# Crank arm length?



## melusive (Sep 18, 2008)

I'm 5'6 with a 29" inseam. Should I use 172.5 or 170 length crank arms?
Suggestion?


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## red elvis (Sep 9, 2010)

go with 170.


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## PJ352 (Dec 5, 2007)

Some info on the topic:
CYCLING PERFORMANCE TIPS -


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## Kerry Irons (Feb 25, 2002)

melusive said:


> I'm 5'6 with a 29" inseam. Should I use 172.5 or 170 length crank arms?
> Suggestion?


Choose any length you want. 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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## Mr. Versatile (Nov 24, 2005)

Kerry's got it right. If you started riding with 170s & somebody changed them to 172.5 or even 175s without your knowledge, you'd never know the difference.


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## velocanman (Jul 15, 2011)

I would say it depends on your riding style. High cadence, 170. Lower cadence, 172.5. But like they say above, you probably couldn't tell the difference if you didn't know.


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## superjesus (Jul 26, 2010)

Mr. Versatile said:


> Kerry's got it right. If you started riding with 170s & somebody changed them to 172.5 or even 175s without your knowledge, you'd never know the difference.


I disagree with this based on my personal experience. I rode 170's all season and switched to 172.5's about a month ago. I immediately noticed something undefinable, but mildly uncomfortable about my pedal stroke, and I noticed my cadence drop roughly 10 rpm. I thought the new cranks were the same length as the old ones, but it was only when I measured the new cranks that my feelings were validated.

I much prefer 170's. FWIW, I am 5' 5", 140 lbs, 30" inseam.


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## acidrane (Aug 13, 2011)

Mr. Versatile said:


> Kerry's got it right. If you started riding with 170s & somebody changed them to 172.5 or even 175s without your knowledge, you'd never know the difference.


I would have to disagree with you as well. Granted, I'm comparing my mtn bike with 172.5's to my road bike which came with 170's. I noticed a difference when mashing. The turning radius felt too short and awkward as if I wasn't being able to put down power. Sitting down, not a lot of difference. On Sunday when I got fitted to my road bike, the tech even mentioned that should i choose to switch out my cranks for lighter/better ones, I should get 172.5's. I'm ~5'9" with a 29.5" inseam. 

Your best bet would be to go to the LBS and test out 2 similar bikes, one with 170 and one with 172.5.


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## Camilo (Jun 23, 2007)

superjesus said:


> I disagree with this based on my personal experience. I rode 170's all season and switched to 172.5's about a month ago. I immediately noticed something undefinable, but mildly uncomfortable about my pedal stroke, and I noticed my cadence drop roughly 10 rpm. I thought the new cranks were the same length as the old ones, but it was only when I measured the new cranks that my feelings were validated.
> 
> I much prefer 170's. FWIW, I am 5' 5", 140 lbs, 30" inseam.


I don't dispute that you might really prefer one crank arm length over another, but did you control for seat height when you changed cranks? Sure the saddle height difference betweeen any two common crank arm lengths is just a very tiny 2.5mm, but that could possibly be the cause of the undefinable discomfort. I do know that my saddle height is probably the most sensitive fit factor I have.


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## Peanya (Jun 12, 2008)

Mr. Versatile said:


> Kerry's got it right. If you started riding with 170s & somebody changed them to 172.5 or even 175s without your knowledge, you'd never know the difference.


I don't 100% agree with that. I believe my knees would know after a ride. Happened to me recently.


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## superjesus (Jul 26, 2010)

Camilo said:


> I don't dispute that you might really prefer one crank arm length over another, but did you control for seat height when you changed cranks? Sure the saddle height difference betweeen any two common crank arm lengths is just a very tiny 2.5mm, but that could possibly be the cause of the undefinable discomfort. I do know that my saddle height is probably the most sensitive fit factor I have.


I appreciate your position about saddle height. I too am quite sensitive to saddle changes. I did not double check my saddle height when I switched cranks. Thinking the new and old cranks had the same arm length, checking the saddle height - which should have remained constant - did not cross my mind. 

After checking the crank lengths, I did reposition my saddle accordingly. Still, whether actual or merely psychological, I felt a difference in the cranks and my pedal stroke.


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## Mr. Versatile (Nov 24, 2005)

There's a very experienced cyclist on these forums. He's ridden for decades. During the winter he tore his bike down for an overhaul & when he reassembled it he accidentally put a different length crank arm on one side. I don't know the length, but I think it was 170 on one side & 172.5 on the other side. He rode the entire season like that & didn't discover it until he tore the bike down again the next winter. I honestly don't remember who it was, but maybe he'll see this & post.


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