Chapter 4 — Brake systems: discs, drums, ABS, ESC and AEB

Modern braking systems are designed to convert kinetic energy into heat through friction. While the interface between the driver and the road begins at the brake pedal, the process relies on Pascal’s Law: pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid. In a UK-specification car, this hydraulic system is split into two independent circuits. This redundancy, mandated by European safety standards and preserved in UK law, ensures that if a leak occurs in one line, at least two wheels—usually in a diagonal configuration—retain braking force.

01The Mechanics of Friction: Discs and Drums

Most modern passenger vehicles use disc brakes on the front axle, where the majority of braking force is required due to weight transfer during deceleration. A hydraulic piston pushes sacrificial friction material (the pads) against a cast-iron rotor (the disc). Higher-performance vehicles may use ventilated discs with internal vanes to dissipate heat more rapidly, reducing the risk of 'brake fade'—a condition where high temperatures cause the bonding resins in the pads to outgas, creating a microscopic film of gas that prevents solid contact.

Drum brakes are now largely relegated to the rear axles of smaller hatchbacks or EVs. They operate by pushing two semi-circular 'shoes' outwards against the inner surface of a rotating drum. While less efficient at dissipating heat than discs, they are cheaper to manufacture and provide a highly effective mechanical parking brake. However, drums are more susceptible to 'water fade' if submerged, and the enclosed design tends to collect brake dust and debris, which can lead to sticking components over time.

02Anti-lock Braking Systems (ABS)

The ABS is not designed to shorten stopping distances in ideal conditions; rather, its primary purpose is to maintain steering control during emergency deceleration. When a wheel locks, it loses lateral grip, meaning the driver can no longer steer the vehicle. ABS uses inductive proximity sensors—often called tone wheels—at each hub to monitor rotational speed. If the Electronic Control Unit (ECU) detects a wheel is about to stop rotating while the vehicle is still in motion, it triggers a series of solenoid valves.

• The pressure in that specific brake line is momentarily reduced.
• The valve then holds the pressure to prevent further locking.
• Finally, the pressure is increased again to maximise braking force.

This cycle happens up to 15 times per second. Drivers will feel this as a distinct pulsing through the brake pedal. It is a common MOT failure point; any illumination of the ABS warning light results in an immediate 'Major' defect, as the system is integral to the vehicle's safety type-approval.

03Electronic Stability Control (ESC) and Traction Control

Building upon the ABS hardware, Electronic Stability Control (ESC)—also known by proprietary names like ESP or DSC—is perhaps the most significant safety advancement since the seatbelt. It introduces a yaw sensor and a steering angle sensor to the mix. These sensors allow the car to compare the driver’s intended direction with the vehicle’s actual path. If the car understeers (ploughs straight on) or oversteers (the rear slides out), the ESC system can automatically apply braking force to individual wheels to pivot the car back onto the intended line.

Under Section 11 of the Road Vehicles (Construction and Use) Regulations, these systems must be maintained in good working order. ESC is particularly effective at preventing 'run-off-road' accidents and rollovers. It works in tandem with Traction Control, which prevents wheelspin during acceleration by either braking the spinning wheel or requesting the engine ECU to reduce torque output.

04Autonomous Emergency Braking (AEB) and EBA

Autonomous Emergency Braking (AEB) represents the shift from passive safety to active intervention. Using a combination of radar, LIDAR, or forward-facing cameras, the car monitors the distance and closing speed to the vehicle or pedestrian ahead. If the system calculates an imminent collision and the driver fails to react, the AEB system will automatically apply the brakes. Most systems function in two stages: an audible and visual warning, followed by full hydraulic application if the collision remains likely.

• Forward Collision Warning (FCW): The initial alert to the driver.
• Emergency Brake Assist (EBA): If a driver hits the brakes but lacks the force required, the system provides maximum pressure instantly.
• Full Autonomous Braking: The car takes over entirely to mitigate or avoid the impact.

While AEB significantly reduces low-speed rear-end shunts, it is not infallible. Reflections, heavy rain, or thick fog can 'blind' the sensors, and the system may occasionally trigger 'phantom braking' if it detects a bridge or a metallic sign as a stationary hazard.

05Component Failure and Maintenance

Braking systems fail in distinct ways. Hydraulic failure is usually indicated by a 'spongy' pedal, often caused by air in the system or aged brake fluid. Brake fluid is hygroscopic, meaning it absorbs moisture from the atmosphere. Over time, this lowers the fluid's boiling point, which can lead to total brake failure during heavy use, such as descending a steep hill. This is why most manufacturers recommend a fluid flush every two years.

Mechanical failures include seized callipers, often caused by road salt corrosion in the UK, and 'warped' discs, which manifest as a vibration through the steering wheel. Electronic failures are usually sensor-based; a cracked ABS ring or a fouled sensor will disable the entire electronic suite, reverting the car to 'base' braking. In this state, the car will stop, but it will lack the anti-skid and stability protections of a fully functioning modern vehicle.

Understanding these systems reveals that while the hardware provides the stopping power, the electronics provide the stability. Regular inspection of pad thickness and fluid clarity remains the most effective way to ensure these safety-critical systems function as intended.