On some of our bicycles, the speed sensor is on the right rear stay, and on others it is on the left rear stay. For the Compass trike, the speed sensor is on the left leg of the fork.
The magnet fastened to a spoke on the rear wheel passes a speed sensor mounted on the inboard side of the rear stay, triggering an electrical signal that the bike uses to count wheel rotations and calculate speed and distance.
It is important that the magnet passes the sensor with the correct alignment and spacing. The smooth side of the magnet should pass between the two + signs marked on the flat face of the speed sensor (try lining up with the + mark closer to the wire end of the sensor). The distance between the two should be between 5-8mm.