Drum Brake: Definition, Types, Parts, Benefits & More
What is a Drum Brake?
A drum brake is a type of brake used in automobiles and other vehicles. It consists of a curved metal drum that rotates with the wheel. Lining inside the drum provides the friction surface for the brake shoes. When the brakes are applied, the shoes are pushed outward against the drum surface to slow or stop the vehicle.
The key difference between drum brakes and disc brakes is that disc brakes have a rotor that is attached to the wheel hub, along with brake pads that squeeze the rotor. Drum brakes rely on the friction between the brake shoes and the drum interior when the brakes are applied.
When the brakes are applied in a drum brake system, the brake shoes are pushed outward by a wheel cylinder. This creates friction between the shoes and drum, slowing the rotation of the drum and wheel. As friction increases, the car or vehicle slows down or comes to a stop.
Compared to disc brakes, drum brakes tend to be less expensive to manufacture and maintain. However, disc brakes typically provide better stopping power and dissipate heat more quickly. That’s why most modern vehicles use disc brakes on the front wheels at a minimum, with drum brakes sometimes used on the rear.
Types of Drum Brakes
There are several different types of drum brake designs that have been used in automotive applications over the years. The main categories of drum brake types include:
Leading/Trailing Drum Brakes
Leading drum brakes are located on the front wheels and provide braking when the vehicle is moving forward. Trailing drum brakes are located on the rear wheels and engage when braking while the vehicle is traveling in reverse.
Single/Dual Drum Brakes
Single drum brakes have one brake drum per wheel hub. Dual drum brakes contain two brake drums per wheel hub, one for the front brakes and one for the rear. Dual drum brakes provide increased stopping power.
Self-Energizing/Non-Energizing Drum Brakes
Self-energizing drum brakes use the forward motion of the vehicle to aid in braking. As the vehicle moves forward, the rotating brake drum creates a self-energizing effect that assists with slowing the vehicle down. Non-energizing drums do not utilize this effect.
Parts of a Drum Brake
Drum brakes have a relatively simple design with just a few key components:
Brake Shoes
The brake shoes are curved metal pieces lined with friction material that press against the inside of the brake drum to create friction and slow the vehicle. When the brakes are applied, hydraulic pressure pushes the brake shoes outward into contact with the drum.
Wheel Cylinder
The wheel cylinder contains a piston with seals that pushes the brake shoes apart when the brakes are applied. This creates the friction between the shoes and drum. The wheel cylinder converts hydraulic pressure into mechanical force.
Return Springs
Return springs pull the brake shoes back away from the drum when hydraulic pressure is released. This allows the drum to spin freely again when the brakes are not applied.
Anchor Plate
The anchor plate provides a fixed mounting point for the ends of the brake shoes and return springs. It connects the shoes to the backing plate.
Adjusting Screw
The adjusting screw is used to adjust the rest position of the brake shoes. It ensures the shoes are the proper distance from the drum when the brakes are not applied.
How Drum Brakes Work
Drum brakes operate through friction to slow down and stop vehicle wheels from spinning. When the brake pedal is pressed, it activates a hydraulic system that pushes brake shoes lined with friction material outward into contact with a cylindrical drum attached to the wheel. The friction material rubbing against the drum creates drag that resists the rotation of the wheel, slowing the vehicle down.
Specifically, when the brake pedal is pressed, it forces brake fluid through the hydraulic lines to the wheel cylinders at each brake drum. This applies force to brake shoes positioned on either side of the drum, pushing them outward. The curved brake shoes press against the inner surface of the rotating drum, creating friction which converts the kinetic energy of the moving wheel into heat. This friction force between the drum and brake shoes counteracts the rotational motion of the drum, causing it to slow down until it stops spinning entirely, stopping the wheel’s rotation.
Once the brake pedal is released, the brake shoe springs pull the shoes back away from the drum surface, allowing the drum and wheel to freely spin again. The basic principle of drum brakes transforms the hydraulic pressure from the brake pedal into friction within the drum-and-shoe system to control wheel speed.
Drum Brake Pros and Cons
Drum brakes have been used in vehicles for decades and still have some advantages over disc brakes in certain applications. However, disc brakes have become more prominent as the preferred brake type in most modern passenger vehicles. Understanding the pros and cons of drum brakes is helpful when selecting the right brake system.
Advantages of Drum Brakes
Some of the main benefits of drum brakes include:
- Less expensive to produce and maintain than disc brakes
- Provide ample stopping power for light-duty vehicles
- More resistant to water, mud and debris than disc brakes
- Self-applying parking brake when used as rear brakes
- Self-adjusting mechanism compensates for brake pad wear
- Enclosed design protects brake components from elements
Disadvantages of Drum Brakes
The downsides of drum brakes include:
- Lower maximum braking force than disc brakes
- More prone to brake fade from overheating
- Heavier than equivalent disc brakes
- Harder to inspect brake components for wear
- Require more maintenance to keep adjusted
- Less responsive and slower to apply than disc brakes
Drum Brake Maintenance
Proper maintenance is crucial to keep drum brakes working safely and effectively. Here are some key maintenance tasks:
Inspecting Drum Brakes
Inspect drum brakes regularly, such as during an oil change or tire rotation. Check for issues like wear, overheating, leaks, and loose parts. Tap the brake drums with a rubber mallet to check for thickness. Thinner, warped drums should be replaced. Inspect brake shoes for thinning, glazing, or embedded debris. Check springs, clips, levers, and other hardware.
Adjusting Drum Brakes
As brake shoes wear down, the distance between the shoes and drums increases, reducing braking power. Adjustment restores the proper clearance. Use a brake spoon to twist the starwheel adjuster at the bottom of the backing plate. Adjust each wheel until the shoes lightly drag against the drum when spun.
Cleaning and Lubricating
Clean the brake assembly with brake cleaner spray to remove grease, oil, and debris. Wipe components dry and apply high-temperature brake lubricant to contact points between the shoes, hardware, and backing plate. Lubricate wheel cylinders and the parking brake cable. This prevents sticking and ensures proper operation.
Drum Brake vs. Disc Brake
Drum brakes and disc brakes are the two main types of brake systems used in most vehicles. Both have distinct advantages and disadvantages.
Disc brakes use calipers that squeeze pairs of pads against a rotor to create friction. This allows them to dissipate heat quickly and provides even, consistent braking power. Disc brakes are generally superior in wet conditions since the exposed rotors allow water to be flung off as they spin. They also tend to be less prone to brake fade at high temperatures. However, disc brakes and their rotors are more expensive to manufacture than drum brake components.
In contrast, drum brakes use curved brake shoes lined with friction material to press outward against the inner surface of a drum. This enclosure can allow brake dust, dirt and water to collect inside the drum, reducing effectiveness and increasing wear. Drum brakes do not dissipate heat as quickly, making them more susceptible to brake fade. However, the drum and shoes are very inexpensive to produce, making drum brakes ideal for the rear brakes in many vehicles. The self-applying nature of drum brakes also makes them suitable as parking brakes.
In summary, disc brakes typically provide stronger braking power and better performance, especially in wet conditions. But drum brakes are simple and cost-effective. For optimal balance, many modern vehicles use disc brakes on the front wheels for better braking performance and drum brakes on the rear for affordability.
Drum Brake History
Drum brakes have been around for over a century, with origins dating back to wagon brakes in the 1800s. The first modern drum brake was patented in 1902 by Louis Renault, founder of the French automaker Renault. His design used two brake shoes that pressed outward against the inner surface of a drum attached to the wheel hub.
In the early 1900s, drum brakes became standard equipment on most cars and trucks. Key innovations that improved drum brake performance over the years include:
- Self-energizing design in the 1920s – Brake shoes were curved so friction helped expand them outward for better contact.
- Hydraulic actuation in the 1930s – Hydraulic brake fluid systems allowed more precise brake control.
- Duo-Servo design in the 1950s – A secondary hydraulic cylinder boosted the braking force applied to shoes.
- Power brakes in the 1950s – Vacuum boost increased pedal force for easier braking.
- Disc brakes in the 1950s – Discs provided better performance but drums were still common.
While drum brake designs improved over the decades, the basic principles remained the same. By the 1950s, the modern self-energizing duo-servo drum brake had emerged and remains similar to what’s still used today.
Drum Brakes in Modern Vehicles
Despite the predominance of disc brakes on most modern passenger vehicles, drum brakes are still commonly used on certain types of vehicles today.
Drum brakes are most often found on lighter weight vehicles such as entry-level compact cars and small SUVs. Some examples include the Nissan Versa, Toyota Yaris, Honda Fit, Chevrolet Spark, and Ford EcoSport.
They are also still widely used on the rear wheels of pickup trucks, where their lower cost and simplicity are advantageous. Full-size trucks like the Ford F-150, Chevrolet Silverado, RAM 1500, and GMC Sierra rely on drum brakes for the rear.
In addition, drum brakes are nearly universal on school buses and other commercial buses. The reasons are mainly economic – bus fleets require simple, low-maintenance brakes to minimize downtime and operating costs.
Finally, many heavy duty vehicles like large delivery trucks and construction equipment still utilize drum brakes, which can better withstand frequent heavy braking. Drum brakes are simpler, easier to service, and cheaper to produce in the large sizes required for these heavy vehicles.
So while disc brakes provide superior performance, the basic and economic nature of drum brakes ensures they will continue to have a place in modern automotive applications, especially for light duty trucks, buses, and basic transportation vehicles.
The Future of Drum Brakes
Drum brakes have been around for over a century and have proven to be a reliable and cost-effective braking solution. Even with the rise of disc brakes, drum brakes are still commonly used on the rear wheels of passenger vehicles and as the primary brake on medium and heavy-duty trucks and buses.
This is likely to continue into the foreseeable future, as drum brakes offer some advantages over discs in certain applications. The enclosed design protects drum brake components from water, dirt and debris – important factors for the heavy-duty vehicles that utilize them. They also tend to be simpler and cheaper to manufacture than disc brake designs.
However, that doesn’t mean there won’t be innovations in drum brake technology moving forward. Manufacturers continue to optimize the design to improve performance. For example, some modern drum brakes utilize two internal shoes per wheel cylinder, which provides more braking surface area and power. The composition of friction materials is also evolving, with new high-performance formulas that withstand heat better and reduce brake fade.
Advancements in wheel cylinder and actuator technology may also emerge, allowing more precise brake pressure control. And lighter weight composite drum materials could potentially replace traditional cast iron drums. While complete phase out is unlikely, drum brakes will continue adapting to meet the needs of modern vehicles.