Brakes: Drum vs. Disc

Disc brakes and drum brakes are the two primary brake systems used in today's automobiles. Both braking systems are different in operation and design, and both have their strengths and flaws. What began in the '60s as a serious attempt to provide adequate braking for performance cars has ended in an industry in which brakes range from supremely adequate to downright phenomenal. The introduction of components like carbon fiber, sintered metal and lightweight steel, along with the adoption of antilock technology, has together contributed to reduced stopping distances and generally safer vehicles.

One of the first steps taken to improve braking came in the early '70s when manufacturers, on a widespread scale, switched from drum to disc brakes. Since the majority of a vehicle's stopping power is contained in the front wheels, only the front brakes were upgraded to disc during much of this period. Since then, many manufacturers have adopted four-wheel disc brakes on their high-end and performance models as well as their low-line economy cars. Occasionally, however, a manufacturer will revert from a four-wheel disc setup to drum brakes for the rear of the car to cut both production costs and purchase price. New cars like the Volkswagen ID.4 EV have a rear drum setup because most modern electric cars rely on strong regenerative braking, meaning super-strong mechanical stopping force isn't required.

Friction and heat

Before you can appreciate the difference between drum and disc brakes, you have to understand the common principles that both systems use when stopping a car: friction and heat. By applying resistance, or friction, to a turning wheel, a vehicle's brakes cause the wheel to slow down and eventually stop, creating heat as a byproduct. The rate at which a wheel can be slowed depends on several factors including vehicle weight, braking force and total braking surface area. It also depends heavily on how well a brake system converts wheel movement into heat (by way of friction) and, subsequently, how quickly this heat is removed from the brake components. This is where the difference between drum brakes and disc brakes becomes pronounced.

Drum brakes

Early automotive brake systems used a drum design at all four wheels. They were called drum brakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the brake shoes, while the shoes themselves were made of a heat-resistant friction material similar to that used on clutch plates.

This basic design proved capable under most circumstances, but it had one major flaw. Under high-effort braking conditions, like descending a steep hill with a heavy load or repeated high-speed slowdowns, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat buildup within the drum. Remember that the principle of braking involves turning kinetic energy (wheel movement) into thermal energy (heat). For this reason, drum brakes can only operate as long as they can absorb the heat generated by slowing a vehicle's wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disconcerting to the driver.

In today's automotive market, many hybrid and electric vehicles come with regenerative braking systems. This new technology uses the car's electric motor to slow it down. This reduction in speed means that there's less overall braking force required, so using drum brakes at the rear, along with front disc brakes, is enough.

Disc brakes

Though disc brakes rely on the same basic principles to slow a vehicle (friction and heat), their design is far superior to that of drum brakes. Instead of housing the major components within a metal drum, disc brakes use a slim rotor and small caliper to halt wheel movement. Within the caliper are two brake pads, one on each side of the rotor, that clamp together when the brake pedal is pressed. Once again, fluid is used to transfer the movement of the brake pedal into the movement of the brake pads.

But unlike drum brakes, which allow heat to build up inside the drum during heavy braking, the rotor used in disc brakes is fully exposed to outside air. This exposure works to constantly cool the rotor, greatly reducing its tendency to overheat or cause fading. Not surprisingly, it was in racing that the weaknesses of drum brakes and the strengths of disc brakes were first illustrated. Racers with disc brake systems could carry their speed "deeper" into a corner and apply greater braking force at the last possible second without overheating the components. Eventually, as with so many other automotive advances, this technology filtered down to the cars driven by everyday people on public roads.

Disc brake systems are considered more effective and practical but come at a higher cost. Even though disc brakes have fewer components than drum brakes, they usually cost more to repair and service. Much of the difference in cost concerns the manufacturing of the parts and the overall materials used in both braking systems.

Drum brakes and disc brakes today

In today's automotive pantheon, it's common to find four-wheel disc brakes as standard equipment. There are still new vehicles, however, that continue to use a front-disc/rear-drum brake setup. What does this say about the current state of braking systems? Are these manufacturers sacrificing vehicle safety in order to save a few bucks by installing disc brakes on only the front wheels?

While a "yes" answer would certainly be great for increasing Town Hall traffic, the truth is that today's disc/drum setups are completely adequate for some new cars. Remember that both disc and drum brake designs have been vastly improved in the last 20 years. In fact, the rear drum brake systems on today's cars would provide better stopping performance than the front disc setups of the '70s. And today's front disc brakes are truly exceptional in terms of stopping power. Combined with the fact that between 60% and 90% of a vehicle's stopping power comes from the front wheels, it's clear that a well-designed, modern drum brake is all that's required for most rear-wheel brake duty.