Auto Questions: How Brakes Work

This brief summary will help you understan dyour braking system.

The braking system on a car is easily the most important safety feature. Over the years, these systems have been developed and refined by manufacturers to make them more dependable and effective in everyday driving. But how does the brake system work and enable a driver to press a pedal and retard a vehicle's speed?

The most common brake system used on modern passenger cars employs disk brakes on the front wheels and either drum or disk brakes on the rear. The disk or drum brakes are connected to the master cylinder through a closed network of hoses and lines usually constructed of reinforced rubber.

This closed network of lines forms a hydraulic system and is filled with brake fluid. When the brake pedal is depressed by the driver, a plunger or piston in the master cylinder is pushed down on, causing an increase in pressure throughout the entire hydraulic system as brake fluid (or any fluid for that matter) is not easily compressed. This is also the reason why the brake fluids in cars should be pure, as air bubbles and other impurities can be more easily compressed reducing the effectiveness and efficiency of the system.

As this hydraulic pressure created by the depressed brake pedal is transferred through the entire system of brake lines, it culminates at the braking unit located at each wheel. These braking units work by causing the brake pads/shoes to squeeze against the disc or drum when pressure is applied by a piston(s) at the end of the brake lines at each wheel. As the brake pads/shoes are constructed out of a very hard and rough material, the car's speed is retarded as the pads/shoes create friction when dragged against the disk/drum. The result of this friction between the pad/shoe and disk/drum is the generation of heat which is the form of energy that the speed of the car is converted into.

This friction produced between the pads/shoes and the disk/drum is what causes these braking components to wear down over time and eventually need replacement. Excessive heat under continuous and prolonged braking (e.g. driving down a steep mountain road) is what causes 'brake fade' or the overheating of the braking system resulting in its efficiency being diminished. As brake fluid is a liquid, it has a boiling point (in most brake fluids it is around 350 Fahrenheit) at which point the liquid expands and turns into a gas. As the brake fluid approaches this boiling point, it starts to expand rapidly, and as the braking system is mostly connected through a series a rubber hoses, these lines start to expand as well. In the end, pressure applied to the braking system by the pedal/master cylinder only causes the lines to expand or 'balloon' even more, rather than transferring that pressure to the braking unit at each wheel.

During panic stops, when the driver depresses the brake pedal as far as it will go and pressure in the hydraulic system is at its highest, the brake pads/shoes can actually lock up against the disk/drum. When lock up occurs, the tires skid over the road as they can no longer roll. This skidding causes a loss of steering input (i.e. control) and increases the braking distance of the car significantly. One system employed on modern cars is called ABS (anti lock brakes) and this lock up is prevented as the pressure applied throughout the system is quickly pulsated when lock up is detected, causing the pads/shoes to release the disk/drums momentarily. This rapid release allows the wheels to continue to roll, thus maintaining control and keeping braking distances as short as possible.

This is in essence the braking system that you will find on any modern car, but some manufacturers of high performance cars have made improvements such as using stainless steel braking lines instead of the common rubber hoses as this material better resists expansion at high temperature.

© High Speed Ventures 2011