The car is indeed one of the vehicles in which it has many constituent components, and each has a variety of functions. Brakes are a vital component of a car because they can stop or slow down the car when moving or backward. There are two types of brakes in cars, the most popular of which are disc brakes and drum brakes.
Both types of brakes have the same purpose: to stop the vehicle, but the disc brake design with drum brakes is made differently. The working system is also different because the various components it carries are different.
There are still many people who own cars but are not familiar with the concept of drum brakes. Drum brakes consist of various components. Let’s understand what is drum brake and how it works.
What is a Drum Brake?
A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating bowl-shaped part called a brake drum.
The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake.
Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or “band” wrapping around the outside of a drum.
Related: What is Brake?
Diagram of Drum Brakes
Drum brakes are a brake system with brake drums (rotor) that rotate with the wheels. Inside each drum are brake shoes fitted with brake linings (friction material). Pistons (pressure mechanism) press against the drums from the inside to generate braking force, thus making it possible to decelerate and stop the vehicle.
How Do Drum Brakes Work?
The drum brake may look complicated, and it can be pretty intimidating when you open one up. Let’s break it down and explain what each piece does.
Like the disc brake, the drum brake has two brake shoes and a piston. But the drum brake also has an adjuster mechanism, an emergency brake mechanism and lots of springs.
First, the basics: When you hit the brake pedal, the piston pushes the brake shoes against the drum. That’s pretty straightforward, but why do we need all of those springs?
This is where it gets a little more complicated. Many drum brakes are self-actuating. As the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force.
The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.
Drum Brakes Parts
Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.
#1. Backing plate.
The backing plate provides a base for the other components. The backplate also increases the rigidity of the whole set-up, supports the housing, and protects it from foreign materials like dust and other road debris.
It absorbs the torque from the braking action, and that is why the backplate is also called the “Torque Plate”. Since all braking operations exert pressure on the backing plate, it must be strong and wear-resistant. Levers for emergency or parking brakes and automatic brake-shoe adjusters were also added in recent years.
#2. Brake drum.
The brake drum is generally made of a special type of cast iron that is heat-conductive and wear-resistant. It rotates with the wheel and axle. When a driver applies the brakes, the lining pushes radially against the inner surface of the drum, and the ensuing friction slows or stops the rotation of the wheel and axle, and thus the vehicle. This friction generates substantial heat.
#3. Wheel cylinder.
One wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. The leading shoe (closest to the front of the vehicle) is known as the secondary shoe. The trailing shoe is known as the primary shoe.
Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes, forcing them against the drum. When the driver releases the brakes, the brake shoe springs restore the shoes to their original (disengaged) position. The parts of the wheel cylinder are shown to the right.
#4. Brake shoe.
Brake shoes are typically made of two pieces of steel welded together. The friction material is either riveted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage, and self-adjusting components.
All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three “V”-shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the shoes are installed. Each brake assembly has two shoes, a primary and secondary.
The primary shoe is located toward the front of the vehicle and has the lining positioned differently from the secondary shoe. Quite often, the two shoes are interchangeable, so close inspection for any variation is important.
Types of Drum Brakes
There are three types of drum brakes depending on how the brake shoes are pressed onto the drums; leading/trailing shoe type, twin leading shoe type, and duo-servo type.
1. Leading/Trailing Shoe type. The leading shoe is the one that is present in the direction of rotation of the drum. The one on the other side is a trailing shoe. The leading shoe is dragged into the friction surface of the drum for achieving braking force. The trailing shoe is dragged away from the friction surface, thus not contributing to the braking. It is equally effective in forward and reverses braking. Generally used on rear wheels of passenger automobiles.
2. Twin-leading shoe. There are two leading shoes. One shoe has a self-servo effect. So, it provides maximum braking force as both shoes provide friction to the drum. Used in Trucks and other commercial vehicles.
3. Duo Servo. Duo-servo-type drum brakes are an improved version of twin leading construction, where two brake shoes are linked. When a primary side leading shoe is pressed against the drum by the wheel cylinder, the force that makes it rotate together with the drum wedges the secondary side leading shoe into the drum to achieve braking action. It is used in commercial vehicles requiring large braking forces.
Advantages of Drum brake
Here are some advantages that drum brakes have over disc brakes:
- Drum brakes can provide more braking force than an equal-diameter disc brake.
- Drum brakes last longer because drum brakes have an increased friction contact area than a disc.
- Drum brakes are cheaper to manufacture than disc brakes.
- Rear drum brakes generate lower heat.
- Wheel cylinders are simpler to recondition than with disc brake calipers.
- Brake shoes can be remanufactured for future use.
- Drums have a slightly lower frequency of maintenance due to better corrosion resistance.
Drum brake disadvantages
- Excessive heating can happen due to heavy braking, which then can cause the drum to distort, and thus cause vibration under braking.
- Under hard braking, the diameter of the drum increases slightly due to thermal expansion, the driver must press the brake pedal farther.
- Brake shoes can overheat to the point where they become glazed.
- Excessive brake drum heating can cause the brake fluid to vaporize.
- Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction is enough, the brake will stay engaged due to self-application, even when the external application force is released.
- Another disadvantage of drum brakes is their relative complexity.
- Maintenance of drum brakes is more time-consuming, compared to disc brakes.
Issues with Brake Drums
Brake drums can occasionally develop problems because of their hollow, enclosed construction.
- When a large amount of heat cannot be released, a “Long Pedal” occurs. This is more likely to happen if you brake too hard when driving down a hill. The heat generated by the friction will cause the drum to expand, as almost all materials that have been heated do.
- The driver must push the pedal substantially further to achieve the same amount of contact to slow the car as the drum walls expand away from the brake shoes. This will reverse when the brake drum slows down, although it’s best to avoid extended braking when traveling downhill. Avoiding extending braking is a recommendation for both drum and disc brakes. The brake drum’s design is relatively simple but may take longer to service or replace than a regular disc brake.
- Asbestos was previously used in the brake shoes of antique cars as part of the friction material. This is no longer an issue with current cars but may resurface if work is done on an older vehicle.
- Water can become trapped inside the drum and get stuck between the brake shoes and drum walls, reducing friction and braking ability.
How Do I Know if it is Time to Replace Brake Shoes?
You can do a visual inspection of the friction material on your brake shoes through the rivet holes in the shoe body. If the shoes are not at least 2 or 3 millimeters thick, they should be replaced.
TIP: Always replace shoes in pairs to prevent pulling when you brake. Other indicators are squeaking and squealing if a worn pad is making metal-to-metal contact with the drum. And, because the parking brake is linked to the rear drum brakes, you can also test the brakes by parking your car in neutral on a hill, applying the parking brake, and confirming that the car does not roll.