Bike Forums - crank arm length

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Quote:

Originally Posted by dookie (Post 8567635)

have you tried different arm lengths on an otherwise identical bicycle? if not, your post is absurd. i assure you, the differences described by mr. cox are plenty apparent.

Your argument is absolutely watertight. I mean, there's no way it could seem easier unless it actually was easier, right? Excuse me for trying to understand it.

Quote:

Originally Posted by schnee (Post 8570057)

That's assuming people are biomechanically set up to produce proportional torque at different leg extensions in relation to the circle described by the arc of the crank. We're not. Once you take the whole system into account, it's far more complex than just a single torque number on one pivot.

Sheldon Brown's site seemed to indicate that more work is required when the legs are closer to the center of rotation (in his section on saddle height), which runs counter to the argument. Again, correct me if I'm wrong. Is it really possible that at 5mm difference you're going to be using a different set of muscles or somehow using the same muscles differently?

Five millimeters of crank length increases the distance the pedals travel in every revolution by a little more than 31mm.

Going from 175mm cranks to 165mm cranks decreases the distance the pedals travel by almost 2.5" or 63mm.

Bottom line, 165mm cranks can spin 6% faster than 175mm cranks.

(63mm / 1037mm = .06, or 6%)

What kind of additional effort do 165mm cranks require in comparison to 175mm cranks?

Six percent.

(Weight times Arm equals moment; or, 10kg X 165mm = 1650kg; 10kg X 175mm = 1750kg; 1750 - 1650 = 100; 100 / 1650 = .06, or six percent)

So, by reducing your crank length by 10mm, you can increase your spin by six percent, but you will also increase the effort by six percent.

since im expending much less energy with my knees not breaking as much at the top of the rotation and my range of motion being fit more optimally for me, theres a substantial difference. a 15mm change in crank arm length is a 30mm change in range from the top and bottom of the stroke, or any two opposite points.

Quote:

Originally Posted by cyrano138 (Post 8570337)

Your argument is absolutely watertight. I mean, there's no way it could seem easier unless it actually was easier, right?

so, have you tried it?