Torque sensor vs cadence sensor
A torque sensor measures the force you put through the cranks and scales the motor to it, so assistance arrives in proportion to your effort. A cadence sensor only detects that the cranks are turning and applies a preset power level, which is why cadence bikes surge away from junctions and feel like a throttle with extra steps.
We use a torque sensor, and it is the single most expensive decision in the bill of materials — roughly €40 per bike against a cadence pickup. It buys back most of that in range, because the motor stops paying for effort you were not making.
| Criterion | Torque sensor | Cadence sensor |
|---|---|---|
| What it measures | Force through the bottom bracket, 1,000× per second | Crank rotation, typically 12 magnets per revolution |
| Response at a standing start | Proportional and immediate | Half a crank revolution of delay, then full preset power |
| Feel | Like being stronger | Like being pushed |
| Range from the same battery | 10–25% further in stop-start traffic | Burns the same power whether you need it or not |
| Behaviour on a climb | Scales with how hard you are working | Fixed until you change the assist level by hand |
| Cost in the BOM | Around €40 more per bike | Cheapest option available |
| Service complexity | Sensor integrated in the motor assembly | Replaceable magnet ring |
Why cadence sensing feels wrong in traffic
The failure case is a junction. You put a foot down, the light changes, you push off — and for the first half revolution the motor does nothing, because no magnet has passed the sensor yet. Then a magnet passes, the controller applies whatever assist level you left it in, and the bike surges into a gap you had not committed to.
Nothing about that is dangerous on an empty road. In a city, it is the reason people ride assisted bikes in the lowest setting: not because they want less help, but because they want less surprise.
What the sensor actually changes about range
Range figures are usually quoted at the lowest assist level on flat ground, which is a number nobody experiences. The difference between sensor types shows up in the way the energy is spent rather than in the headline: a cadence system delivers its preset power on the descent as readily as on the climb, because rotation is rotation.
A torque system tapers as your input tapers, so the descent costs nothing. Across a real commute with lights, corners and a following wind, that is worth 10 to 25 per cent of the pack — the same effect as two extra cells, for less money and no extra weight.