The brakes on modern vehicles are truly great engineering accomplishments. They are designed to operate under a wide range of tolerances, they are often the most neglected maintenance item, and they are designed to just-plain last longer than all other vehicle systems. Long after the transmission is toast, the engine is blown and the wheels fall off, it is likely that the calipers and shoes of the braking system will still be holding their respective rotors and wheel cylinders in place. So just how do brakes work?
Keep in mind that this article is about passenger car and truck brakes, not those found in freight trucks or other automobiles.
The principle behind braking is simple – by using friction between surfaces, the wheels of a car are slowed or stopped. But the linking of the two ends of the braking system from the brake pedal to the wheels is a different story. Let’s start at the pedal and end at the wheel.
When you press the brake pedal, a linked rod and piston are pushed into a device called a master cylinder. However, this can be very difficult if the vehicle is operating at high speeds. Modern vehicles have a brake booster between the pedal and master cylinder which uses vacuum pressure from the engine to assist in pressing the brake pedal. The brake booster uses vacuum to multiply the force that the driver’s foot places on the pedal and applies that force to the master cylinder piston. Older vehicles which lacked “power brakes” required the driver to come up with this force.
The master cylinder is under the hood, often directly in front of the driver and is below the brake fluid reservoir. When the piston is pressed, brake fluid is forced from the reservoir into the brake lines, one for each wheel. The master cylinder creates redundancy in the brake design by separating itself into two chambers, one for each pair of wheels. If a brake line leak on one cylinder threatens a vehicle’s ability to slow, the other chamber will still likely be operational and enough for an emergency stop.
Brake fluid is a hydraulic fluid which transfers force from the master cylinder to the brakes, causing them to operate. Because the fluid is not very compressible (the molecules cannot be pushed closer together under the force applied by braking), it transfers force very well. Also, the fluid is able to withstand the very hot temperatures that result from hydraulic action and the subfreezing temperatures experienced in cold climates. Keep in mind that brake fluid is both hygroscopic and corrosive – it will absorb water from the atmosphere over time and eat paint. Brake fluid that has absorbed too much water will steam under high temperatures and compress and must be replaced.
Once the pressurized brake fluid makes its way to the brakes, it is injected into either one of two systems – drum brakes or disc brakes. Drum brakes are operated when a shoe inside a wheel cylinder pushes outward against the cylinder, causing friction and slowing. Most drum brakes are used on the rear wheels of trucks and some cars. Disc brakes work as a caliper (imagine a vice-grip) squeezes down on a rotor which is connected to the wheel assembly. The caliper piston is forced inward by the hydraulic pressure and causes two brake pads, one on either side of the rotor, to squeeze against the rotor. The result is friction and a slowing of the vehicle.
While brake systems are not incredibly difficult to understand, they arguably perform the most vital safety function on a vehicle. And they do this in spite of high speeds, careless drivers and poor maintenance. They have built in redundancy and use a fluid and parts which last many years without assistance, all for the sake of safety despite carelessness. The next time you brake to avoid a collision, think about how great those brakes really are.