What determines crank length?

Question

Without getting really technical what determines the length of the cranks that you would use on your bike? I was wondering if longer cranks without being ridiculous would act as a fulcrum and make climbing easier in a higher gear ratio.

Sadly cranks longer than 175mm are uncommon because they're expensive, and they're expensive because they're uncommon. Repeat. I suspect more people would use them if they were more available. — Criggie, Commented Feb 22, 2019 at 5:41
@Criggie Longer cranks make ground strikes more likely and most people aren't actually very sensitive to crank length. So, for most people, cranks longer than 175mm have a disadvantage but no real compensating advantage. — David Richerby, Commented Feb 22, 2019 at 13:32

OraNob · Accepted Answer · 2019-02-22 15:27:07Z

As an opener to your question....

Femur length. Theoretically. The longer the femur - the longer the crank length. The old pros used to favour the longer crank lengths 175mm to 180mm being common. Iirc Pantani who was a short rider even favoured a slightly longer 172.5mm for reasons of leverage as you mention above. There has been a trend towards shorter crank lengths - which lend themselves to a slightly faster cadence.
Natural cadence speed. Most riders find when they switch to shorter cranks, their cadence increases.
Flexibility. A longer length will bring the upper leg at the top of the stroke closer to the abdominal cavity.

Argenti Apparatus · Accepted Answer · 2019-02-21 14:27:51Z

3

You are correct that a longer crank arm acts as a longer lever resulting in greater peak torque output on the power stroke of the pedaling cycle. That might lead us to think that longer cranks are better. However, the optimum crank arm length is that which enables a rider to produce maximum power which does not necessarily correspond to maximum peak torque.

The most important factor determining optimum crank arm length is obviously the length of the femur (between hip and knee joints), but there are other physiological factors that mean a given rider may prefer a slightly longer or shorter crank arm.

Really, there's no simple way of determining optimum crank arm length. Pro riders who really want to do the optimization will do controlled tests with different crank arm lengths and a power meter.

answered Feb 21, 2019 at 14:27

Argenti Apparatus

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Wow, thanks guys. I really didn't expect that much interest.
– Andrew
Commented Feb 22, 2019 at 6:59
1

I'll add to that, that you should rarely use full peak force anyway. If the lever of your cranks is relevant to your power, your gear is too high. Concentrate on power output, and let your gears convert that into the right torque at the wheel. That's what gears are for.
– cmaster - reinstate monica
Commented Feb 22, 2019 at 22:24