Pedal Assist (PAS)

Summary

Pedal Assist, often abbreviated as PAS, refers to a control system in electric bicycles where the motor provides power only while the rider is pedaling. It uses sensors to detect pedaling input and proportionally augments rider effort, offering a natural, bicycle-like experience enhanced by electric assistance.

Key Facts

  • Introduced: Late 1990s – early 2000s (mainstream adoption followed)
  • Category: E-Bike System / Concept
  • Also known as: Pedal-activated assist, pedelec mode
  • Used by / Found on: Class 1 and Class 3 e-bikes globally
  • Sensor types: Cadence sensors, torque sensors, or a combination of both
  • Legal relevance: Defines e-bike classification in most regulatory systems
  • Primary advantage: Delivers assistance only during active pedaling for a controlled, intuitive feel

Overview

Pedal Assist (PAS) is a foundational concept in e-bike technology—so much so that it now defines how electric bicycles are regulated, classified, and experienced around the world. Unlike throttle-controlled systems, which engage the motor independently of the rider’s effort, PAS requires the rider to pedal. The bike then responds by applying motorized power in tandem with that effort.

The idea is straightforward: you ride as you would on a regular bicycle, and the motor provides support based on how hard—or fast—you’re pedaling. It’s an evolution of cycling, not a replacement of it. Pedal Assist allows for smoother hill climbs, easier acceleration from a stop, longer-range commuting, and less fatigue on extended rides—all while preserving the essential mechanics of pedaling.

Modern PAS systems vary widely in their implementation. Basic cadence-based systems sense when you’re pedaling and trigger a pre-set level of motor output. More advanced torque-sensing systems adjust motor output in real time based on how hard you’re pushing the pedals. Either way, PAS has become the default for everything from entry-level commuter bikes to high-performance e-MTBs.

More than just a convenience feature, PAS reshapes the entire riding experience. It encourages more pedaling, enhances exercise potential, and delivers a more integrated interaction between human and machine.

How It Works

The functionality of a PAS system centers on three core components: sensors, a controller, and the motor.

1. Sensors

The PAS system needs to detect when and how you are pedaling. There are two main types:

  • Cadence Sensor: Detects if the pedals are turning. Usually a ring of magnets and a sensor near the crank, this setup sends a signal when the rider is pedaling, turning on the motor. It does not measure how hard you’re pedaling—just that you are.
  • Torque Sensor: Measures how much force you’re applying to the pedals. This offers a more nuanced response: the harder you push, the more assist you get. Often combined with cadence sensors in high-end systems.

Some bikes also include a speed sensor, which helps modulate power delivery and ensures the motor cuts out at regulated speed limits (usually 20 mph or 25 km/h).

2. Controller

The controller is the bike’s “brain.” It receives input from the sensors and determines how much power the motor should deliver. On cadence-only systems, it may simply toggle the motor on or off based on pedal rotation. On torque-based setups, it continuously adjusts output based on rider effort.

Controllers also let riders choose assist levels via a handlebar display—typically labeled as Eco, Tour, Sport, or Turbo. Each level modifies the motor’s output multiplier.

3. Motor

The motor receives signals from the controller and applies assist to the drivetrain. In mid-drive systems, the motor turns the crank or chainring. In hub motors, the motor adds force directly at the wheel.

The integration of these components defines how smooth, natural, and responsive the system feels. Advanced PAS setups can react within milliseconds, offering seamless transitions between assist and no-assist.

Comparison with Throttle & Impact on Ride Feel

Pedal Assist differs fundamentally from throttle-based e-bike systems in both user experience and regulatory classification.

PAS vs Throttle

  • Pedal Assist:
    • Requires pedaling for motor activation
    • Feels more like traditional cycling
    • Encourages consistent exercise
    • Typically quieter and more efficient
    • Complies with European pedelec laws and most Class 1 / Class 3 standards in North America
  • Throttle Control:
    • Engages motor regardless of pedaling
    • Useful for starts, hill boosts, or rider fatigue
    • Reduces exercise input
    • Often regulated as mopeds in EU; Class 2 in the U.S.

Ride Feel

Torque-based PAS systems feel incredibly natural—almost like a tailwind pushing you forward. Riders describe the experience as “intuitive” or “invisible.” The power fades in as you push and fades out when you coast. There’s no jerking or surging, especially on modern mid-drive units.

Cadence-based systems, though less refined, still serve well on budget models. They’re particularly helpful for urban commuting or for riders with limited leg strength, since assistance kicks in as soon as pedaling begins—regardless of force.

Notable Implementations

  • Bosch Performance Line CX: One of the most respected PAS systems on the market. Offers multi-sensor torque detection with instant response. Smooth, strong, and widely used in e-MTBs and trekking bikes.
  • Shimano STEPS EP8: Torque sensor-based system with natural feel, tuned for performance and trail use. High customization via E-Tube app.
  • Specialized SL 1.2: Lightweight PAS platform with subtle assist behavior. Feels like riding a road bike with superhuman legs.
  • Fazua Ride 60: Combines cadence and torque sensing in a removable drive system. Offers light, fitness-oriented assist tuned for gravel and urban use.
  • Yamaha PWseries SE: Mid-tier PAS platform offering strong low-cadence support and smooth transitions.

Related Terms

References

  • Bosch eBike Systems: Technical Manuals and PAS Integration
  • Shimano STEPS: Developer Documentation and User Guides
  • Specialized Turbo SL: Ride Feel and Assist Curve White Paper
  • E-Bike Regulations by Region – Class Definitions
  • Yamaha E-Bike System Specs and Assist Behavior
  • ElectricBikeReview: Pedal Assist vs Throttle User Comparisons
  • CyclingTips: “How PAS Changes Rider Behavior and Fitness”
Scroll to Top