Originally Posted by
dsbrantjr
The one thing you need to be aware of is that the magnet, like all magnets, has two poles and you do not want to mount it so that both poles get counted by the sensor; this will give you double the speed and distance readings. you can use another magnet or a compass to determine just where the poles are; make sure one is close to and one is far from the sensor.
I've never had this problem with any wheel magnet but, I guess if you mounted it "sideways" (i.e. with the flat face of the magnet at 90° to the frame), the pickup could see both poles as separate pulses.
Edit: A couple of magnet ideas:
If you want to mess with your riding buddy, sneak a second magnet on his wheel, 180° away from the first. It's unlikely to be noticed at first but he will be astounded by his new-found speed.
I've seen the two magnet idea used intentionally on a reproduction high wheel bike by a rider who wanted to know his distance and speed. The front wheel was so large the cyclometer calibration numbers didn't go that high. He mounted two magnets and used a calibration number 1/2 of the true wheel circumference.