On my Strida, where the speed tops out somewhere around 16 mph and my feet are spinning insanely fast, my super short crank arms (127) help; I can spin them a little faster, and I can therefore go a little faster. But while I do get a rather neglibible increase in top speed, it comes at a significant decrease in accelleration, especially from a stop.
Is the pedaling rhythm perhaps confusing the issue here? If we forget about cadence for a moment then if you are going faster for a given amount of foot travel (in inches, not revolutions) that means you are pushing harder; it's just that more revolutions are going by as it happens.
Is it possible that it's just difficult to think about it as a "higher gear" in the face of so many revolutions? Because increased top speed and hampered acceleration sure sounds like a higher gear to me! What would your top speed and acceleration be like with an equivalent "gain ratio" created with sprockets and normal cranks?
Originally Posted by
wahoonc
I wonder if SB's gain ratios is one of those things that is based on sound physics but in the real world is only a fraction of a difference? Kind of like the difference between spinning at say 80rpm vs 85 rpm on a given gear. You also have to factor in the physiological differences to really make gain ratios work. Things like femur to tibia ratio to overall height.
Well, "gain ratio" compares the distance traveled by the foot/pedal to the distance traveled by the bicycle.
However, I'm thinking the mistake is that it doesn't account for the fact that the deadspot of the pedal stroke is wastes part of that foot travel and, thus, must be subtracted. So perhaps shorter cranks do increase the effective gearing, but they do so in a highly nonlinear fashion which is different from changing sprocket ratios in two important ways:
1. Calculation is only approximate because the deadspot of the pedal stroke is difficult to estimate and cannot be computed with only simple mathematical operations.
2. The relative size of the approximation error compared to the "gain" is always large because, unlike sprocket ratios, large changes in crank length quickly produce untenable deadspots.
In other words perhaps slightly shorter cranks increase gearing and slightly longer cranks decrease gearing, but to maintain efficiency the change in effective gearing has to remain small and even then no one can say exactly how much it is.
I think rhm's experiences bear this out...they're just all tangled up with biomechanics and the cyclical point of view, but maybe he can comment.