How about mounting an optical mouse so the bottom is facing and very near, but not touching, the surface of the wheel?
The only issue here might be that the wheel might be turning too fast for you to get an accurate or reliable reading.
If that is the case then you could perhaps mount the mouse pointing at the side of the wheel, and put some kind of cardboard disk on it with contrasting lines with whatever spacing makes the rotation appear slow enough for the mouse to work properly.
I think gearing the wheel down is also a good idea, but if the above works, it would be much simpler to construct. And you don't neccesarily have to destroy a moust to test it.
The only issue here might be that the wheel might be turning too fast for you to get an accurate or reliable reading.
If that is the case then you could perhaps mount the mouse pointing at the side of the wheel, and put some kind of cardboard disk on it with contrasting lines with whatever spacing makes the rotation appear slow enough for the mouse to work properly.
I think gearing the wheel down is also a good idea, but if the above works, it would be much simpler to construct. And you don't neccesarily have to destroy a moust to test it.