An electric bicycle translates stored chemical energy into rotational force through a closed-loop feedback system connecting the battery, controller, sensor, and motor. Rather than replacing traditional mechanical drive, an e-bike operates as a real-time force multiplier—monitoring mechanical input and delivering electrical assistance within milliseconds.

Core drivetrain layout of an e-bike. Resource：Alla Koala / Getty Images

1. The Tri-Component Power Loop

At the center of every e-bike system are three primary electrical components:

• Lithium-Ion Battery Pack: Houses multiple lithium cells in series and parallel configurations, delivering direct current (DC) typically rated at $36\text{V}$ or $48\text{V}$. Total capacity is measured in Watt-hours ($\text{Wh}=\text{Volts}\times \text{Amp-hours}$).

• Motor Controller: Acts as the central processing brain. Using a bank of MOSFET switches, it modulates power delivery via high-frequency Pulse Width Modulation (PWM), adjusting voltage levels to regulate speed and torque.

• Brushless DC (BLDC) Motor: Replaces traditional carbon brushes with electronic commutation. Alternating current fed through stator coils creates rotating magnetic fields that push against permanent neodymium magnets on the rotor, generating rotation.

2. Mechanical Sensing: Cadence vs. Torque

The controller requires continuous feedback to determine exactly how much current ($I$) to send to the motor windings.

Cadence and torque sensor components.Resource：VICTRIP EBike

• Cadence Sensors: Utilize a magnet ring mounted to the crank spindle alongside a stationary Hall-effect sensor. It reads pedal rotation speed (RPM). Power output functions as a threshold trigger based on rotation rather than rider effort.

• Torque Sensors: Employ precision strain gauges integrated directly into the bottom bracket axle. They measure torsional deflection caused by pedal force. The controller calculates instantaneous input torque ($\tau$) and scales motor power proportionally using the electrical power relation:

3. Delivering Force to the Road: Hub vs. Mid-Drive

Once electrical energy converts into rotational power, it reaches the road through one of two primary mechanical architecture configurations:

Direct-drive rear hub motor assembly. Resource：Ruslan Sitarchuk / Getty Images

When hydraulic brake levers are compressed, integrated reed switches or Hall sensors trigger an electronic motor cutoff circuit, overriding all throttle or pedal assistance to prioritize braking friction.