Crank arm length?

[Wheels]

Does anyone have a magic formula for determining what crank arm length you should be using?(or do you just go by feel)

My old bike has 175mm and the new bike is 172.5mm. What are the pro's and con's to using the differing lengths?

Thanks

[Chris Helwig]

There are some formulas out there to predict optimal crank arm length, but like many things there is as many opinions as there is formulas. I ride 172.5 cm cranks which is standard for someone around our height and inseam length. The theory goes that longer crank arms give you more leverage while shorter arms allow you to spin better. Most track riders ride 165 mm arms while TT riders ride 175-180 mm arms. For you 172.5 is a good length. 175 mm is standard MTB length for your height and inseam.

Graydon may want to weigh in on this, he rides 180 mm crazy long crank arms.

[Graydon]

Chris is right, there are many formulas out there. At some basic level, it makes sense that crank length should scale to leg length (i.e. why would I ride the same length cranks, for example, as my wife who is 5'4"?).

The difficultly with any available formula, is that they are all oversimplifications of a very complex system. The factors that will influence "optimal" crank length might include femur length, tibia length, foot length, seat height, seat fore-aft, muscle/tendon flexibility, how powerful a rider you are, your muscle fibre type makeup, and what cadence range you freely choose when riding. You have to decide what factors are important to you, and what the effect of changing crank length will have on the rest of the system.

One more thing, despite what the manufacturing industry would have you believe, the difference between 170, 172.5, and 175 is tiny and negligible. Ever put on thick socks in the fall? Could be close to 2.5mm right there. The reason carbon and most other cranks only ever come in 170, 172.5, and 175mm sizes is because for manufacturing purposes, they are all made from the same form, and just punch the pedal axle holes +/- 2.5mm from the 172.5 position. Only companies like the big S and Campy, who make a zillion cranks a year, can afford to have several presses and forms to offer a range from 165-180. That said, one or two "exotic" after market companies offer custom cranks up to 220mm I believe, but they aren't cheap, and often look like they were made in some guys machine shop down the road.

So, if you are going to experiment, you should try at least +/-5mm increment. Ideally, you find someone with adjustable cranks, and play around on a computrainer for a week or so. I can tell you from my own experience, the effects are pretty minimal, if at all, performance wise, however your knees may disagree with any changes very quickly. In that respect, remember to raise/lower seat height whenever you change cranks, and possibly adjust fore/aft.