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Crank arm length(3 posts)

Crank arm lengthcurtis
May 17, 2002 4:26 PM
I serviced my bottom bracket yesterday and noticed something that has escaped my attention for ALL of the 17 years I have owned my Peugeot. I have 170mm cranks. No big deal and certainly, I have gotten along just fine with them, but I would hate to waste the opportunity to spend money on my bike, so........are there any REAL benefits to longer cranks? Colorado Cyclist's fit pages say that either 170s or 172.5s are appropriate for my frame size.

Whaddya think?
law of physicselviento
May 17, 2002 6:18 PM
The law of physics dictates that longer cranks give you more leverage, but your feet will need to move at a bigger circle, basically a zero sum game. Obviously spinners may find it hard to spin at big circles, while mashers may need the extra leverage.

For lack of a better expression, I think it's a matter of comfort. As long as the saddle height is adjusted accordingly, your legs will feel the same at the bottom of the pedal stroke, no matter what the crank length. The difference is at the 3 o'clock, 9 o'clock, and 12 o'clock, especially the last. If you are a short rider with long cranks, you legs will be feel cramped at the top of each pedal stroke, and may be uncomfortable and might even prevent an aero position.

Interestingly, short cranks hardly ever seem to be a problem. Here is proof: a rider using a 64 frame will typically have legs 22% longer than someone using a 50 frame, but the former will typically use a 180 crank and the latter a 170 crank, the former crank being only 6% longer than the latter. You can also see tall guys' legs don't really bend very much at the top of each stroke. Yet not that many tall riders are complaining.

On the other hand, if you watch a women's race, you will notice shorter women's legs seem to revolve at much bigger circles relative to their size, and sometime their knees even start bumping their chests when going aero.

So to figure out the perfect crank length takes either
a) extremely thorough understanding of human physiology and physics; or
b) test ride!

At 5'8" I have used 172.5 and 170s extensively and finally decided on 170s although the preference is not super strong. But I can tell 175s are noticeably uncomfortable due to larger circles I have to pedal at. The 175s for all on MTBs is simply rediculous.
Crank arm length choicesKerry
May 18, 2002 6:24 AM
It is generally the case that longer cranks make it harder to spin, and high cadence is the best way to minimize knee problems. That said, an extra 5 mm in crank length may only take away 3-5 rpm of spin, so it is not a large effect.

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. What little research has been done on crank length suggests that people adapt to different crank lengths and there is no optimum or formula related to body proportion. 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.

The short answer is: if it feels better and measures faster, do it. If it doesn't feel better but measures faster, maybe you should do it and hope to adapt. And even if it doesn't measure faster, you may adapt and get faster. Simple, eh? NOTE: measures faster means repeat timed distances on different days in different weather, not just "faster on my nightly ride" which can be masked by the effects of weather, fatigue, and the placebo effect of riding on "faster" equipment.