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.

How can I get this to fit ?

Hire shop help.

I’m willing to wager that wasn’t the response he was looking for.

Que ?

Diagnosis in the dark 3000 miles away aint workin'. you aint showing the Issue , too much assumption.

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.

One option that some have used is to modify a combo sensor. Most have arms for the wheel sensor that can be removed and the two fine wires extended using some basic electrical/soldering skills.