I'm looking at buying a cateye wireless cadence computer with the combined cadence and speed sensor (ISC10). However on my small wheeled bike (406mm/20") the correct place to mount for cadence means the speed sensor is just forward of the wheel. Or if I mount the speed sensor by the spokes it is too far back from the crank for the cadence sensor.
Sounds like the crank arm and wheel spokes don't overlap due to the small wheel size. If so, then you have two choices:
1: Change the laws of physics such that the magnetic field of one of the magnets is displaced sufficiently from its physical location to allow it to trigger the sensor.
2: Go with separately mounted cadence and speed sensors instead of a combination speed/cadence sensor.
I suspect option 2 will be more easily accomplished.