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Hydraulic Brakes
Anti-Lock Braking (ABS)

Anti-lock Braking System (ABS) — Operating Principles

Jump to:
1. What the System Does
2. How It Works – Step by Step
3. Key Components Involved
4. Common Misconceptions
5. Why This Matters

abs warning light red
Anchor 1

How it Works - Step by Step

An anti-lock braking system (ABS) prevents the wheels from locking during heavy braking. By automatically regulating brake pressure at each wheel, ABS allows the tyres to continue rotating while braking, maintaining steering control and improving stability. The system operates by keeping wheel slip within a range that maximises tyre-road friction.

How it Works - Step by Step

Anchor 2

  1. Braking input
    When the driver applies the brake pedal, hydraulic pressure is generated in the braking system as normal.

  2. Wheel speed monitoring
    Speed sensors at each wheel continuously measure rotational speed and transmit this data to the ABS control unit.

  3. Detection of impending lock-up
    If a wheel decelerates more rapidly than expected, the control unit identifies that it is approaching lock-up.

  4. Brake pressure modulation
    The hydraulic modulator momentarily reduces brake pressure at the affected wheel, preventing it from fully locking.

  5. Controlled wheel rotation
    With pressure reduced, the wheel continues to rotate rather than slide, maintaining a rolling contact with the road surface.

  6. Pressure reapplication
    Brake pressure is then reapplied. This cycle of pressure reduction and reapplication occurs many times per second.

  7. Slip control
    By repeatedly adjusting pressure, ABS maintains wheel slip near the range where tyre friction is highest, allowing effective deceleration while preserving steering control.

  8. Vehicle stability support
    By preventing individual wheels from locking, ABS reduces the risk of loss of control during braking, particularly on uneven or low-grip surfaces.

Anchor 3

Key Components Involved

  • Wheel speed sensors
    Measure the rotational speed of each wheel.

  • ABS control unit (ECU)
    Processes sensor data and determines when brake pressure must be adjusted.

  • Hydraulic modulator
    Contains valves and a pump that rapidly increase, hold, or reduce brake pressure.

  • Brake hydraulic circuit
    Transfers braking force from the master cylinder to the wheel brakes.

  • Electrical power supply and wiring
    Support sensor operation, control logic, and valve actuation.

Common Misconceptions

Anchor 4

  • “ABS always shortens stopping distance”
    ABS improves control and stability; stopping distance depends on surface conditions and tyre grip.

  • “ABS prevents all skidding”
    ABS prevents wheel lock, not loss of grip caused by very low friction surfaces.

  • “ABS replaces good driving technique”
    ABS assists braking but does not overcome excessive speed or poor road conditions.

  • “The brake pedal pulsing means a fault”
    Pedal pulsation during ABS operation is normal and indicates pressure modulation.

Why This Matters

Anchor 5

Steering control during braking depends on the tyres rolling rather than sliding. Locked wheels cannot generate the lateral forces required for steering. By maintaining controlled wheel rotation and optimising slip, ABS allows the driver to steer while braking and helps keep the vehicle stable during emergency stops and split-grip conditions.

Quick Reference

  • Primary function: Prevent wheel lock during braking

  • Control method: Hydraulic pressure modulation

  • Key principle: Slip ratio optimisation

  • Main benefit: Steering control and braking stability

Related Topics & Videos

Part of the Braking System​

 

→ Back to Braking Systems overview

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