Where the sensor sits on the bike

Where's the sensor? Fair question. You're looking at your bike and can't see it. It isn't always visible, and it isn't always in the same place from one bike to the next.

The spots reference sources name

One reference lists where force can be measured: from a force sensor on the pedals, the pedal crank, the chain or at the wheel.¹ Pedaling speed can be read from a rotary speed sensor at the crank or at another suitable location.¹

Two more details from the same page. Torque sensors can sit near the crankset. Strain gauges can sit on the axles of hub motors.¹ And the speed of the bike is measured at the wheel.¹

What the bottom bracket is

The bottom bracket is the part of the frame between your feet. One reference says the crankarms and chainring are attached to the spindle of the bottom bracket, which is installed in the bottom bracket shell.²

The sources saved for this site don't say a sensor sits there.

Mid-drive and hub motors

The motor's layout often goes along with the sensor's place, though the sources don't set out a firm rule.

• Mid-drive. One reference describes a mid-drive system as one where the motor isn't in a wheel. It drives the chain and is usually mounted in or under the bottom bracket shell.³ That puts the motor in the same area as the crank.
• Hub motor. In a hub motor, the motor is built into the wheel hub. The same source says hub motors are usually located in the rear wheel only, though front hub and dual motor bikes exist.³ For some hub motors, the pedaling force is read by strain gauges on the axle.¹

Why it matters

Location decides what the sensor can see.

A sensor on the crank sees the cranks. It can read how fast they turn. A strain gauge on a hub axle sees force there instead. A road-speed sensor at the wheel does a different job. It helps cut power at the legally set speeds.¹

Trying to work out why a bike behaves as it does? Find out what sensor it has and where it sits. Types are compared on the differences page.

A note on dropouts

A dropout is the slot where a wheel axle fits into the frame. The sources saved for this site don't describe a sensor in that spot, so this page makes no claim about it. If your bike's manual shows one there, trust the manual.

Before you go looking

Don't pry, unscrew or rebuild anything near the motor or sensors to find them. Check the manual for your own model, then ask a dealer. The error code and repair basics start on the first page.