You are supposed to put one magnet on every spoke on the side of the sensor, which would be 18 magnets for a 36 spoke wheel and 16 for a 32. Putting only one magnet on your wheel to save weight reduces the accuracy of the speedometer as it will only read once per revolution instead of 16-18 times and is a potential safety hazard, throwing the wheel off balance, ruining handling and potentially causing injury or death.