Although I may not have mentioned it before, I often experiment with different lengths of crank on my various unis.
I have always adjusted the height of the seat in the traditional bicyclist’s way: sit on the seat, and you should be able to place your heel on the pedal at its lowest point of travel with your knee straight but not locked.
However, I have noticed that if I change cranks, the distance I move the seat to compensate is not the same as the difference between the lengths of the cranks. For example, I took off 125mm cranks and put on 150s, but the ideal seat height was not exactly 25mm higher. (150 - 125 = 25, so the lowest point of travel of the pedal should be 25 mm higher.)
The rule used by bicyclists assumes that standard 170mm bicycle cranks are used. As unicyclists, we use cranks varying from about 89mm up to about 175mm. The top end of the range is almost double the bottom end.
I guess that the leg muscles have the most power and fine control at a certain degree of leg extension. With very short cranks, if the seat height is the ‘traditional’ distance from the bottom of the pedal stroke, then even at the top of the pedal stroke, the leg would be pretty straight COMPARED to using 170mm cranks.
I’m not sure this lends itself to scientific analysis, because the ideal seat height can change with the type of riding to be done and so on. However, I FEEL as if I need the seat that bit lower (compared to the bottom of the pedal travel) with short cranks.
I wondered if anyone had similar experiences, and if anyone had better explanation.