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Speed sensor

The speed sensor reads a magnet on the wheel so the controller knows your speed. Most problems are the magnet or gap; a replacement must be made for your system.

Last edited Nov 8, 2026 by Pedalwise team Edit History Add a subpage

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A speed sensor works with a magnet on the wheel to measure how fast you are going. Bafang's dealer manual for its SR SD021 sensor explains that the sensor mounts on the frame, the magnet mounts on a spoke, and the controller works out riding speed from the pulses it receives (Bafang SR SD021 dealer manual).

A speed sensor is swapped at its plug for the part made for your system, or by a pro. Never cut and splice its cable, and remove the battery before you unplug it.

What it's used for

If the display shows no speed, or assist behaves oddly, check the magnet and the gap first with Align speed sensor magnet. Replace the sensor when its body or cable is damaged, or a shop confirms it has failed. See also No pedal assist and E-bike error codes.

What to look for

  • The right sensor for your system. Bafang sensors are sold by motor family. California Ebike lists a speed sensor for the Bafang Ultra M620 and G510 and a separate kit for the BBS02 or BBSHD (California Ebike cables and sensors).
  • The correct gap. For the SR SD021, Bafang specifies 10 to 20 mm between the sensor and the magnet (Bafang manual). Other sensors can differ, so check your own manual.
  • Cable length. Speed sensor extension cables are sold for some Bafang kits (California Ebike). Use one made for the system rather than splicing.
  • Keep magnets away. Bafang's handling notes say to keep the sensor away from strong magnets and magnetic materials and not to knock it during fitting (Bafang manual).

Typical price

  • California Ebike lists the Bafang speed sensor for Ultra M620 G510 at $34.95 (read Nov 9, 2026) (California Ebike).
  • California Ebike lists the Bafang speed sensor extension cable for BBS02 or BBSHD from $9.95 (read Nov 9, 2026) (California Ebike).

Where to buy

  1. Your bike maker's parts store or support team.
  2. For Bafang systems, a Bafang service center such as California Ebike.
  3. Independent specialists in Where to buy e-bike repair parts.
  4. A local shop, through find a shop.

Guides that use it

Sources

Stuck after these checks? Find a shop near you.

Need a pro?

If these steps didn't fix it, find a shop near you in our partner network. For battery or wiring trouble, look for shops marked Battery and electrical.

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