For decades, drum brakes were the standard braking system used on cars and light trucks. But over the last 30 years, disc brakes have become increasingly more common, especially on the front wheels. So how do these two braking systems compare when it comes time to service and replace brake parts? Generally speaking, drum brakes tend to be more difficult to work on than disc brakes.
Drum Brake Design and Operation
Drum brakes work by pressing curved brake shoes against the inner surface of a brake drum. When the brakes are applied, the shoes spread apart and create friction with the drum, slowing the rotation of the wheel. The components are enclosed within a round drum housing mounted to the wheel hub.
[Diagram of drum brake components]
The key parts of a drum brake system include:
- Brake drums – metal cylinders attached to the wheel hubs that spin with the wheels. The inner surface provides the friction surface.
- Brake shoes – curved brake pads that press against the drum when the brakes are applied.
- Wheel cylinders – hydraulic actuators that push the brake shoes outward when brake pressure is applied.
- Adjusting mechanisms – manually or automatically adjust the brake shoes as they wear down.
- Return springs – retract the brake shoes away from the drum when brake pressure is released.
- Backing plate – metal plate that provides a mounting point for the brake parts.
When the brakes are applied, hydraulic pressure pushes the wheel cylinders outward, which in turn spreads the brake shoes apart. Friction between the brake shoes and drum slows the rotation of the wheels.
Disc Brake Design and Operation
Disc brakes work very differently than drum brakes. They utilize brake pads that squeeze against a rotor attached to the wheel hub.
[Diagram of disc brake components]
The main components of a disc brake system are:
- Brake rotor – a metal disc that spins with the wheel. The outer surfaces provide the friction surface.
- Brake pads – pad assemblies that clamp against the rotor from both sides when the brakes are applied.
- Calipers – hydraulic actuators containing pistons that push the pads against the rotor.
- Brake lines – carry pressurized brake fluid to the calipers.
When the brakes are applied, hydraulic pressure causes the caliper pistons to extend and clamp the pads on both sides of the rotor. Friction between the pads and rotor slows the wheels.
Compared to drum brakes, disc brakes provide better stopping power, resist fade from heat buildup, and shed water well in wet conditions. However, drum brakes can outperform discs in certain heavy-duty applications.
Why Drum Brakes are Harder to Service
There are several key reasons why servicing and replacing parts on drum brakes is generally more challenging than disc brakes:
The drum housing and backing plate enclose all of the working parts of a drum brake. To access the brake shoes and other internal hardware, you need to remove the wheel and the brake drum first. This adds time and work to the process.
With disc brakes, the caliper, pads, and rotor are all externally mounted and accessible without needing to disassemble components. Pads can be easily checked and replaced with the wheels still on the car in many cases.
Over time as drum brake shoes wear down, the distance between the shoes and drum increases. This can reduce braking effectiveness. Drum brakes use mechanical or hydraulic mechanisms to automatically or manually adjust the shoes closer to the drum surface.
These adjusters add complexity and another thing that can fail or function improperly. Often the auto adjusters need to be manually reset when brake shoes are replaced.
Disc brake pads simply press directly against the rotor surface at all times. No complex adjustment mechanisms are necessary.
Replacing brake shoes on drum brakes requires retracting the brake shoe springs and maneuvering all the hardware inside the cramped drum housing. The curved shoes only fit properly when aligned a certain way. Doing this blindly through the inspection hole can be frustrating.
In comparison, disc brake pads are simple flat pads that only attach via bolts or clips to the caliper. No spring retraction or odd angles are involved. Pads slide into place easily once the caliper is removed.
Uneven Wear Issues
Drum brakes are prone to uneven wear, especially on the leading and trailing edges of the brake shoes which take the brunt of the friction forces. This can cause braking performance issues. The shoes must seat properly across the whole drum surface for even braking.
Disc brake pads wear evenly across their entire swept surface area, maintaining smooth braking performance throughout their lifespan.
To fully inspect drum brake components and replace shoes, the wheel must be removed to drop the brake drum. This adds an extra step compared to disc brakes.
With disc brakes, the condition of the pads and rotor can be visually checked with the wheels still on. Pad replacement can often be done without needing to remove the wheel at all.
When Disc Brakes are Harder
Though disc brakes generally are easier to service than drums, they do have some maintenance challenges in certain circumstances:
- Stuck sliders – Corrosion can cause the caliper slider pins that allow the caliper to float over the rotor to seize up. This prevents even pad wear. Sliders must be cleaned, lubricated, or replaced.
- Stuck pistons – Rust and debris can prevent caliper pistons from retracting properly, causing drag. Pistons may need to be cleaned or forced back into place.
- Pad deposits on rotors – Semi-metallic pads can leave deposits on the rotor surface that need to be sanded or scraped off to prevent brake pulsation.
- Rotor runout – Excessive rotor warp due to overheating or improper torque can cause vibration under braking. Machining or replacing rotors may be required.
- Corroded mounting hardware – Rusty bolts and slides make caliper removal more difficult. Penetrating lubricants or bolt extraction methods may be needed.
Tips for Servicing Drum Brakes
While certainly more involved than disc brakes in most cases, here are some tips to make working on drum brakes easier:
- Use a brake spoon – These simple curved tools can help retract the brake shoe springs safely and easily. Don’t improvise with screwdrivers that can damage components.
- Inspect the drums – Check for deep grooves, cracks, or excessive wear. Machine or replace drums as needed for smooth operation.
- Lubricate contact points – Lightly lubricate the brake shoe anchor pins, adjusters, springs and other sliding surfaces with high temp brake lubricant.
- Check the wheel cylinders – Look for leaks or weeping that indicates worn parts that need replacement.
- Adjust the brakes properly – Reset automatic adjusters and manually adjust the shoes to the proper clearance from the drums.
- Reassemble carefully – Make sure springs are seated and shoes aligned right to avoid issues after reassembly.
- Break them in properly – Follow the break-in procedure to transfer pad material and seat the shoes evenly.
Warning Signs of Drum Brake Problems
Be alert for these common signs of potential issues with your drum brakes:
- Brake squealing or scraping noise when stopping
- Significant brake pedal pulsation or vibration when braking
- Increased stopping distance and reduced braking power
- Brake pull to one side during stops
- Grease or fluid leaks around the drum backing plate
- Visible cracks or discoloration on the friction surface of the drum
Any of these symptoms typically indicates worn shoes, defective drums or wheel cylinders, or improper adjustment. Have your drum brakes inspected right away if you experience these issues. Continuing to drive on faulty drum brakes is unsafe.
Are Drums Here to Stay?
Drum brakes have been around since cars first hit the roads, while disc brakes have continually improved over the decades. This has led many to wonder if drum brake systems might fade away entirely in favor of the superior disc brake.
While discs have taken over as the predominant brake type, especially on the front wheels, drum brakes still have some inherent advantages that will likely keep them around well into the future:
Lower Cost – Drum brake parts are typically cheaper to produce than disc components. This makes them attractive for basic vehicle models.
Parking Brake Integration – The self-energizing design of drums makes them well-suited for parking brakes. Discs require a separate system.
Weather Resistance – Drums shed water and perform better than discs in wet conditions due to their enclosed design.
Heat Tolerance – Drum brakes handle repeated high heat applications better, making them ideal for the rear of trucks and tow vehicles.
Simplicity – Drum brakes are operationally simple with fewer parts to fail. This increased reliability is advantageous in certain heavy-duty applications.
While the tide has certainly turned towards disc brakes for everyday vehicles, don’t expect the humble drum brake to disappear from trucks, basic economy cars, and heavy-duty applications any time soon. With proper maintenance, they should continue meeting the needs of drivers for years to come.
In conclusion, drum brakes are generally more challenging to service and replace parts on versus disc brakes. The enclosed design makes accessing the brake shoes difficult compared to the open and easily visible disc brake pads. Drum brakes also have more fiddly components like springs and adjusters that add complexity.
However, being familiar with how drum brakes work and having the proper tools on hand can make a big difference when attempting to replace brake shoes or wheel cylinders. With methodical disassembly and reassembly, proper lubrication, and careful adjustment, drum brakes can be safely serviced by DIYers to restore smooth and reliable braking performance.
So while often frustrating, drum brakes don’t have to be feared with the right know-how and patience.