Summary
A torque sensor measures twisting force within an e-bike’s pedal or drivetrain system. The controller combines this signal with cadence, speed, assist-mode, and system data to determine motor output. This allows assistance to respond to rider effort rather than merely detecting crank rotation.
Key Facts
- First major production use: Yamaha PAS, 1993
- Category: E-bike control system
- Measurement: Torque, expressed in newton-meters
- Common sensing method: Strain-based measurement
- Common locations: Motor spindle, bottom bracket, crank axle, or rear dropout
- Usually combined with: Cadence and wheel-speed sensors
- Used with: Mid-drive and hub-drive motors
- Main function: Provide rider-effort data to the motor controller
- Legal requirement: Generally no; regulations focus on power, speed, and when assistance operates
Overview
Torque is rotational force. In an e-bike, it represents the turning effort transmitted through the crank, spindle, chainring, or another part of the drivetrain.
A torque sensor detects microscopic deformation in one of these load paths. The resulting signal tells the controller how much effort the rider is applying. The controller then uses software to decide how much motor assistance to provide.
The common description “push harder, receive more assistance” is broadly accurate, but incomplete. Assistance also depends on:
- Selected riding mode
- Pedaling cadence
- Wheel speed
- Motor speed
- Battery condition
- Temperature
- Current and torque limits
- Manufacturer programming
The sensor supplies data; the controller determines how the bike responds.
How It Works
Strain Measurement
Many torque sensors use strain gauges attached to an elastic metal element. Pedaling torque causes this element to twist or bend by a microscopic amount.
As the strain gauge changes shape, its electrical resistance changes. Multiple gauges may be arranged in a Wheatstone bridge to improve sensitivity and compensate for temperature effects.
The sensor electronics then:
- Amplify the small electrical signal
- Compensate for offset and temperature
- Convert it into torque data
- Send that data to the motor controller
Other electromagnetic sensing methods exist, but strain-based designs are common.
Torque Versus Rider Power
Torque alone does not equal rider power. Power also depends on how quickly the cranks are turning:
Rider power = torque × angular velocity
Two riders applying the same torque at different cadences are producing different amounts of power. This is one reason modern systems normally combine torque and cadence measurements.
Controller Mapping
The controller applies an assist map to the sensor data. This map determines characteristics such as:
- Initial motor response
- Assistance ratio
- Maximum motor torque
- Acceleration ramp
- Response at different cadences
- Power reduction when pedal pressure decreases
- Motor cutoff behavior
A high sampling rate can help, but it does not guarantee better ride quality. Filtering, calibration, motor response, and software tuning are equally important.
Sensor Locations
Motor or Crank Spindle
Common in mid-drive systems. The sensor measures strain within the drive unit’s axle or internal torque sleeve.
This arrangement provides close integration between the rider, transmission, controller, and motor.
Bottom Bracket
A bottom-bracket torque sensor can be paired with either a mid-drive or hub motor. Some designs measure axle twist, while others detect the reaction force created by pedaling.
This is a common approach for higher-quality hub-drive e-bikes that need proportional assistance.
Rear Dropout or Chainstay
These systems measure small frame deflections caused by chain tension. They can provide torque-based control without placing electronics inside the crank spindle.
Their accuracy may depend on correct axle installation, dropout condition, chainline, and frame design.
Crank or Pedal
Crank- and pedal-based sensors resemble cycling power meters. They are less common as primary e-bike controls but may be used in specialized or retrofit systems.
Combined Sensor Operation
Most advanced e-bike systems use several inputs together:
| Sensor | Information supplied |
|---|---|
| Torque | How much turning effort the rider applies |
| Cadence | How quickly and in which direction the cranks rotate |
| Wheel speed | Bicycle speed and assistance-limit control |
| Crank position | Position of the pedal stroke |
| Motor sensors | Rotor position, motor speed, current, and temperature |
Combining these signals allows the controller to distinguish between starting, climbing, spinning, coasting, shifting, and approaching the assistance-speed limit.
Torque Versus Cadence Sensing
| Torque-based system | Cadence-only system |
|---|---|
| Responds to rider effort | Primarily responds to crank movement |
| Allows output modulation through pedal pressure | Output depends more heavily on selected assist level |
| Usually requires the rider to contribute meaningful effort | Can provide substantial assistance with light pedaling |
| More complex and expensive | Simpler and less expensive |
| May require calibration | Usually requires little calibration |
Torque sensing is not automatically better for every rider. Cadence-controlled systems may suit riders who want consistent motor output with minimal physical effort. Torque sensing is generally preferred where precise modulation and a conventional cycling feel are priorities.
Rider Experience
Starting
A torque sensor can detect pedal pressure before the cranks complete much rotation. Some systems therefore engage quickly from a stop, while others deliberately wait for cadence or crank-position confirmation.
This safety programming is why two bikes using similar sensors may start differently.
Climbing
Increasing pedal pressure normally increases assistance. This gives the rider fine control on loose or technical climbs.
However, pushing a large gear at very low cadence can place heavy loads on the chain, cassette, motor, and battery. Torque sensing does not eliminate the need to select an appropriate gear.
Shifting
Easing pedal pressure reduces the torque request and may cause the controller to reduce motor output. This can make shifting smoother, but riders should still avoid forcing conventional derailleur shifts under excessive combined rider and motor load.
Battery Range
Proportional control can prevent unnecessary motor output, but it does not automatically increase range. A strongly tuned assist mode can consume more energy than a lower-output cadence system.
Terrain, speed, cadence, rider input, temperature, tire pressure, and assist settings remain major factors.
Calibration and Zero Offset
A torque sensor must establish what zero load looks like. If the system initializes while a rider is pressing on a pedal, it may record an incorrect zero point.
Shimano instructs riders to keep their feet off the pedals while switching on certain STEPS systems. Failed initialization can produce a warning and reduced or disabled assistance.
Calibration varies by manufacturer:
- Some systems establish zero during startup
- Some recalibrate automatically while riding
- Some require dealer software
- Some need calibration after drive-unit or bottom-bracket service
Always follow the procedure for the specific motor system.
Common Symptoms and Diagnosis
Possible torque-sensor problems include:
- Weak or missing assistance
- Assistance that surges or pulses
- Uneven response through the pedal stroke
- Excessive delay before engagement
- Incorrect rider-power readings
- Torque-sensor initialization errors
These symptoms do not prove that the torque sensor has failed. Speed-sensor alignment, cadence detection, wiring, firmware, battery output, drivetrain condition, or motor problems can produce similar behavior.
A practical diagnostic sequence is:
- Check stored fault codes
- Restart the system without pressure on the pedals
- Verify speed-sensor and magnet alignment
- Confirm cadence and torque data through diagnostic software
- Inspect wiring, connectors, crank installation, and drivetrain condition
- Perform the manufacturer’s calibration procedure
- Test the bike under controlled load
Torque sensors inside sealed drive units are rarely repaired at component level. Depending on the system, the sensor module or complete drive unit may require replacement.
Legal Classification
Torque sensing is not specifically required by the European pedal-assist definition. European rules instead specify conditions including continuous rated power, assistance cutoff when pedaling stops, and progressive reduction before the regulated speed limit.
Common U.S. e-bike class systems also focus on motor operation, throttle use, and assistance speed rather than requiring a particular sensor type. State definitions vary and should be checked individually.
A cadence sensor can therefore operate a legally compliant pedal-assist bicycle when the complete system meets the applicable requirements.
Notable Implementations
- Yamaha PAS: The original 1993 production system combined torque and speed sensing with computerized assistance control.
- Bosch Performance Line: Integrates torque and motion data at high sampling rates to control mid-drive assistance.
- Shimano EP801 and EP6: Combine torque, cadence, and speed sensing for assist control and compatible automatic shifting.
- FAZUA Ride 60: Measures torque and cadence within its integrated lightweight drive unit and supports automatic torque calibration.
Related Terms
- Cadence Sensor
- Pedal Assist
- Mid-Drive Motor
- Hub Motor
- Strain Gauge
- Motor Controller
- Power Meter
- Wheel-Speed Sensor