Complete Guide to Disc Brakes and Drum Brakes
When it comes to driving safety, nothing is more critical than your tires and brakes. Here’s a guide to the two types of passenger-vehicle brakes, disc and drum. We explain how they work, how they’re different and alike, why you may have both types on the same vehicle, what kind of wear to expect and what parts will need maintenance.
Braking System Basics
Disc and drum brakes are both based on a hydraulic pressure system. Braking starts with a mechanical force — your foot pressing the brake pedal.
- A piston compresses brake fluid inside the master cylinder located under your vehicle’s hood near your engine. This creates a lot of hydraulic pressure, generating a much bigger force than that of the small effort of pressing down on the pedal.
- The pressure is transferred via the brake fluid through the brake lines then through brake hoses (flexible tubes) that connect the lines with brake assemblies at each wheel.
- There, wheel cylinders convert that hydraulic pressure back to mechanical force. Brake friction material is pushed against the brake disc or drum, slowing or stopping your vehicle.
Basics of Disc Brakes
Disc brakes are found on most vehicles today. They are mounted on the front axle and often the rear as well. To stop a wheel (and your car), a disc brake uses a caliper fitted with brake pads to grab a spinning disc, or rotor.
The caliper is an assembly mounted to the vehicle with a bracket so it frames the rotor. It looks and functions like a c-clamp. It contains:
- Brake pads: metal plates bonded with material that provides stopping friction.
- One or two pistons to push the brake pads against the rotor when you brake.
- A bleeder screw to allow for servicing the brakes and replacing the fluid.
- A rubber piston seal that prevents brake fluid leakage and retracts the piston when the brakes release.
- A dust boot to keep contaminants out of the cylinder.
- Anti-rattle clips that keep the brake pads stable.
The rotor is made of cast iron or a steel/cast iron composite. It’s attached to the wheel hub and turns with the wheel. It’s the surface the brake pads contact. When you step on the brakes, pressurized brake fluid pushes against the pistons inside the caliper, forcing the brake pads against the rotor. As the brake pads press against both sides of the disc, the friction stops the wheel’s rotation.
Rotors can either be solid or vented. Vented ones have more surface area and can more easily dissipate heat.
Two Types of Disc Brakes
There are two types of disc brakes, named after the type of brake caliper used: floating and fixed.
A floating caliper (also called sliding) is the most common type. It has one or two pistons. When the brakes are applied, the inner brake pad is forced against the disc while, at the same time, the caliper body moves closer to the rotor. This action forces the outer brake pad against the rotor.
The fixed caliper design has one or more pistons mounted on each side of the rotor. The caliper itself doesn’t budge: It’s rigidly fastened to a brake caliper bracket or the spindle. When the brakes are applied, only the caliper pistons move, pressing the brake pads against the disc.
Basics of Drum Brakes
Drum brakes are an older style of brake, not common on today’s vehicles. When they are used it is only on the rear axle.
They don’t use brake pads as the friction material. Instead of a caliper that clamps brake pads against a rotor, a drum brake system has a wheel cylinder with pistons that push brake shoes out against the inside of a spinning drum. This contact slows and stops the rotation of the brake drum and the wheel.
Which Is Better?
Although they both operate with the same basic hydraulics, the two types of brakes perform differently. Disc brakes are more efficient, provide better stopping power, dissipate heat easier and work better in wet conditions, all while being less complex.
Most of today’s vehicles have disc brakes at all four wheels. Some base models have disc on the front axle and drum on the rear, to keep costs down. In these models, why are disc put on the front and drum on the rear? It’s due to weight factors. A typical, unloaded vehicle is already about 10 percent heavier in front due to the engine. Then when you hit the brakes, the weight of the car transfers to the front. More braking power is needed there, making it a job for disc brakes.
Here’s more on how disc and drum brakes compare.
Stopping power. Disc brakes apply more braking force faster, resulting in shorter stopping distances.
Heat management. Since they are exposed to air, disc brakes cool better. Drum brake components aren’t as exposed to the air so they take more time to cool down after braking. This can cause brake fade, a loss of stopping power when friction material overheats.
Wet performance. Disc brakes perform better in wet conditions because they are open to the air and can sling water off easily. Plus, the rotors get dried by the pads dragging across them. When water gets inside a drum brake it tends to get trapped inside the drum, so it takes longer for the friction material to dry out.
Weight. Discs are lighter than drum brakes designed to apply the same force.
Emergency brake. A vehicle’s emergency brake is usually applied to the rear axle. This feature is easier to install on a drum brake than to a caliper or inside the hub of a disc brake rotor.
Cleaning. Disc brakes are self-cleaning. The brake pads “wipe” the rotor off when they’re engaged. Drum brakes are closed and are prone to brake dust collecting from the shoes, so they need periodic cleaning.
Repairs. Drum brakes have more hardware and can be more complex to service. But drum brake shoes and wheel cylinders typically cost less to replace than disc brake pads and calipers.
Since a lot of heat is generated by the braking system, plenty can go wrong. The act of braking converts kinetic (moving) energy of the vehicle into thermal energy (heat), subjecting many parts to very high temperatures.
This means a lot of wear and tear even in normal conditions. Some brake components will need to be replaced over the life of a vehicle. There’s no set interval for this since it depends on your driving style, climate and road conditions.
The solution is simply to get regular checks and replace pads, shoes and other components before braking is compromised or other parts get damaged.
Disc brake pads slow the rotor through friction and they wear with normal use. Eventually, they become too thin to function properly. Same thing for drum brake shoes. The friction material on the shoe gets worn out and braking is compromised.
These components should be inspected regularly. You don’t want to wait until pads/shoes wear down to the metal and grind against the rotor or drum.
Other items in the braking system are just as important to keep in good repair. Routine brake service should also include the following.
The brake system should be checked regularly for leaks and fluid should be replaced every few years (usually when the brakes are serviced). Any leak in the master cylinder, the brake fluid reservoir, the wheel cylinders, lines or hoses will reduce the hydraulic pressure that’s created when brakes are activated. Basically, the system can’t generate sufficient force needed to create braking power. You’ll notice you have to push your brake pedal a lot further in order to slow or stop.
Changing out brake fluid occasionally is also essential. This liquid is specifically formulated to prevent corrosion of the brake hydraulic components. But time and moisture contamination can damage its ability to do this important job.
Moisture that infiltrates the fluid will mix with the brake fluid, lowering the boiling point. Even though it resists evaporation, brake fluid will then be more likely to boil and turn into vapor when it gets hot. There will be less pressure in the hydraulic system, causing a low — possibly very low — brake pedal.
Along with moisture, it’s also very common for impurities like rust, road grit or brake dust to get into the fluid, causing internal damage to parts and reducing braking performance.
These rubber rings keep the hydraulic fluid from leaking and protect it from moisture and contaminants. They also cause the piston to return to its off position so the brake pads disengage properly when you release the brake pedal. If this doesn’t happen, you could experience brake drag and premature wear and the vehicle may pull to one side when you brake.
Brake lines are steel tubes that connect the master cylinder to the brake hoses. A spongy brake pedal could mean air has gotten into a line.
Brake hoses carry the hydraulic pressure from the brake lines to the wheel cylinders and calipers. The rubber brake hoses flex, allowing the wheel cylinders and calipers to move up and down with the wheels in relation to the vehicle's frame. If the rubber wears out, your vehicle may pull to one side during braking or you may even get fluid loss and brake failure. If there’s wear inside the hose, small rubber particles can restrict the flow of fluid, causing a brake pull or drag.
The rotor surface can thin unevenly from the brake pad not releasing, leaving the pad in contact even when the brake pedal isn’t activated. When this happens, you’ll experience shaking or wobbling in the steering wheel when you brake.
Brake components are constantly exposed to road debris and brake dust. The dust boot prevents grime from entering the caliper piston. If it fails and can’t do its job, piston damage can occur, causing brake drag, pulls and premature wear.
Failing master cylinders can leak internally. In this case, you may get a low or fading pedal without visible fluid loss. Regular fluid maintenance is important for prolonging cylinder life.
NOTE: There are different approaches to brake service. Get informed about why it’s important to maintain more than just the brake pads or drum brake shoes.
Disc and drum brakes are built differently, with somewhat different advantages. Your vehicle may have both or just disc brakes. Both work as part of the hydraulic brake system. This is a system that’s under high pressure, is subject to lots of heat and can be compromised by road grime, air, brake dust and moisture.
It’s important to get regular brake inspections to keep everything in proper working condition. Refer to your owner’s manual for a recommended schedule. Remember that funny brake sounds, smells or performance are indicators you need to get your vehicle to the shop right away.