E-MTB speed sensors

What does an electric mountain bike speed sensor do?

The speed sensor tells the motor how fast the bike is travelling. It triggers the motor assistance cut-off at 25 km/h, as required by European regulations. If it is misaligned, dirty or faulty, the motor assistance may cut out erratically or fail to engage, and the display will show an error code.

Two types from Shimano

• Spoke magnet : SM-DUE10, EW-SS300 and EW-SS300 Torx. The sensor mounts on the chainstay, and the magnet clips onto a spoke.
• Brake disc magnet : SM-DUE11, EW-SS301 and EW-SS302. The magnet is integrated into the brake disc, and the sensor reads it near the calliper. A neater setup that is much less exposed to branches.
• Switching from one type to the other also means changing the brake disc or refitting a spoke magnet.

Adjustment matters as much as the part itself

• The gap between the magnet and sensor must stay within the manufacturer's specified range — just a few millimetres.
• An engraved mark on the sensor shows where the magnet should pass: align it with that mark, rather than guessing.
• Simply removing the rear wheel and refitting it slightly out of alignment is enough to shift the setup.

Before replacing the sensor

A speed reading fault often comes from the cable or its connector rather than the sensor itself. Check the cable routing, that the E-Tube connector is clean, and that the cable is not pinched near the derailleur hanger before ordering a replacement.

To find out more: How to maintain your electric mountain bike properly

See also: Electric mountain bike brake discs · Electric mountain bike displays and mounts · Electric mountain bike batteries