Also called: wheel sensor, spoke magnet
What it does
The motor needs it to know your speed and stop assisting at the legal limit. If the signal is missing or odd, assist is reduced or cut.
Signs of a problem
- Speed shows 0 while riding
- Assist limited or missing
- A speed sensor error code
Safe things to check
- Check the magnet is still on the spoke or rotor and lines up with the sensor. Don't bend anything.
- Check the sensor's cable is plugged in.
Looking after it
- Knocked magnets are the most common cause; check after transporting the bike.