Circling back to crank length

Ha, circling.

A couple months ago we talked about shortening cranks for improved pedaling and shredding: Shorter cranks for my Stumpy EVO?

Since then I’ve been thinking about crank length as is relates to leg length and biomechanics. And then our friend Anne sent in this question:

Hi Lee,

You’ve met me [we’ve been working skills for years]. I’m average height for a female 5’4/5’5, with a 28.5in inseam. I run 175mm cranks and run a small frame. The question is would I benefit by having shorter cranks?

I sometimes get hip pain from taking long strides when I walk. When I take shorter strides with faster RPMs (yes, when I walk) I find that my hip doesn’t hurt. Would the same thing apply to pedaling with shorter cranks (say, 170mm) or would it just make me miserable? What would be the benefits vs the drawbacks? I know DH riders run shorter cranks usually, but that’s mainly for clearance.

Thanks,

-Anne

Anne!

Thanks for the great question.

First

If you have pain — especially in your joints — see a medical expert. If you ignore a structural issue, it will get worse, and the surrounding tissues will compensate in stupid and ultimately painful ways. This I know well.

Optimal crank length

The bike industry does not crank out cranks for every possible rider. They build what they can sell, and they try to sell as few versions as they can. Why? 1) The more they make of a certain model, the less each item costs. 2) The more options they sell, the more time/money gets spent keeping track of them.

Most mountain bikes come with 175mm cranks because that size fits the theoretical average rider. But what if you’re bigger or smaller?

Lennard Zinn is a very smart, very experienced bike fitter, builder and writer. His theory, which I totally buy, is that crank length should be proportional to leg length. When a shorter rider turns longer cranks, her legs are forced through a longer-than-optimal range of motion. Pedaling is likely less efficient, and the chance of injury can be increased.

For general cycling, Lennard uses this formula:

Inseam in millimeters x 0.216

Based on this formula, your optimal crank length is 156mm.

Whoa. Not close to 175mm.

One sure sign of too-long cranks: The rider’s hips rock side to side when pedaling out of the saddle. You see this on kids at the BMX track. I’ve also seen this with you, and you might remember me mentioning shorter cranks. I believe the too-big pedal circle puts extra strain on your hips and lower back.

Benefits of shorter cranks

• Pedaling might be easier, more powerful and less painful.

• Increased ground clearance for shredding.

• You get to buy sweet new cranks.

Drawbacks of shorter cranks

• It might take time to adjust to the new pedaling pattern. I believe you’ll like it right away.

• To get the same pressure at the pedal, you’ll need lower gearing.

Learn more at Shorter cranks for my Stumpy EVO?.

Things you can try

• Ride some 165mm cranks. I’ll bet you’ll like ’em. Shimano XT is a great value and comes in this size. Your local bike shop, Trail Head Cyclery, will dial you right in.

• Lennard can make custom cranks down to 130mm.

Have fun out there. Tell me how it goes!

Lee

Know more. Have more fun!

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