Robust long-distance Hall sensor - magnet pair for rear wheel speed measurement
Bicycle speed measurement is generally done with a spoke magnet and a reed relay sensor. If this is done on the rear wheel, there is a large spacing between the spoke magnet and the chain stay, where the sensor is mounted on. Therefore, the sensor has to be mounted on a relatively long bracket to minimize the distance. Since the sensor has to be adjusted precisely, this makes it vulnerable. With the use of a strong ring magnet and a sensitive Hall effect sensor, the distance is not critical anymore and the sensor can be mounted directly on the chain stay. A sensor-magnet distance of 30mm is allowed.
Strong ring magnet
Here, we use a ring magnet because it can be mounted robustly on the spokes with four small cable ties. After that we have to glue the ring magnet and the cable ties together with 2 component epoxy. A suitable magnet is the neodymium 15mm diameter ring magnet R-15-06-06-N from the company Supermagnete.
Sensitive Hall effect sensor
Instead of a reed relay, which is widely used, we use a high sensitive Hall sensor, for instance the AH3362Q. It just needs 45 Gauss. I have mount the Hall sensor on a breadboard PCB. Seal it afterwards with 2 component epoxy. Mail me if you would like to buy a small PCB with the AH3362Q on it, if there is a lot of demand then I may produce it.