For most cars, drum brakes work in conjunction with disc brakes to stop a car. Although disc brakes are a more efficient means of braking, drum brakes are less expensive to manufacture. Car makers install drum brakes in the rear of most cars due in part because of their low cost relative to disc brakes, but also because they work well as a parking brake. High end cars may come with stock disc brakes in the rear, but they are usually added as an aftermarket part. Comparing the two types of brakes, you will see that there are pros and cons to both.
Drum brakes are named because of the round drum that comprises part of the wheel housing. Although open from the backside, from the front the large cylinder resembles a drum. The way they work is as follows: When the brake pedal is applied, pressurized brake fluid enters into the individual wheel cylinder in each of the two drums. This fluid pushes two brake pistons out which in turn push two brake shoes. The shoes are pushed up against the interior of the drum. An adjuster arm opposite the wheel cylinder lets the shoes rotate slightly as pressure is exerted. This creates better stopping power and allows the shoes to wear evenly on the inside of the drum.
Disc brakes, by comparison, consist of flat metal rotor that surrounds the wheel housing and rotates with the wheel. When the brakes are applied, the pressure forces brake calipers to clamp down on two brake pads, one on either side of the disc.
The Pros and Cons
In general, it is thought that disc brakes are a better means of stopping the car than drum brakes. This is due to three primary reasons. The heat energy that is transferred to the brakes better dissipates with discs, brake fade occurs more slowly with disc brakes and they tend to stay drier in wet weather. Drum brakes tend to get hotter with use, they lose their effective ability to stop the car faster and water can gather on the drum’s interior between the lining and the shoes.
Drum brakes, on the other hand, function well as a parking brake. A mechanism is built right into drum brakes that activates them when the parking brake lever is pulled. On disc brakes, an additional device must be installed to accommodate a parking brake. The other benefit is the low cost of drum brakes. Cars fitted with disc brakes on all tires will cost more off the lot, so the use of drum brakes in the rear equals savings conferred to the consumer. When having brake work done, the replacement of drums or shoes is less expensive than that of calipers or discs. Front brake pads are a relatively inexpensive fix, but the other two main components up front can get pricey.
It is not really a matter of drum brakes and disc brakes competing with one another. It is agreed that disc brakes stop cars better. For high performance vehicles, the ability to slow down and stop from high speeds is vital. For the average car, though, the combination of drums and discs is a more than adequate means of braking.