Originally Posted by
11.4
There's no formula that will determine your ideal crank length, and you're a perfect example of why. There's a theory (mostly just theory) of how a particular crank arm length gives you the most power translation, but more importantly is how well your joints (knee and hip in particular) accommodate a circle of rotation for a larger crank arm. If you have some limitation (whether bone impingement or muscle or tendon inflexibility or something else), you won't be able to power through the top or bottom of your stroke correctly and you'll have that coasting moment you describe. Second, if your thigh is hitting your pelvis or your stomach/chest at its high point, it may just coast through because it's being blocked from free rotation at that point.
Yes, your fixed drive will push your foot through the top and bottom extremes of the pedal stroke, but you don't have to leave it that way. It's always likely to be much weaker than the mid-point power points in the crank rotation (and you may be feeling weakness, not inability to push at all). I'd look at your position to make sure you aren't so low that you are impinging on your legs' ability to power through the rotation. You don't have to go nuts about crank arm experiments -- you may well be having the same problem at shorter crank arm lengths as well, your sensation of smoothness notwithstanding (it's very hard to feel whether you are genuinely applying power at the top or bottom of the stroke).