The analysis above also plays in to crank length selection.
Being that cranks are levers, generally speaking, the longer the better. But, too long is too long.
How long is too long?
Ride the longest cranks that will allow you to:
- Hit the cadence ranges that you need to hit (see above)
- Don't interfere with your aero position. Longer cranks require more clearance of the torso.
175mm cranks come into the chest area TWO centimeters higher than 165mm. Why 2cm instead of one? 1 of the 2 centimeters is easy to account for. At the top of the pedal stroke, the knee is 1 cm higher based on the longer crank arm. The other 1 cm comes from the fact that the bottom of the pedal stroke is
also 1cm longer...further away. This means that you have to lower your saddle 1cm to keep your knee and ankle angles the same. When you lower the saddle 1cm, the knee at the top of the stroke is 1cm closer to your chest. 1+1 = 2.
This is also why switching to shorter cranks allows for a more aero position. Higher saddle and less knee into the torso area means that the rider can lay his/her back down lower. Whether lower is better is a matter of debate. But, that's one way to get lower.
EDIT:
Also, crank length affects muscle firing rates.
Given the same circumvential foot speed (speed at which your foot is moving in a circle), the longer cranks will have you pressing less often...giving you a few more milliseconds of "micro -rests". This is good. This means that you can press harder for longer.