Cadence Sensor

Summary

A cadence sensor detects crank rotation in an e-bike pedal-assist system. It tells the controller when the rider is pedaling and may also report crank speed and direction. In a basic cadence-only system, motor output is usually determined by the selected assist level rather than by how much force the rider applies.

Key Facts

• Adoption: Became widespread in e-bike systems during the 2000s
• Category: E-Bike Technology
• Also Known As: PAS sensor, pedal-rotation sensor, PAS magnet ring
• Common Locations: Crank spindle, bottom bracket, crank arm, or inside a drive unit
• Core Function: Detects crank rotation for pedal-assist activation
• Typical Design: Magnetic ring paired with one or more Hall-effect sensors
• Typical Applications: Hub-motor e-bikes, conversion kits, utility bikes, and combined-sensor mid-drives
• Does Not Measure: Pedal force or rider power unless paired with a torque sensor
• Legal Classification: Can be used on Class 1, Class 2, or Class 3 e-bikes

Overview

Cadence sensors are one of the simplest ways to control an e-bike motor from the pedals. When the cranks begin rotating, the sensor sends electrical pulses to the controller. Once the controller recognizes forward pedaling, it activates the motor according to the selected assistance level.

The sensor does not normally determine motor output by itself. It supplies information about crank movement, while the controller decides how much current or power to send to the motor. On many basic systems, the rider can turn the cranks with very little force and still receive full assistance for the selected level.

This behavior makes cadence-based pedal assist useful for riders who want the motor to perform much of the work, including commuters, recreational riders, and people with limited strength. The tradeoff is reduced control through the pedals: pressing harder does not necessarily produce more assistance, and easing pedal pressure may not reduce it until crank rotation stops.

Cadence sensing is not limited to inexpensive e-bikes. Premium mid-drive systems also measure cadence, but they typically combine it with torque, wheel-speed, crank-position, and motor-speed data. Bosch, for example, lists separate torque and cadence inputs in its Performance Line CX system. Bosch Performance Line CX

How It Works

1. Magnet Ring and Sensor

A common external cadence sensor consists of:

• A rotating disc containing several magnets
• A stationary Hall-effect sensor mounted near the disc
• A cable connecting the sensor to the controller

As each magnet passes the sensor, the magnetic field switches the sensor’s output. The controller counts these pulses to determine whether the cranks are turning and, when required, calculate cadence in revolutions per minute.

Cadence rings may contain approximately 5 to 32 magnetic poles. A higher pulse count gives the controller more frequent crank-position updates, potentially allowing faster engagement and cutoff. However, magnet count alone does not determine response quality. Controller programming, signal filtering, motor ramp rate, and required pulse count also affect how the system feels.

Some cadence sensors are built into the bottom bracket rather than fitted externally. Bafang, for example, offers bottom-bracket cadence sensors producing 32 pulses per crank revolution. Bafang cadence-sensor specifications

2. Direction Detection

A PAS system should distinguish forward pedaling from backward crank movement. Otherwise, rolling a bike backward or rotating the cranks during service could unintentionally start the motor.

Basic systems may infer direction from magnet polarity or sensor placement. More advanced sensors use two signal channels, sometimes called quadrature sensing, to identify both rotation and direction.

A replacement sensor therefore must match more than the connector. Direction, signal type, operating voltage, pulse count, connector pinout, and controller compatibility may all differ.

3. Controller Response

The controller determines what happens after pedaling is detected. Common control strategies include:

Speed-based assistance: Each PAS level corresponds to a target speed or assistance cutoff. The motor may accelerate strongly until that speed is reached.

Current- or power-based assistance: Each PAS level limits motor current or wattage. Lower settings provide gentler acceleration, while higher settings permit more output.

Cadence-responsive assistance: The controller adjusts output according to measured crank speed. This is possible with a cadence sensor but is less common in basic systems.

Some manufacturers refine cadence-based assistance through software. The Lectric XP Lite 2.0, for example, uses a cadence sensor with power-regulation programming that assigns a power level rather than relying only on speed limits. Lectric XP Lite 2.0

Because the controller has such a large influence, two bikes using similar magnet-ring sensors can feel substantially different.

Cadence Sensor vs. Torque Sensor

Many torque-sensing systems contain a cadence sensor as well. Cadence establishes whether and how quickly the rider is pedaling, while torque determines how hard the rider is pushing. The controller combines both signals to calculate motor assistance.

Some newer bikes allow the rider to select between torque-responsive and cadence-style assistance. This provides proportional support for active riding and low-effort assistance when the rider wants the motor to contribute more.

Ride Characteristics

Advantages

Low activation effort: The rider generally needs only to rotate the pedals, making the system suitable for limited strength or mobility.

Lower cost: Magnetic cadence sensors and their supporting electronics are inexpensive to manufacture.

Simple operation: The rider selects an assist level and pedals without having to maintain a particular pedal force.

Easy retrofit installation: External magnet rings can often be added to conventional cranksets for hub-motor conversions.

Steady motor contribution: Current-controlled systems can provide consistent assistance for commuting and relaxed riding.

Limitations

Limited effort sensitivity: Pressing harder does not automatically increase motor output.

Possible startup delay: The cranks may need to rotate through several degrees before the controller receives enough pulses to activate assistance.

Motor overrun: Assistance may continue briefly after pedaling stops while the controller confirms that crank rotation has ended.

Low-speed surging: Strong preset output can make tight turns, crowded paths, and technical terrain harder to control.

Ghost pedaling: Riders may rotate the cranks with minimal effort while the motor supplies most of the propulsion.

Cadence sensing is not inherently less battery-efficient than torque sensing. Efficiency depends on controller programming, assistance level, speed, terrain, gearing, motor type, and rider contribution. A poorly mapped cadence system may waste energy by applying unnecessary power, but a current-controlled system used at a low setting can be efficient.

Safety and Legal Use

Cadence sensing does not determine an e-bike’s legal class. It can be used on pedal-assist-only bikes or on bikes that also have a throttle.

Under the common U.S. three-class framework:

• Class 1 uses pedal assistance up to 20 mph.
• Class 2 may use a throttle up to 20 mph and may also include PAS.
• Class 3 uses pedal assistance up to 28 mph.

European pedelec regulations require assistance to stop when the rider stops pedaling and progressively cut off by 25 km/h, but they do not require torque sensing.

Brake cutoff switches are commonly paired with cadence-based hub-motor systems. They interrupt motor output when a brake lever is pulled, which can help control systems with noticeable engagement delay or overrun.

Troubleshooting and Service

A cadence-sensor fault may cause no assistance, intermittent operation, delayed engagement, assistance during backward crank rotation, or an incorrect cadence display.

Common causes include:

• Excessive gap between the magnet ring and sensor
• Magnet disc installed backward
• Cracked, loose, or missing magnet ring
• Sensor moved during crank or bottom-bracket service
• Damaged cable near the crank
• Corroded or partially seated connector
• Incorrect replacement sensor or connector pinout
• Controller settings that do not match the sensor’s pulse count
• Faulty brake cutoff switch preventing motor operation

A wheel-speed sensor problem can also disable assistance or produce a system error, so it should not be confused with the crank cadence sensor.

When diagnosing a separate Hall-effect sensor, mechanics may check its supply voltage, ground, and pulsing signal. Wire colors and pin assignments are not universal, so the manufacturer’s diagram should be confirmed before applying voltage or probing connectors.

Before working around the crank or rear wheel, switch the e-bike off and remove the battery when the system allows it. A functioning cadence sensor can start the motor unexpectedly when the cranks are turned.

After crank or bottom-bracket work:

1. Confirm that the magnet ring faces the correct direction.
2. Set the sensor gap to the manufacturer’s specification.
3. Check cable routing and connector seating.
4. Test forward and backward crank rotation.
5. Verify that assistance stops promptly when pedaling ends.
6. Check brake cutoffs and speed-sensor operation if assistance remains disabled.

Some drive units use internal cadence sensors that cannot be serviced separately. Diagnosis may require manufacturer software, live sensor data, or replacement of a larger sensor or motor assembly.

Buying Considerations

A useful test ride reveals more than the sensor description. Buyers should check:

• How much crank movement is required before assistance starts
• Whether power arrives gradually or abruptly
• How quickly assistance stops
• Behavior during slow turns and hill starts
• Whether PAS levels control speed, power, or both
• Whether brake cutoff switches are fitted
• Whether cadence and torque modes can be selected
• Availability of replacement sensors and diagnostic support

Cadence-based PAS remains a practical choice when affordability and low rider effort matter most. Torque-based control is generally preferable when precise pedal modulation, technical riding, or a conventional bicycle feel is the priority.

Notable Implementations

• Bafang PAS Sensors: External and bottom-bracket-mounted cadence sensors used with numerous OEM and conversion systems.
• Lectric XP Lite 2.0: Uses a cadence sensor with power-based assistance programming.
• Lectric XP Trike2: Available with cadence-based control, while some versions provide selectable torque and cadence behavior.
• Bosch Performance Line CX: Uses cadence as one input within a combined torque-, cadence-, and speed-sensing system.
• Grin Technologies Conversion Systems: Support configurable cadence sensors, including directional and quadrature designs.

Related Terms

Pedal Assist
Torque Sensor
Hall-Effect Sensor
Hub Motor
Motor Controller
Throttle Control
Brake Cutoff Sensor

References

Bafang Sensor Technical Specifications
Bosch E-Bike System Documentation
Grin Technologies: Pedal-Assist Sensor Theory
Lectric E-Bikes: PAS and Power-Regulation Documentation
Applicable National and Regional E-Bike Regulations