The idea of ‘knocking together’ a set of cranks from odds ‘n’ sods is
nice, but simplistic. It is absolutely vital that the pedal spindle
axis is exactly parallel to the axis of the hub. Even a tiny error is
detectable by the rider, as the pedal appears to ‘seesaw’.
Here’s an idea that has probably been thought of before, could be
quite heavy and probably not very practical; a 4"-6" variable length
Each pedal would be screwed into its respective 3" diameter disc about
1/2" from one edge. Both discs would have machined slots around the
edges or split into halves for mounting in to the pedal ends. Once
securely mounted into each crank, the discs can be slowing rotated in
the same direction to vary the effective crank length from 4" to 6".
The center of each disc would be 5" from the axle axis. Next a
locking mechanism is added to tighten the discs firmly to the crank
arm for riding. The end of each crank would have a 4" ring (3" hole
to accommodate the disc.
To change the length, the locking mechanism is unlocked, each disc is
rotated the same amount in the same direction so the line between the
pedal axles intersects the hub axle, and finally the cranks are locked.
Ken Fuchs <firstname.lastname@example.org>