What Is Drum Brake?- Parts, Working, and Diagram

What is 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 cylinder-shaped part called a brake drum.

A brake drum is a cylindrical drum that is attached to the inside of a car’s wheel, and so rotates at the same speed as the wheel. The drum surrounds a set of brake shoes that are coated in materials that generate friction.

When the brake pedal is pressed, these shoes are pushed outwards against the inner surface of the brake drum, generating friction and so slowing the car down. A brake drum is usually made of iron, rendering it resistant to wear.

Drum brakes have been around since 1900 when they were used on a Maybach vehicle, although the idea wasn’t patented until 1902 by Louis Renault. However, drum brakes can be susceptible to heat generation, which can have an adverse effect on braking performance.

Related: What is Brake?

If the drum is subjected to a long period of hard breaking the heat generated can cause the drum itself to expand, meaning that the brake shoes have to be pressed farther outwards to press against the drum’s inner surface. This gives the car what is termed as a ‘long pedal’ because the driver has to press it farther down to get the same braking performance.

Drum brakes can also be susceptible to water ingress. If water gets into the drum it can end up being compressed between the brake shoe and the inside of the brake drum and will have an adverse effect on the length of time it will take a vehicle to stop.

Unlike a drilled disc brake, there’s nowhere for the water to go, so it will continue to affect braking performance until sufficient heat is generated to vaporize the water. Only then will full braking be resumed. Another downside of the drum brake is its relative complexity. They take more time to understand, and due to the number of moving parts they contain, take longer than disc brakes to service and overhaul.

Related: What is Disc 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.

Diagram of Drum Brakes

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.

How Do Drum Brakes Work?

A drum brake system consists of hydraulic wheel cylinders, brake shoes, and a brake drum. When the brake pedal is applied the two curved brake shoes, which have a friction material lining, are forced by hydraulic wheel cylinders against the inner surface of a rotating brake drum. The result of this contact produces friction which enables the vehicle to slow down or stop.

When you press the brake pedal, pneumatic brake fluid is squeezed through the brake lines under pressure and into the brake cylinder. This forces a pair of springs in the cylinder against a piston at each end of the cylinder.

Each piston is driven against one of the two long, curved brake shoes, forcing each shoe against the drum a drum-shaped housing that is coupled to the wheel via the lug nuts. The shoe consists of a metal backing and a friction-material pad that contacts the drum and causes the wheel to slow and stop.

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.

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.

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 increased friction contact area than a disc.
  • Less expensive to produce
  • Rear drum brakes generate lower heat.
  • The built-in self-energizing effect requires less input force (such as hydraulic pressure).
  • Wheel cylinders are somewhat simpler to recondition compared to calipers.
  • Brake shoes can be remanufactured for future use.
  • A slightly lower frequency of maintenance due to better corrosion resistance compared to disks.

Drum brake disadvantages

  • Their components require a break-in period. Unlike disc brake pads, brake shoes require a longer break-in period. Therefore, when replacing these components, it is advisable to avoid sudden braking during the first 300 kilometers. In addition, it is also not advisable to carry out an MOT brake test, since braking is not as effective.
  • Poor heat dissipation. This ventilation problem occurs because drum brakes do not have the capacity to disperse the heat generated by friction. This causes the brake system to overheat quickly. Consequently, the brake drum ends up deforming and, because of this, as drivers, we have to press the brake pedal harder to bring the vehicle to a stop.
  • Under hard braking, the diameter of the drum increases slightly due to thermal expansion, the driver must press the brake pedal farther.
  • Excessive brake drum heating can cause the brake fluid to vaporize.
  • Another disadvantage of drum brakes is their relative complexity.
  • Maintenance of drum brakes is more time-consuming, compared to disc brakes.