XII. Sensor response

1. Equipment

    A 32-spoke wheel in carbon fiber forks was fitted with Sigma Sport magnets (2002). Tests were done with and without metal sheaths. Values in parentheses refer to the latter tests. Sensors were 2002 models from Sigma Sport and Planet Bike (Protégé 8).

2. Magnet clearance

    With the Sigma Sport sensor magnets remained effective for up to 20 (15) mm (center to center) or almost as far as they could be twisted away from the sensor. The Planet Bike sensor remained effective for up to 10 (8) mm even though the set-up violated instructions given by the manufacturer. In summary: clearance adjustment is not at all critical.

3. Sigma Sport sensor

    An impulse is produced by the magnet when it has gone 20 (15) mm (center to center) past the sensor. This happens whether wheel is moving forward or backward. Thus, four impulses are produced in forward direction at 0, 0.25, 0.5, and 0.75 rev on the rim and four impulses in backwards direction at 0.022   (0.016) rev behind these positions. To avoid ultimately producing spurious impulses, do not allow the wheel to rotate backwards through any of the above points.

4. Planet Bike sensor

     An impulse is produced by the magnet when it is 13 (9) mm (center to center) before the sensor. This happens whether wheel is moving forward or backward. Thus, four impulses are produced in forward direction at 0, 0.25, 0.5, and 0.75 rev on the rim and four impulses in backwards direction at 0.009   (0.005) rev before these positions. To avoid ultimately producing spurious impulses, do not allow the wheel to rotate backwards through any of the above points.

5. Avoidance of spurious impulses

    Do not allow the wheel to rotate backwards near the points on the rim at 0, 0.25, 0.5, and 0.75 rev. Problem points can be marked on the tire. In between these points the wheel can be allowed to rock backwards and forwards to a large degree without affecting measurements. If the only method  used is riding continuously between splits and rezeroing before each split, no attention to spurious impulses is necessary.