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Old 07-13-18, 01:18 AM
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Originally Posted by prathmann View Post
I agree with your first line above. Since you can't expect a high leg speed from the OPs stoker you should give her shorter cranks to reduce her leg speed when the OP is wanting to pedal at a higher cadence. That is a 'mechanical fix' that works for many people to adapt when they have a different preferred cadence. I don't notice much difference either when switching between 170mm and 175mm cranks, but when I ride with 165mm or shorter cranks I definitely prefer a higher cadence and correspondingly lower gears for a given riding speed. Given the large difference the OP reports between him and his stoker there should be a very substantial reduction in her crank length to get her leg speed down to an acceptable rate so she can contribute her share of the power.
With all due respect, I am suggesting that it is because you KNOW the theory and believe it to be sound why you find shorter cranks more amenable to a higher spin rate. Your results are being influenced by your expectations. Someone less knowledgeable simply feels their feet going round and round and dang it, to them 75 rpm with a 165mm crank feels a whole lot like 75 rpm with 170mm cranks. I think 5mm works out to like 1/4". I can perceive 1/4" with my fingers readily enough. With my feet? Not so much. So would a much larger difference in crank length make a difference? Likely. How much then? Would 150mm cranks reduce the foot speed enough? Personally I doubt it. I believe one's perception of cadence has nothing to do with the persons linear foot speed around the rotation axis and everything to do with the reciprocating motions of the articulating joints of the riders legs. These motions are frequency dependent. 90rpm is 90 rpm. Period. Well that's my take on it anyway. What is less theoretical to me is the fact that a change to 150mm cranks would reduce the torque the Stoker could apply by ~25%. They are probably going to notice that more than the fact that they no longer have to move their feet as fast. I say probably because I don't really know. But I do know that cranksets cost money and changing them out costs more money if you can't do the work yourself. It's getting better but it still isn't yet the time where you can find cranks in 2mm increments at your LBS or even on Amazon. Once you get to about 160mm on the short end your options rapidly diminish. Considering all that ... I don't know ... 'standard' cranks just don't seem like such a jail sentence to this writer.
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