Originally Posted by
prathmann
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.
Going from 170mm to 165mm decreases leg speed at the same cadence by 3%. However, it also decreases crank torque by the same 3% at the same pedal force. I don't see how she comes out on it in terms of power. After a period of adaption during which the stoker's power might well be lower, the captain's cadence might go up by 3%, but I don't see a power advantage there, either. High cadence was popularized by EPO using cyclists with artificially high VO2max which allowed them to pedal fast while still producing power. Now it's become a fad. I've ridden with LD cyclists who pedal 95 on the flat in a paceline while my HR is like 108 and I'm pedaling 75-80 at low effort. They're just wasting energy looking cool.
The fact that generations of cyclists of various sizes have "made do" with the same stock 170 cranks with no complaints rather argues for adaptation being the important thing rather than a particular mechanics.
For a much more extensive, better reasoned, and more experienced discussion of crank length vs. leg length vs. power, etc. than I can give, see Steve Hoggs:
https://www.stevehoggbikefitting.com...gth-which-one/
Warning: the good stuff is near the bottom of this long article, so read all the way down and the comments.