Bicycle Mechanics - crank arm length
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09-21-10, 08:25 AM
I have a question concerning the best crank arm length for my current build. I am 5'11" and have a 29" inseam. The bike frame (TT bike) is a medium size (55 cm) and has 650C tires. My current MTB has 175 cm cranks and this seems perfectly fine, but is there perhaps some sort of equation or magic that can be used to calculate the optimum crank arm length for this bike? Or is this best left to trial and error?
If you think you'll spin more than on the MTB, 172.5s might be more appropriate.
09-21-10, 09:08 AM
I don't really understand.
09-21-10, 09:28 AM
take it with a grain of salt.
09-21-10, 09:48 AM
Q: Pedaling style.... are you a Masher, or a Spinner?.. foot flat across BDC?
or toes point down. ?
I no longer worry about 'Best'; have 170s and 180s, and 175s
on different bikes , and ride them all.
Oysterboy, Because the circumference is less with a 172.5 mm crankarm vs. a 175 mm crankarm there is less distance to pedal for each 360 d. of rotation allowing for a slightly higher cadence for a given amount of time.
All in all I think cyclists are pretty adaptable to crankarm length. What is the crankarm length on your road bike?
09-21-10, 02:25 PM
Keep in mind that usually you're going to want longer cranks on a mountain bike than a road bike.
29" is your cycling inseam or your pants size? Not that a 29" inseam on a 5'11" person is unusual, it's just not average. That's why I'm asking. You measure your cycling inseam by straddling a level while barefoot and pulling it up to your soft tissues to simulate where the saddle would be, then measuring to the floor.
I have a 30.25" inseam and I ride 165mm cranks but I'm a spinner. I'm usually pedaling over 90rpm. If your cadence is lower then shorter cranks are going to feel very odd. I personally would put 170mm cranks on a mountain bike as there is more mashing involved, but I can't say what is right for you.
09-21-10, 02:29 PM
I'm supposed to ride 175's but currently ride 170's. They spin fine and I couldn't care less. As someone above said, most peope are pretty adaptable to a general ballpark range of lengths.
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