What an E-Bike Sensor Does and Why Assist Needs One

A pedal-assist e-bike has no way of guessing that you want help. It needs a sensor to tell it.¹

On a bike of this type, the electric motor is regulated by pedaling. That's the plain definition of pedal assist. The motor helps while you pedal, and the bike needs a sensor to detect your pedaling speed, your pedaling force, or both.¹

The sensor replaces the button

Compare that with the other main kind of e-bike. There, the motor is activated by a throttle, usually on the handlebar. You twist or press, and the motor runs.¹

A pedal-assist bike works differently. It uses a control system for the motor with sensors that detect the speed and/or the torque of the cranks, instead of a twist-grip controller or buttons.²

Torque is a word you'll see a lot. It means twisting force. Here it's how hard your legs push the cranks around. Speed in this context means how fast the cranks turn. The next pages cover each in turn: cadence sensors read how fast you pedal, and torque sensors read how hard.

What happens after the sensor reads you

The sensor doesn't move anything itself. It only produces a signal. That signal goes to a controller, which decides what the motor should do.²

One source describes the signal as being processed mechanically or electronically, then used to switch the motor on and off or to regulate a control function.²

The amount of help also depends on the support level you pick. Makers usually offer several levels, from zero to maximum. So the sensor says you're pedaling, and the level says how much help to give.²

A second sensor job: the wheel

Pedaling isn't the only thing measured. The speed of the vehicle is measured at the wheel as well. The point is to reduce or cut the motor power at the legally specified speeds.²

That's why a bike can feel as if the help fades out as you go faster. It's the wheel's speed reading doing its job, not your legs getting weaker. The speed sensor page goes into how that reading is taken.

Why the type of sensor matters when you shop

Not every pedal-assist bike measures the same thing. The sources list force sensors on the pedals, the crank, the chain or at the wheel, and speed counters at the crank or another suitable spot. Some designs measure both.²

So when a listing says "pedal assist" and nothing more, you haven't learned much. Ask which of these the bike measures. A maker's manual or spec sheet is the place to look it up. The torque vs cadence page sets the two main types side by side.

Some pedal-assist bikes also have a throttle, so don't assume the two ideas never share a frame. One source notes that some manufacturers offer both systems at once.²

What a sensor can't tell you

A sensor reading is only as good as the part and its position. Its signal tells the controller about pedaling. It says nothing about road, hills or your plans. That's why the support level is still your choice, and why assist can feel different from one bike to the next.

If your bike behaves oddly, read the maker's manual for your own model and ask a bike shop that works on e-bikes. Don't open sealed motor or sensor housings yourself.