Drum Brakes vs Disc Brakes: What’s the Difference?
With all the advancements allowing vehicles to speed up, manufacturers have had to develop better ways to slow them down. Today’s cars and trucks are designed with different brake configurations, but what’s the difference when it comes to drum brakes vs disc brakes, and how does a manufacturer decide the best fit?
The “Brake” Down
Both disc and drum brakes operate by converting kinetic energy into heat through friction. They each rely on the pressure of a slowly degradable material, in the form of shoes or brake pads, applied to a metal component attached to the wheels. This last component is what gives its name to the brake type — drum or disc — and the rate at which it’s able to get rid of the friction-caused heat determines that brake type’s efficiency.
Drum brakes are composed of three key parts — slave cylinder (sometimes also called the wheel cylinder), shoes and drum. When the brake pedal is depressed, fluid is directed to the slave cylinder, positioned between two curved brake shoes. As the cylinder expands, it pushes the shoes outward against the inside of the spinning drum. The drum is attached to the wheels, so when enough force is applied to the drum, the whole car stops. The amount of force needed to stop a moving vehicle depends on several variables like size and speed, but as you can imagine, this sequence of events produces a lot of heat.
When heat buildup is more than friction components can handle, you get a condition known as brake fade, and that’s not a place you want to be. Also, because heat causes metal to expand, too much heat will expand the drum itself, meaning the shoes have to travel even further to stop it, greatly reducing efficiency. The upside to drum brakes is that they’re cheaper to manufacture (a benefit passed on to the consumer) and easy to service. Generally all that’s needed is to pluck out a few springs and replace the shoes — a task that a pair of skilled hands could perform in a matter of minutes.
Disc brakes also have three key components: calipers, pads and rotors. Rotors, like drums, are attached to the wheels, but where drums act as a kind of enclosure around the assembly, rotors are flat (hence “disc” brakes) and spin in the open air. Calipers hug, but aren’t attached to, the rotor and contain inward-facing pistons. When the brake pedal is depressed, fluid fills the caliper and the piston shoots out to apply pressure to the rotor via a pair of brake pads. The great advantage of disc brakes vs. drum brakes is their ability to dissipate heat. Since rotors are exposed, they cool as they spin. The centrifugal force of the spinning rotor is also better at removing water.
Disc brakes’ superior braking ability gives them a clear win over drums in terms of performance, but many manufacturers continue incorporating them because of price. Luckily, this does not come at the cost of safety in normal driving applications. Drum brakes themselves have come a long way through advancements in design and materials used. Also, placement is key: the majority of work involved in stopping a vehicle is done by the front axle, so drums are often used on rear wheels only, as a kind of compromise between function and affordability.
For more information on drum brakes vs disc brakes, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Wikimedia Commons