Usually, we don't think much about our car's brakes. We step on the pedal and the car slows down or stops, and that's it. Consider, though, that a driver steps on the brakes tens of thousands of times in a year's worth of driving. Sometimes it's going to be a light tap on the pedal, and other times a hard stop.
Considering that brakes wear slowly and can last 40-60,000 miles according to your driving style, it can be hard to know when they're wearing down to a point where they need replacement. That's why it's important to have a good understanding of how your brake system is designed, and how it works.
Disc brakes are similar in principle to the handbrakes on a bicycle.
Your wheels are bolted to smooth steel discs, called rotors. The rotors then have a caliper that encloses them and holds two pads of hard ceramic or semi-metallic friction material. When you step on the brake pedal, hydraulic pressure from the brake master cylinder (located on the firewall in the engine compartment) travels through the brake lines and activates a piston in the brake calipers.
When the piston activates, it forces the brake pads against the rotor's surface, slowing the vehicle down via friction.
Drum brakes are an older system that has fallen out of favor for new vehicles since disc brakes are simpler in design with fewer parts and also offer better braking performance. Still, there are plenty of vehicles on the road that still uses drum brakes, especially for the rear wheels.
Drum brake systems use a pan-shaped drum, with the wheel studs passing through the drum. Inside the drum is a pair of curved brake shoes that fit the contour of the drum's lining. When you step on the brake pedal, a hydraulic wheel cylinder forces the shoes outward against the drum's lining, and they then retract again with stiff return springs.
Since the 90s, antilock brakes have been a feature on most cars and are now standard equipment on many. ABS systems were developed for aircraft and are designed to enhance control by preventing lockup and skidding. ABS systems include sensors that monitor the rotational speed of each wheel, sending several pulses per second to a processor. When any wheel starts to slow down more than the rest and seems like it could be on the verge of locking up, the processor and a series of pumps and valves meter the braking force differently to the wheel(s), heading off the likelihood of a skid.
you take the time to think about it, it’s pretty amazing that we can stop the forward motion of a four thousand pound car by just pressing on a pedal with one foot. Of all the things we ask of our vehicles, please stop is the most important request of all. So how is it possible that so little effort can generate so many results? The answer is hydraulics.
Hydraulics uses pressurized liquid to create power. This works because liquids cannot be compressed. So if you try to compress a liquid that is inside a sealed container, it will push out on all sides of the container, looking for a place to go.
Think of squeezing a plastic water bottle. If the cap is removed the water will shoot out of the top. If the cap is tightly secured, you will only be able to squeeze the bottle so far. Since water doesn’t compress, hydraulics stops you from collapsing the bottle. If you leave the cap loose and squeeze the bottle real hard, hydraulic pressure will blow the cap right off the bottle.
So when you step on your brake pedal, you are pressurizing the entire brake system, applying the brakes.
The end game of all this hydraulic pressure is kinetic energy. Kinetic energy is created by friction, and friction is what stops the vehicle. All four wheels of your car are equipped with either a brake rotor or a brake drum. The rotor or drum is attached to, and spin with the wheels.
When the rotor or drum slows down, so does the wheel. So when you apply your brakes, high-temperature resistant brake shoes or brake pads press against the rotor or drum creating friction, slowing down, and eventually stopping your car.
On drum brakes, the shoes are actuated by hydraulic wheel cylinders. Disc brakes use hydraulic calipers. Wheel cylinders and calipers are the sealed containers that we talked about earlier.
They are filled with brake fluid. When the fluid is pressurized, it pushes out on all sides of the container, forcing out a piston that engages the pads or shoes. Much like squeezing the water bottle caused the loose cap to pop off. The pads or shoes press against the drum or rotor, stopping the car.
The force that pressurizes the wheel cylinders and calipers comes from the master cylinder. The force that pressurizes the master cylinder comes from the brake pedal. Which makes your foot the initial force that puts this whole chain of events into motion.
The master cylinder is another sealed container filled with brake fluid. When you push the brake pedal you are pushing a piston up against the fluid, pressurizing it. This pressure is forced out of the master cylinder through the brake lines, much like when we squeezed the bottle with cap removed. Some sections of the brake lines are steel, other sections are rubber. The lines attach to the wheel cylinder and calipers. So by pressurizing the master cylinder, the whole brake system is pressurized.
When the brake pedal is released the piston pulls back in the master cylinder, relieving pressure. This allows the pistons in the wheel cylinders and calipers to pull back, releasing the brakes.
Here’s the difference between a drum and disc brakes. A brake drum is shaped kind of like a bowl and spins with the wheel. The brake shoes are located inside the drum and are pushed out against the drum by the wheel cylinders.
Disc brakes use rotors which are heavy metal discs, usually around an inch thick. They too spin with the wheels. The brake pads are attached to the caliper, one on each side of the rotor. So when the caliper is pressurized, the pads clamp down on the rotors from the outside.
Either way, it’s friction causing kinetic energy, stopping the car.
We tend not to think much about our brakes till they stop working. It’s easy to take for granted that the car will stop every time we push on the brake pedal. If you’ve ever had brakes fail, you’d never feel that way again. Having the brake pedal go straight to the floor without even slowing the car down is an experience that you will never forget.
Unusual brake noises or a low brake pedal are your brakes giving you a heads up that something bad could happen. So heed the warning signs.
Your car's brake fluid is hygroscopic, meaning it can attract water. Unlike oil, which is heavier and will separate from water, brake fluid can support it, leading to problems with corrosion and damage to rubber seals. In extreme cases, droplets of water in the brake fluid can "boil off," leading to brake fade during hard braking.
Check your brake fluid level and condition regularly. Brake fluid should be light amber and clear in color. If your brake fluid is cloudy or dark, you'll need to flush the system completely, refill it with the correct brand and rating of fluid and then bleed the system of any air bubbles. From time to time, scrape a screwdriver blade along the bottom of the brake fluid reservoir in the master cylinder. If the screwdriver has any gunk on it when you pull it out, that's a sign that the fluid's contaminated.
Since your brakes wear down so slowly, it can be tough to recognize when it's time for brake service. These are all signs of worn brakes, however:
It's also important to listen to any noises the brakes might make. Groaning, screeching, or squealing noises are all trouble signs. A metallic screech is especially worrisome - many brake pads are designed with a steel tab that protrudes from the backing plate and drags on the rotor when the pads' friction material reaches a minimum thickness. This wear indicator is designed to alert the driver that it's time to change pads.
On other pad designs that don't feature a wear indicator, a metal-to-metal grinding noise indicates that the friction surface is worn clear through to the rivets and backing plate. In that case, the only braking ability you have is coming from metal-to-metal contact, which is obviously very dangerous and will quickly destroy the smooth surface of the brake rotors.
Another troubling sign is a brake pedal that slowly goes to the floor while you're holding the car at a stop, in gear. Power brakes rely on engine vacuum to multiply braking effort when you step on the pedal. If you find your brake pedal sinking or if it's suddenly as hard as stepping on a rock, chances are your power brake booster is on its way out or there's a vacuum leak between the booster and engine.