A Little Bit About Automotive Brake Systems...

The cars we know today have been refined tremendously over the years. We have gone from the early days of steam-powered, self-propelled vehicles to the gasoline and/or "hybrid" engines of modern vehicles. Over this time, cars have become ever more reliable and dependable - so much so that most people have forgotten to appreciate the technology behind their driving wheels - out of sight, out of mind so to speak. The same thing applies to the brake system that our lives depend on every moment we are on the road. Most people's knowledge of brake system is limited to the single brake pedal that they step on to slow down or stop their cars.

The information that follows provides some information into what happens when you step on the brake pedal in your car.

Have you ever wondered how, with relatively little energy, your foot can bring a car weighing more than 1 ton roaring down the highway at 110-km/hr (a law abiding Malaysian driver on the PLUS highway!) to a screeching stop? The answer is through the physics principle of Leverage (mechanical advantage) and Pascal's Law (hydraulic force multiplication). Pascal's Law states that "a change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container." Simply put, the principles above allow a cleverly designed brake fluid circuit (to transmit brake fluid through brake hoses and pistons, to convert a 10-kg force stepping the brake pedal down 3-cm, into a whopping 1,000-kg force or more acting on the brake disc at the wheels! Of course, the catch is the brake piston can only extend a few millimeters…

Figure 1. Leverage and Pascal's Law on Brake System
Figure 2. Typical Non-ABS Brake System

A typical brake system consists of:

  1. Brake Pedal - uses leverage to transfer the multiplied effort from the driver's foot to the master cylinder.
  2. Master Cylinder - located under the hood connected directly to the brake pedal, acts as a holding tank for brake fluid until it is needed. When the brake pedal is depressed, the master cylinder forces fluid to each of the vehicle's wheels.
  3. Metal Brake Lines and Flexible Brake Hoses - connects the master cylinder to the wheel or slave cylinder located at each wheel.
  4. Brake Fluid - to transmit pressure and motion in the brake system. Due to the intense heat generated by components of the brake system, brake fluid must have a high boiling point. Brake fluid must also have a low freezing (melting) point to ensure proper operation at any low temperatures that the vehicle may encounter. 2 types of brake fluids are available, namely glycol based (DOT-3 and 4) and silicone based (DOT-5); rated by the Department of Transportation (DOT), USA based on their respective boiling points.
  5. Brake Booster - available in the power-assisted brake system that uses the engine's energy to add pressure to the master cylinder.
  6. Disc Brake - uses a clamping action to produce friction between the brake disc and the brake pads mounted in the caliper attached to the suspension members. When the brake pedal is pushed, brake fluid from the master cylinder compresses the brake pads against the brake discs. The friction between the stationary brake pads and the revolving brake disc causes the brake discs and wheel to slow and stop. Disc brakes work using much the same basic principle as the brakes on a bicycle; as the caliper pinches the wheel with pads on both sides, it slows the bicycle.
  7. Drum Brake - brake drum is a heavy flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub. The inside surface of the drum is acted upon by the linings of the brake shoes. When the brake pedal is pushed, pressure from the master cylinder causes the wheel cylinder to push the brake shoes against the brake drums which are attached to the vehicle's wheels. The friction between the stationary shoes and the revolving drums causes the drums to slow and stop the wheels.
Figure 3. Disc Brake Assembly
Figure 3. Drum Brake Assembly

In summary, a brake system is essentially an energy conversion device that converts kinetic energy (car in motion possessing momentum) to potential energy (car stopping) by dissipating heat and noise to the surrounding air. The friction surfaces of the brake pads on a disc brake, or the brake shoes on a drum brake convert the forward motion of the vehicle into heat. Heat is what causes the friction surfaces (linings) of the pads and shoes to eventually wear out and require replacement. When it is time to replace brake pads or brake shoes, there will generally be an audible squealing noise during braking.

As mentioned earlier, brake systems are often neglected by most drivers. In actuality, we should pay the most attention to our brake systems as our safety on the road very much depends on the proper functioning of the brake system. At a minimum, the brake system should be inspected by a qualified mechanic once a year for pad or lining wear (to check if replacement pads or linings are necessary), brake fluid leakage, brake disc thickness variation, broken seals and other wear and tear.

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