On some Auroras the speed sensor is on the right rear stay and on others its on the left rear stay depending on generation.
The magnet fastened to a spoke on the rear wheel passes a speed sensor mounted on the inboard side of the rear stay.
It is important that the magnet is passing 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.