Ralph is using my Garmin Edge 705 bike computer & I decided to try fitting the cadence sensor. (Owing to the Brompton small wheels, it’s not possible to make use of the wheel speed component of the Garmin sensor but that was ok; the 705 gives me wheel speed via the GPS.)
The usual position for the sensor would be on top of the chain stay arm of the rear frame, with the crank arm magnet on the end of the crank. However, Ralph has a widened rear frame to fit the Alfine 11speed hub & there is no clearance for the crank arm magnet when Ralph is folded. I tried mounting the sensor on the vertical leg of the frame but discovered that the magnet then passes the sensor in the wrong plane (giving erratic/doubled cadence values). I nearly gave up but then tried cobbling up the sensor onto the frame joint (see pic) – & so far, it seems to be working!