26/04/2008
Brakes are arguably the most critical safety system on any vehicle, and understanding how they work is fundamental to ensuring your car remains safe and reliable on the road. While modern vehicles often feature disc brakes on all four wheels, many still utilise drum brakes, particularly on the rear axle. This comprehensive guide will delve into the intricate workings of drum brakes, explain their role in your vehicle's overall braking system, and provide essential advice on when and how to service them.
At their core, brake drums operate on the same hydraulic principles as their disc brake counterparts. However, instead of a caliper squeezing a disc, the hydraulic fluid within a drum brake system pushes pistons outwards from a component known as the wheel cylinder. These pistons, in turn, press specially designed brake shoes against the inside surface of the rotating brake drum, creating friction that slows and eventually stops the wheel.
How Brake Drums Work: A Detailed Look
The drum brake system is a self-contained unit, with all its primary components enclosed within the cylindrical brake drum itself. This drum rotates in unison with the wheel. When you depress the brake pedal, hydraulic fluid is sent under pressure from the master cylinder to the wheel cylinder located within each drum brake assembly. The wheel cylinder contains one or two pistons, which are then forced outwards by the hydraulic pressure.
As these pistons extend, they push the brake shoes – curved pieces of metal with friction material bonded to their outer surface – against the inner circumference of the brake drum. The friction generated between the shoes and the drum converts the vehicle's kinetic energy into heat, effectively slowing the rotation of the wheel. When you release the brake pedal, return springs pull the brake shoes back into their original position, away from the drum, allowing the wheel to rotate freely once more.
This design offers a significant mechanical advantage due to what's known as 'self-energisation'. As one shoe (the leading shoe) is pushed against the drum, the friction itself helps to push the other shoe (the trailing shoe) further into contact with the drum, amplifying the braking force. This characteristic makes drum brakes particularly effective for parking brake applications, as they can hold a vehicle securely with relatively little effort.
Key Components of a Drum Brake System
- Brake Drum: A cast-iron cylindrical component that rotates with the wheel. It provides the friction surface for the brake shoes.
- Brake Shoes: Curved metallic structures with bonded friction material (lining) that press against the inside of the drum.
- Wheel Cylinder: A hydraulic cylinder containing pistons that push the brake shoes outwards when brake fluid pressure is applied.
- Return Springs: Coil springs that pull the brake shoes back to their resting position when the brake pedal is released, preventing continuous contact with the drum.
- Self-Adjuster Mechanism: A system (often a star wheel adjuster) that automatically compensates for wear in the brake shoes, maintaining the correct clearance between the shoes and the drum.
- Parking Brake Lever/Cable: A mechanical linkage that directly applies the brake shoes to the drum, independent of the hydraulic system, for parking.
- Backing Plate: A sturdy metal plate mounted to the vehicle's axle that serves as the foundation for all the drum brake components.
When to Service Your Brake Drums: Signs and Intervals
Like all components of your vehicle's braking system, drum brakes require periodic inspection and servicing to ensure optimal performance and safety. Ignoring warning signs can lead to reduced braking efficiency, increased stopping distances, and potentially dangerous situations.
Common Warning Signs of Drum Brake Issues:
- Squealing or Grinding Noises: A high-pitched squeal often indicates worn brake shoes or a build-up of dust and debris. A grinding noise, particularly a metallic one, suggests severe wear where the metal backing of the shoe is contacting the drum, which can cause significant damage.
- Reduced Braking Performance: If your car takes longer to stop, or the brake pedal feels spongy or requires excessive force, it could point to problems with the drum brakes, such as air in the hydraulic system or worn components.
- Pulling to One Side: If your vehicle consistently pulls to the left or right when you apply the brakes, it could indicate uneven braking force between the wheels, possibly due to a seized wheel cylinder, contaminated brake shoes, or an improperly adjusted self-adjuster on one side.
- Vibration or Pulsation: While more common with disc brakes, a pulsating feeling through the brake pedal can sometimes be caused by an out-of-round or warped brake drum.
- Excessive Handbrake Travel: If your handbrake needs to be pulled up much higher than usual to engage, it could indicate worn brake shoes or a need for adjustment of the parking brake cable or the drum brake self-adjuster.
- Leaking Brake Fluid: Any visible leaks around the rear wheels, particularly on the inside of the tyre, could indicate a leaking wheel cylinder, which is a serious safety concern as it will compromise braking performance.
Recommended Service Intervals
While specific intervals can vary based on your vehicle's make, model, and your driving habits, a general guideline for drum brake inspection is every 20,000 to 30,000 miles, or at least once every two years. However, it's always best to consult your vehicle's owner's manual for precise recommendations. During a routine service, a qualified mechanic will typically remove the drums, inspect the brake shoes for wear, check the wheel cylinder for leaks, examine the drum for scoring or warping, and ensure all springs and the self-adjuster are functioning correctly.
What Servicing Involves
A standard drum brake service usually includes:
- Inspection: Checking the thickness of the brake shoe lining, the condition of the drum, the wheel cylinder, and all springs and hardware.
- Cleaning: Removing accumulated brake dust and debris, which can cause noise and reduce efficiency.
- Adjustment: Ensuring the self-adjuster is working correctly and manually adjusting the brakes if necessary to maintain the proper shoe-to-drum clearance.
- Lubrication: Applying high-temperature grease to contact points to prevent seizing and noise.
- Replacement: Replacing worn brake shoes, damaged drums, leaking wheel cylinders, or corroded hardware as needed.
Drum Brakes vs. Disc Brakes: A Comparative Analysis
The evolution of braking technology has seen a gradual shift from drum brakes to disc brakes, particularly on the front axles of vehicles. This change, largely popularised from the 1970s onwards, was driven by performance and safety considerations. Understanding the differences between these two systems is key to appreciating why certain vehicles have a particular setup.
Historically, early automobiles used drum brakes on all four wheels. While effective for the speeds and demands of the era, their design presented a notable challenge: heat dissipation. When the brake shoes press against the inside of the drum, the heat generated by friction becomes trapped within the enclosed drum assembly. This heat build-up, especially during aggressive braking or prolonged braking on steep descents, can lead to a phenomenon known as 'brake fade'. Brake fade occurs when the friction material overheats, leading to a significant reduction in braking effectiveness and potentially making the brakes feel spongy or completely unresponsive. This design flaw led to drum brakes being deemed less safe for high-performance or demanding driving conditions.
Enter disc brakes. Car manufacturers observed that racing vehicles were achieving superior braking performance with a new design where a disc (or rotor) was exposed to the outside air. In disc brake systems, braking is achieved by a caliper squeezing brake pads against both sides of a rotating disc. The open design of disc brakes allows heat to dissipate much more readily into the surrounding air, drastically improving their resistance to fade and maintaining consistent performance even under heavy use. This ability to shed heat efficiently made disc brakes the preferred choice for the front wheels, where the majority of braking power is required.
Most modern vehicles are front-wheel drive, and it's a well-established principle that the front wheels perform 60-90% of a vehicle's braking, especially during hard stops due to weight transfer. Consequently, the switch to disc brakes on the front axle significantly enhanced overall vehicle safety and performance. Today, it's common to find a setup with disc brakes on the front and drum brakes on the rear, or disc brakes on all four wheels. Only very old vehicles will typically feature drum brakes on all wheels.
How to Identify Your Brake Type
If you're unsure whether your vehicle has drum or disc brakes, there are a couple of straightforward ways to check:
- Visual Inspection (Front Wheels): Look through one of the holes in the top of your front wheel. If you see a shiny, smooth metal surface, that's your disc rotor. If you see a rusty and less smooth, rounded surface, that's your brake drum. (Note: some rotors may have light rust speckling, which is normal).
- Rear Wheel Inspection (Hoses): Look at the backside of the wheel. A disc brake will have a rubber hose leading to the brake caliper, whereas a drum brake will typically have a rigid metal tube connecting to the wheel cylinder.
Comparative Table: Drum Brakes vs. Disc Brakes
| Feature | Drum Brakes | Disc Brakes |
|---|---|---|
| Heat Dissipation | Poor (heat trapped within drum) | Excellent (exposed to air) |
| Brake Fade Resistance | Lower (prone to fade under heavy use) | Higher (more consistent performance) |
| Complexity | More complex internal components (springs, adjusters) | Simpler, fewer internal moving parts |
| Cost | Generally lower manufacturing cost | Generally higher manufacturing cost |
| Maintenance | Can be more time-consuming to service due to internal components | Generally easier and quicker to service |
| Parking Brake Integration | Often integrated directly into the drum mechanism | Often requires a separate small drum brake or caliper mechanism |
| Common Application | Rear wheels of many modern vehicles, older vehicles (all wheels) | Front wheels of most vehicles, all four wheels on many modern cars |
Beyond the Main Braking System: Other Brake Types
While disc and drum brakes handle the primary stopping duties, your vehicle features other braking systems designed for specific purposes:
- Emergency Brake (Parking Brake): Also known as a handbrake, this system is designed to bypass the normal hydraulic braking system. It typically uses levers or cables to mechanically engage the rear brakes (often via the drum brake mechanism, or a small separate drum inside a rear disc rotor) to hold the car stationary, or to provide a backup in case the main brakes malfunction.
- Anti-lock Braking System (ABS): Found in newer models, ABS is a safety system that prevents the wheels from locking up during hard braking. Sensors monitor wheel speed, and if a wheel is about to lock, ABS rapidly modulates the brake pressure to that wheel, allowing the driver to maintain steering control. ABS works in conjunction with both disc and drum brakes.
Frequently Asked Questions about Brake Drums
Are drum brakes dangerous?
While older drum brake systems on all four wheels could be prone to fade under extreme conditions, modern drum brakes, particularly when used on the rear axle in conjunction with front disc brakes, are perfectly safe. They have been significantly improved since the 1970s and are well-suited for the less demanding braking requirements of the rear wheels. The 'danger' aspect primarily relates to their susceptibility to fade during prolonged or heavy braking, which is why disc brakes are preferred for the front wheels.
How often should drum brakes be replaced?
The lifespan of drum brake shoes varies widely depending on driving habits, vehicle weight, and road conditions. Generally, brake shoes can last anywhere from 60,000 to 100,000 miles or even more. However, regular inspection (as part of your service schedule) is crucial. They should be replaced when the friction material reaches its minimum thickness, or if they become contaminated with oil or brake fluid.
Can I convert my drum brakes to disc brakes?
While technically possible, converting rear drum brakes to disc brakes is a complex and often costly modification. It typically involves replacing the entire hub assembly, mounting new calipers, discs, and potentially modifying brake lines and the master cylinder. It's generally not recommended unless you have specific performance needs, as the factory-fitted drum brakes are usually more than adequate for rear braking requirements and are designed to work seamlessly with your vehicle's existing braking system.
What causes brake drum noise?
Common causes of noise from drum brakes include worn brake shoes (metal-on-metal grinding), excessive brake dust accumulation (squealing), contaminated shoes (oil or grease), a warped or scored drum, or loose/worn hardware components (rattling or clunking).
Are drum brakes cheaper to replace?
Generally, replacing drum brake shoes and drums can be less expensive than replacing disc brake pads and rotors. This is primarily due to the lower cost of parts and, in some cases, less frequent replacement intervals for the shoes compared to pads. However, the labour involved in disassembling and reassembling drum brake components can sometimes be more intricate.
Conclusion
Brake drums, despite being an older technology, continue to play a vital role in the braking systems of countless vehicles on UK roads. Understanding how they function, recognising the signs of wear, and adhering to recommended servicing schedules are paramount for maintaining your vehicle's safety and performance. Regular inspections by a qualified mechanic will ensure that your brake drums, whether working in tandem with disc brakes or as part of an older all-drum setup, provide reliable and effective stopping power when you need it most.
If you want to read more articles similar to Understanding Your Car's Brake Drums, you can visit the Brakes category.