All brake types tend to use the same basic principle: a flat-surfaced component is pressed against a spinning flat part of the wheel in motion, causing friction and slowing it down. Both drum and disc brakes work in this manner, with brake shoes pressing against a flat metal portion of the wheel to slow or stop its motion.
While disc brakes involve the shoe pressing against a rotor, drum braking involves the shoe pressing against an enclosed structure called a drum. A lot of cars feature disc brakes on the front tyres and drum brakes on the rear.
Drum brakes tend to be harder to service than disc brakes, as they have more parts and components. On the plus side, they are less expensive to manufacture than disc type brakes and incorporate easily within emergency brake lever mechanics.
The Anatomy of a Drum Brake System
Just like disc brakes, at their core drum brakes are comprised of a piston and two brake shoes. However, drum brakes include an adjuster mechanism, more springs and often an emergency brake for the car that is also attached to its functioning.
When the brake pedal is applied, the drum brake's pistons press the brake shoes to the drum, causing friction and slowing the vehicle. However, that doesn't address what all of the springs within a drum brake mechanism are for. Enter the drum brake adjuster system.
The Drum Brake Adjuster
For optimal functioning and braking within a drum brake, its brake shoes need to stay in close proximity to the drum, but not actually touch it. If the brake shoes are too far from the drum, such as when the pads start to wear and become smaller, the brake piston will then need significantly more brake fluid in order to travel that distance. The driver will likely begin to notice that the brake pedal tends to sink down more when brakes are applied. Because of this risk, most drum braking systems are equipped with an automatic adjuster. The automatic brake adjuster applies a self-acting principle via spring mechanisms to do its job, responding the braking needs of the moment.
As the brake shoe's pad wears and becomes smaller and thinner, there will naturally be more space between the brake shoe and the brake drum. When this distance grows large enough, the brake adjuster lever rocks and then eventually advances by one of its threads, just as a screw moves when it is turned. The advancing adjuster helps to assure that the brake pads and shoes stay close to the drum so that they can be effective during the braking process.
Drum Systems and the Emergency Brake
A car's emergency brake mechanism is often linked with the drum braking system; however, it is activated via a different source than the brakes. The emergency brake is usually controlled via a cable that allows it to be engaged or released. When activated, the cable pulls a lever that pushes the two brake shoes away from each other.
Some cars use the drum brake adjuster system when the emergency brake is applied. However, this mechanism can fall out of alignment if the emergency brake isn't used very frequently. For optimal functioning, the emergency brake should be used a minimum of once per week.
Service for Drum Brake Systems
The most frequent type of service that is needed for a drum braking system is the removal of worn brake shoes and the installing of new ones.
How does one know if their car's drum brakes need to be replaced? Some systems have a hole that can be peered through on the back so that the condition of the current pad can be determined. Brake pads in a drum braking system should generally be replaced when they are worn to within 0.8 mm of the brake shoe's rivets. If the shoe does not employ rivets as a fastener, then pads should be replaced once they're down to 1.6 mm in thickness for optimal safety.
If a very-worn pad is used too long, its rivets can actually wear scores or grooves in the metal drum. This can sometimes be fixed with a drum refinishing process; however, drum brakes have a maximum diameter that is considered safe, so only a certain amount of drum material can be ground away (disc brakes have a corresponding minimum thickness that is considered safe for their rotors).
All of these components of a drum brake allow the system to effectively slow and stop a vehicle. Drum brake systems and components can be found on eBay at great prices, along with cutting edge electronics, sporting goods, and much, much more.