Originally Posted by d_D
All of mine have used
reed switches. Sometimes you can hear the contacts close when you move a magnet to the sensor.
As there is no current induced it can't get stronger when moving the magnet faster.
QFT. If it used induction, it would have trouble picking up slow speeds.
Honestly, I installed my magnet pretty much halfway on my commuter hybrid. This is where I've seen it installed on almost every bike I've seen, so that's why I put it there.
When I bought my new road bike and a rack, computer, headlight, and seat bag, the shop manager just grabbed everything and tossed it on for me. The computer's magnet ended up being pretty much in the middle of the spoke.
There's one guy I ride with on Monday nights, whose computer has a single "brick" that goes over the non-drive side chainstay. It has a sensor on the inside for the rear wheel and the outside for crank cadence. On his bike, the magnet is about 2-3 inches from the rim. He's never had a problem, either.