Most of us probably know how disc brakes work. But what about the drum brakes? The operation of these humbler parts is almost certainly something less known, something that has not been helped by the fact that their use in vehicles has decreased over the years (they have been in use since 1902), relegated to being installed in smaller cars with modest engines.
If your model equips them, it is always on the rear wheels, with disc brakes at the front, since they only have to take care of 25-30% of all braking power in that location. Drum brakes have more components, but are also cheaper to manufacture, and even provide certain advantages in electric vehicles. To understand this type of brake in detail, we must first have knowledge about its various parts and their function.
What is a drum brake made up of?
The drum brake may seem complicated and can be quite intimidating when you open one. However, once you find the reason for each component, understanding how it works is not so difficult. These are the main parts of which a drum brake is made up:
Drum: It is the moving part that covers all the innards of the braking system and is anchored to the wheel. It is often made of cast iron and is resistant to heat and wear. This is what you see when looking at a drum-mounted brake, and it is the component upon which braking force is applied to slow or stop the vehicle.
Shoes: It is used as a surface to press the inner face of the drum and create the friction necessary to stop the vehicle. They are made up of organic or metallic compound materials (steel) that come into contact with the drum and wear out with use. Each of them contains two shoes: the main one, closer to the front of the vehicle, and the secondary one, in charge of the rear. Depending on the type and brand, they may be interchangeable.
Hydraulic cylinder: Thanks to the brake fluid, a pressure is exerted that causes the pistons to keep the brake shoes at a constant distance from the drum, either to press them against the drum wall or retract them, even when the lining wears. It is connected to a master cylinder that contains a piston that moves outward when force is exerted on the brake pedal and forces the shoe toward the inner surface of the drum.
Return springs: They are responsible for retracting the shoes after pressing the brake. There are two: one for the primary shoe and one for the secondary shoe. They must have enough strength to carry out their mission, but not too much to hinder braking.
Adjustment mechanism: It maintains the minimum space between the shoe and the drum so that they do not come into contact when the pedal is not pressed. In the event that the first ones wear out and the space between them and the drum increases, they can be adjusted again to maintain the space inside. Once adjusted, it retains the same gap on its own.
Dish: It is the part where the internal components of the drum brake are fixed: the cylinders, the springs, the adjustment mechanism and the shoes.
How does a drum brake work?
Drum brakes work on the same principle as disc brakes: two elements that act as friction components (shoes) and a piston that pushes them against an area that rotates integrally with the wheel (drum) thanks to a hydraulic circuit. But it also has an adjustment mechanism, that of the parking brake, and many springs. When you step on the brake pedal, the piston pushes the brake shoes against the drum. That’s simple, but why do we need all those docks? This is where it gets a little more complicated.
Many drum brakes engage automatically. That is, when the shoes contact the metal surface, a type of wedging action occurs that has the effect of pressing the shoes with more force. That extra braking force allows a smaller piston to be used than disc brakes, but the shoes must separate from the surface when the pedal is released. This is the reason for being recovery springshelping to keep them in place and return them to their resting position after fulfilling their function.
To work properly, the shoes must remain close to the drum without touching it. If they move too far away (as the pads wear, for example), the piston will require more fluid to travel that distance and the brake pedal will sink further when you apply the brakes. That is why most of these systems have a mechanismautomatic adjustment. Some cars are equipped with one that is activated when the handbrake is applied, and can become out of adjustment if not used for long periods of time; You have to move it with a certain periodicity.
The parking brake must be activated by a different energy source than the primary braking system. The drum design allows for simple cable drive mechanism from the lever, handle or pedal itself, depending on the model. When this is activated, the cable pulls a lever that separates the two shoes from the drum core. In the rest position, the shoes are pressed against the cylinder motivated by the retention springs, with the necessary space to compensate for the heat expansion that occurs during braking.
And mentioning this last aspect, the drums are also not great when it comes to high temperature management– With all components inside a metal structure, the accumulated heat has nowhere to go. An increase in temperature means a reduction in friction, making the brakes less effective. They will also not work as intended if they are soaked in water inside. This is because the water takes longer to leave the structure and is responsible for reducing friction between the shoes and the drum itself.
Do drum brakes need maintenance?
Being more complex than disc brakes, they do require a little more maintenance in terms of time investment, because as such, they are not components that require a lot of care. The most common thing required for drum brakes is change the shoes. Some have an inspection hole on the back, where you can see how much material is left. These should be replaced when the friction material is below the 1.6 millimeters of the support plate.
As with discs, drums can have an uneven braking surface if a worn brake pad is used for too long; The rivets that hold the friction material to the plate can wear down some grooves on the inside. Sometimes a badly scarred drum can be repaired. Since the contact surface is the inside of the drum, as you sand, the diameter increases. And as for the price of changing some drums, each one can cost between 50 and 100 eurosalthough it depends on the size and model.
How often should drum brakes be changed?
Drum brake housing is designed to last between 250,000 and 300,000 kilometers under normal conditions, before friction causes its inner diameter to increase enough to affect contact with the brake pads. Generally, the latter can travel around 60,000 km before they wear out and need replacement. Aside from replacing worn parts and ensuring you have the proper amount of brake fluid, little additional maintenance is required.
Advantages and disadvantages of drum brakes
Advantages:
- They are cheaper to manufacture than disc brakes.
- Drum friction pads have more surface area and last longer than disc calipers.
- Unlike disc brakes, they have a “self-energizing” capability where they take advantage of their geometry to increase braking power without increasing pressure.
- Slightly lower maintenance frequency due to better corrosion resistance.
Disadvantages:
- They are not as good as disc brakes at managing high temperatures.
- They do not work properly if they are wet inside.
- Maintenance requires more time compared to disc brakes.
- The asbestos lining of the brake shoe is harmful to humans.
