inch in seem and weigh 220

i guessing i need a size 54 cm bike for my size
what i wounder is why are most the companies have a 172.5mm crank
is that the size i need

i race bmx and it know it 2 diff sport and i run a shamino DXR crank in a 175mm
and some of the MTB i seen at like wallie world and LBS run the 175 cranks
so why would i used a 172.5 crank
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.

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.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
 

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Mountain bikes use longer cranks for leverage. On a road bike, you are most effecient if you are spinning. A longer crank makes it harder to do this, and can make fitting difficult. At your height, I would not go over 170mm cranks. I'm 5'11" and plan to go to 170 from 172.5.
 
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