Torque Sensor vs Cadence Sensor

Every pedal-assist ebike has to answer one question several times a second: should the motor be helping right now, and by how much? There are two ways to answer it, they produce completely different bikes, and the cheaper answer is the one most of the market uses.

Spec sheets bury this. A listing will lead with 750W, quote a top speed, name the battery capacity, and mention the sensor type in a bullet halfway down, if at all. Yet after motor placement, which is covered in mid-drive vs hub motor, this is the specification that most determines whether you enjoy riding the bike.

How a cadence sensor works

A cadence sensor is a ring of small magnets clamped to the bottom bracket spindle or a crank arm, with a fixed sensor on the frame beside it. As you pedal, magnets pass the sensor and it produces a pulse each time. Count pulses over time and you know crank rpm. That is the entire mechanism, and it costs almost nothing.

What the controller does with that signal is where the ride feel comes from. It knows only two things: the cranks are turning, and roughly how fast. It has no way of knowing whether you are grinding up a hill or freewheeling your feet in circles. So it applies whatever the assist level says: level three means level three power, always.

Two consequences follow, and both are recognisable to anyone who has ridden a budget ebike.

Startup lag. The controller usually waits until it has seen a certain number of magnets pass before it accepts that you are genuinely pedalling, rather than rocking the cranks while stationary. With a coarse sensor disc that can mean most of a pedal revolution. You push off, nothing happens, and then the assist arrives all at once. The number of magnets on the disc is what governs this: more magnets means finer resolution and less delay, which is why disc counts of twelve behave better than counts of five.

Overrun. The same logic applies in reverse. When you stop pedalling, the controller has to wait long enough to be sure the pulses have genuinely stopped before it cuts power. That produces a brief continuation of assist after your feet stop, which is mildly disconcerting in traffic and genuinely unhelpful when you are trying to slow for a corner. Brake cutoffs exist partly to paper over this.

How a torque sensor works

A torque sensor measures force rather than motion. The usual implementation is a strain gauge: a component that changes electrical resistance very slightly when it flexes. Bolt one where drivetrain force passes through, and the tiny deflection under load becomes a measurable voltage proportional to how hard you are pushing.

Manufacturers put them in several places, and the location affects both accuracy and cost:

The controller multiplies the measured rider force by the assist level. Push twice as hard and you get twice as much help. Ease off and the motor eases off with you, within milliseconds rather than after a delay. There is no lag to speak of, because the force appears the instant you push, and no overrun, because it disappears the instant you stop.

Better systems combine the torque signal with cadence and wheel speed, using rider force for the magnitude and the other two to smooth delivery and manage the cutoff at the legal speed limit. That combination is what makes the best mid-drives feel less like a motor and more like unusually strong legs.

What the difference feels like

Riding a cadence bike, you learn to manage the motor. You anticipate the lag when pulling away, you learn which assist level is appropriate for which situation, and you use the level buttons the way you would use a throttle. The bike is a machine you operate.

Riding a torque bike, you mostly forget the motor is there. You push harder and more happens; you soft-pedal and less does. Standing on the pedals produces a surge because you are producing a surge. The assist level stops feeling like a speed setting and starts feeling like a multiplier on your own fitness.

Three situations expose the gap most clearly:

• Pulling away at a junction. The cadence bike pauses then shoves. The torque bike moves exactly as hard as you push, which matters when the gap in traffic is small.
• Technical low-speed riding. Tight turns, shared paths, filtering through people. Cadence assist is on or off, which makes fine speed control awkward. Torque assist modulates with your feet.
• Long climbs. On a cadence bike you pick a level and the motor does its fixed share regardless of how you feel. On a torque bike, effort and assist stay proportional the whole way up, so the climb feels like a climb rather than a chairlift.

Why it changes range

Range on an ebike comes down to watt hours consumed per mile, and sensor type moves that figure through rider behaviour rather than efficiency.

On a cadence system, the motor draws the current its assist level commands whenever the cranks turn. Riders quickly work out that turning the pedals lightly is enough to keep the assist flowing, so a great deal of cadence-bike riding is effectively throttle riding with the feet going round. The battery pays full price for every mile.

On a torque system, that strategy does not work. Soft-pedalling produces soft assist by definition. Riders end up contributing more of the power, and consumption falls accordingly. Ten to thirty percent more range from the same battery on the same route is a reasonable expectation, and the gap widens on hilly terrain where cadence systems spend longer at high assist.

This has a practical consequence for conversions. A cadence-sensing kit needs a larger battery than a torque-sensing one to cover the same route with the same confidence, which partly offsets the lower kit price. Choosing a battery for a conversion kit covers sizing, and longest range electric bikes is a useful check on what capacity alone buys.

Where each one shows up

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The pattern is consistent: torque sensing correlates with bikes designed to ride like bicycles, and cadence sensing with bikes designed to be cheap or to be ridden on a throttle. Neither is wrong, but they are aimed at different riders. If you want the motor to do the work, throttle ebikes covers that approach directly, and whether you have to pedal at all covers where the law lands on it.

Can you upgrade a cadence bike

Usually not in any way that makes financial sense, and the reason is the controller rather than the sensor.

A cadence sensor sends simple pulses. A torque sensor sends an analogue signal representing force, which the controller must be programmed to interpret and scale. A controller expecting pulses has no idea what to do with a voltage. So fitting a torque sensor means fitting a controller that reads it, and usually a display that matches that controller, at which point you have replaced most of the electrical system. See what an ebike controller does for why it sits at the centre of this.

There are things worth doing instead on a cadence bike:

• Fit a sensor disc with more magnets if yours is coarse. More pulses per revolution means the controller decides faster, which cuts startup lag noticeably.
• Reprogram the assist curve. On Bafang kits a USB programming cable lets you soften how aggressively each level ramps, which removes much of the shove without removing the power. This is the highest-value change available, and it is covered alongside the kits in Bafang BBS02 vs BBSHD.
• Use lower assist levels and your gears. Level one plus a sensible gear behaves far more like a torque system than level four plus a high gear does.

Which one you actually want

If you are choosing a complete bike, treat the sensor type as a filter before you compare anything else, because no amount of motor or battery compensates for delivery you find irritating. If you are choosing a kit, this is the main axis on which the common options differ, and the best ebike conversion kits lays out which is which. Riders coming from unassisted road riding tend to find cadence delivery hardest to accept, which is worth weighing alongside the best electric road bikes, where torque sensing is close to universal.

Frequently asked questions

Sources and further reading

• Bafang drive systemsBafang
• Bosch eBike Systems: drive units and batteriesBosch eBike Systems
• 15 U.S.C. 2085: Low-speed electric bicyclesLegal Information Institute, Cornell Law School