Automotive brakes are grouped into three basic categories: disc brakes, drum brakes and parking brakes.
Disc brakes use a pair of pads that clamp against the flat rotor or disc, while drum brakes use brake shoes that expand against the inside of a cylindrical drum. These brake types are both hydraulic systems designed to convert the forward momentum of a moving vehicle into heat and slow the vehicle.
Parking brakes function mechanically and use static friction to hold a parked vehicle in place.
Disc Brake Function
There are two types of disc brake system in use today. They can be identified by their caliper piston position.
The floating caliper disc brake system can be identified by caliper piston(s) located in only one side of the caliper assembly. When the brakes are applied by the driver, the inside pad is forced against the rotor. As the force increases, the inside pad pushes against the rotor and forces the caliper to slide on its slide pins and pull the outside pad against the rotor. This is the most common caliper design used today.
The fixed caliper disc brake system is identified by its multiple pistons, with equal number of pistons on each side of the caliper. In this system, as the brakes are applied, the force is applied equally on both sides of the rotor surface. This clamping force is what stops the vehicle.
Drum Brake Function
With drum brakes there are also two types. The type of drum brake system is identified by the position of the self adjuster or star wheel.
When this adjuster is located on the lower portion of the shoe and becomes the connecting point between the shoes, this is a servo drum brake system. On this system, the lower portion of the shoes are connected by an adjuster and spring, and the upper portion is separated by an anchor pin bolted to the backing plate. When the brakes are applied, the wheel cylinder spreads the shoes against the rotating drum. When contact is made, the shoe's natural tendency is to rotate with the drum. As the primary shoe tries to rotate, the force of that rotation is transmitted into the secondary shoe through the adjuster between them. The secondary shoe tries to follow the rotation of the drum as well, but is stopped by the anchor pin at the top of the backing plate. Since it can't rotate, it becomes jammed against the drum surface. This is why the secondary shoe is always bigger then the primary, because the secondary shoe is applying all the force and wears faster.
In a non-servo system, the anchor is located at the bottom of the backing plate. Instead of being a round pin, it's a flat, wedge-shaped plate. At the top of the backing plate, we find the adjuster located just below the wheel cylinder. As the brakes are applied, again the shoes try to follow the rotation of the drum. But on this system, the leading shoe hits the wedge shaped anchor plate and is forced down the wedge into the drum. The trailing shoe is stopped by the wheel cylinder, applying its force into and through the wheel cylinder continuing into the top of the leading shoe. The shoes in this system are the same size, and uneven wear is common on the leading shoe, because it applies all of the braking force going forward.
Parking Brake Function
Parking brake systems are mechanical brakes used to hold a vehicle when it's parked. They have three basic designs.
The conventional drum brake design, works within the basic drum brake and is cable actuated. When the parking brake lever or pedal is set, the cable pulls a lever attached to a brake shoe. A flat steel bar connecting the lever and the secondary shoe forces both shoes against the drum providing the pressure to hold the vehicle.
The actuator caliper, disc brake system is also cable actuated. When the parking brake is applied, the cable pulls a lever attached to the actuator in the caliper. This rotates a worm screw in the caliper and caliper piston, and it actually "unscrews," forcing the piston and pad into the rotor. This provides the needed force to hold the vehicle.
The final system is called drum-in-hat and is used on some disc brake systems. The inside of the rotor, called the hat, is machined in the same way a brake drum is. A small set of auxiliary shoes is attached to the backing plate, and the rotor is slipped into place over them. When the parking brake is applied, the cable pulls a lever attached to the actuator. This spreads the shoes and provides the needed friction to hold the vehicle.
Conclusion
There are several types of brake systems used today. Your vehicle will use any or a combination of these to perform its functions.
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