Drum Brakes vs. Disc Brakes: What’s the Difference?

Drum brakes were the standard before the 1970s, but disc brakes are today’s most common braking system. Most vehicles have a set of disc brakes in front, if not on all four wheels. But folks still ask the drum brakes vs. disc brakes question, half a century later. Lets dive into the basics to find the answer.

Drum brakes vs. disc brakes – components

Traditional Drum Brakes Defined

Drum brakes include a pan (or drum) that fits over the wheel studs. When the brake pedal is pressed, hydraulic pressure causes two curved brake shoes to press against the inside of the drum. The resulting friction slows the car down.

Some small compact cars still use drum brakes, generally on the rear axle, while discs are used on the front axle. That’s because the front brakes get the most workout when braking, so some carmakers make do with the rear drum brakes. Utility trailers and other haulers often use drum brakes as well. They cost less, and manufacturers can easily design a brake system compatible with most tow vehicles.

Drum brakes consist of several parts:

Backing plate: This forms the base for all the other parts and rests against the wheel studs. The other brake components are connected to the backing plate.

Brake drum: The cast-iron drum rotates with the car’s axle. The metal wears away slightly as the brake shoes press against the drum. Over time, the drum will lose its perfect circular shape and must be turned on a lathe to restore its “true” form.

Brake shoe: These curved semi-circular metal pieces have friction material applied to the outer edge. When the shoes press against the brake drum, the friction material pushes against the brake drum causing the vehicle to slow.

Brake springs: The shoes have springs attached to them so that when the hydraulic brake pressure is released, the brake shoes retract from the brake drum and stop the braking effort.

Wheel cylinder: The cylinder has two pistons connected to the hydraulic brake fluid line. When the brake is activated, the pistons press against the brake shoes to force them against the brake drum. Over time and with use, the brake cylinders are prone to leaking.

Brakes and Hydraulic Fluid

Disc and drum brakes both rely on hydraulic fluid. When the brake pedal is depressed, hydraulic fluid is compressed, generating more pressure in the line. That additional pressure causes the brake shoes in a drum brake system to compress against the inside of the drum, slowing the vehicle. In a disc brake system, the increased brake fluid pressure causes the brake pad to squeeze against the rotor and slow the car. As a result, the brake fluid needs to be replaced every two years.

Drum Brake Limits

There are three safety issues with drum brakes. First, when drum brakes are applied, heat builds up quickly inside the drum. Since the drum encloses all the brake parts, this heat builds up rapidly when brakes are used repeatedly. As the heat builds up, the brake lining on the brake shoes loses efficiency. Gripping power is critical on any long and steep downhill stretch. Drum brakes can turn a long descent into a high-speed nightmare.

Second, drum brakes and wet weather don’t mix well. Drum brake shoes are less effective in rain and snow. Also, moisture can get trapped in the drum, resulting in rust and corrosion of moving parts over time.

Third, drum brakes are prone to maintenance issues. That’s because drum brakes have many complex, moving parts, including pistons, levers, gears, hydraulic wheel cylinders, and springs.

This complexity makes replacing brake shoes an unpleasant experience. Springs pop out. Other parts stick together and refuse to budge.

When first introduced, drum brakes required routine maintenance to adjust the brake shoes as they wore down. Then, in the 1950s, self-adjusting drum brakes were developed, eliminating the need for mechanics to adjust the brakes manually.

Disc Brakes Defined

Installing a new slotted rotor

Disc brakes are so standard that you rarely see a four-wheel drum brake setup. Some restored classic cars fake a drum brake setup for aesthetics but secretly run concealed disc brakes.

Disc brakes generally stop a vehicle quicker—typically by 20 to 30 percent. In addition, disc brakes are often integrated more easily into driving systems, such as anti-lock and adaptive braking.

A thin circular rotor is attached to the axle in a vehicle equipped with disc brakes, and a caliper (like a clamp) mounts over the rotor. When the brake pedal is engaged, the caliper’s pads squeeze the rotor, slowing the car.

These are the standard components in a disc brake system:

Brake rotors: The metal disc rotates with the axle and comes in various designs. The solid disc works well, but often, to upgrade performance, enthusiasts will drill holes in the disc or add slots to increase heat dispersal. The disc is commonly referred to as a rotor. Over time the disc will lose metal material and need to be replaced.

Brake pads: The brake pads consist of a friction material and a metal base. When the brakes are activated, the pistons push the brake pad and the friction material against the disc, slowing the car. The pads wear over time and can be easily replaced.

Calipers: The caliper acts as a clamp on the disc and consists of brake pads and pistons. It houses the moving parts responsible for the braking of the car.

Pistons: The pistons move when brake pressure is applied and cause the brake pads to push against the disc. Many systems will have one piston per disc, but performance models might have up to six or more pistons per cylinder. These extra pistons put more even pressure on the brake pads to slow the vehicle faster.

Drum Brake to Disc Brake Conversion

Drum brakes take longer to stop and do not perform as well as a disc brake setup. But they get the job done. You will be okay if the drum braking system is regularly maintained with new brake pads and regular fluid changes. However, if you want to improve braking performance, a conversion to disc brakes is your best option.

1967-1970 Ford Mustang power front disc brake conversion kit from Tomsclsssic

When it comes to the long standing question, drum brakes vs. disc brakes, the answer is clear. Disc brakes have become so common that many firms offer conversion kits to swap a drum brake to disc brakes quickly.