Speed Sensors and Wheel Magnets

The speed sensor watches the wheel, not your legs. Its job is mainly to tell the bike when to ease off.¹

Besides measuring pedaling, a pedal-assist bike measures its own speed at the wheel. The source gives the reason: motor power is cut or reduced at the legally specified speeds.¹

Speed limits differ by place

Which speed counts depends on where you live and how your bike is classed. This page can only report what the sources say.

Take the EU definition of a pedelec. Over 6 km/h (3.7 mph), assistance only engages when the rider is pedalling. The motor cuts out once 25 km/h (16 mph) is reached.²

Rules in other countries and regions are different. Check your local law and your maker's manual. Nothing here is legal advice.

How magnet speed sensors work

The sources describe the sensor types rather than any particular bike.

Hall-effect sensors are used in rotating speed sensors, with bicycle wheels named as an example. They're solid-state, so they're not prone to mechanical wear.³

A reed switch is the other type. A magnetic field from an electromagnet or a permanent magnet causes its reeds to attract each other and close the circuit.⁴

Which one your bike has isn't something general sources can answer. The manual for your model will say, or the shop that serves it.

Fitting and alignment

The sources don't describe how a magnet or sensor is mounted or lined up. Follow your manual. It is the right place to look for anything about position or spacing on your model. If the manual doesn't cover it, ask a shop that works on e-bikes. Don't adjust parts by guesswork.

What about the speedometer?

The sources here talk about the wheel reading being used to limit motor power. They don't cover the display. Don't assume the screen speed and the cut-off speed come from the same reading unless your manual says so.

Pedal sensor and wheel sensor aren't the same part

A bike can use both. One reading says you're pedaling.¹ The other says how fast the bike is going. Keep them apart in your head, because a fault in one doesn't have to mean a fault in the other, and a shop will ask which you mean. The cadence page covers the first, and the opening page of the site shows how the two fit together: the basics.

Care

If a wheel magnet looks loose, or the speed reading seems off, don't improvise. Check the maker's manual for your own model, and have a shop that works on e-bikes look at it. Wheel and brake work affects your safety on the road.