Table of Contents
What are Drum Brakes?
Basic Components of a Drum Brake
Drum brakes are a common type of brake used in automobiles and other vehicles. The key components of a basic drum brake are the brake drum, brake shoes, wheel cylinder, return springs, and various hardware.
The brake drum is a hollow cast-iron cylinder that is attached to the wheel hub and rotates with the wheel. The interior surface of the drum has a machined surface that provides friction when the brake is applied.
The brake shoes are curved blocks lined with friction material that presses against the inside of the drum. When the brakes are applied, the shoes expand to create friction against the drum.
How Drum Brakes Differ from Disc Brakes
Drum brakes differ from disc brakes primarily in that friction is created by pressing shoes against the interior of a drum rather than squeezing pads against a rotor. Drum brakes are typically only found on the rear wheels of modern vehicles, with disc brakes more common on the front.
How Do Drum Brakes Work?
The Science Behind Drum Brakes
The basic science behind drum brakes involves converting the vehicle’s kinetic energy into heat energy through friction. When the brakes are applied, the brake shoes press against the rotating drum. This friction converts the rotational motion into heat, slowing the vehicle down.
When the Brakes are Applied
When the driver presses the brake pedal, hydraulic pressure builds in the brake line system. This pressure pushes the wheel cylinders on the brake assembly outward. The wheel cylinders then activate the brake shoes, pushing them outwards into contact with the drum.
The friction between the brake shoe lining material and the drum produces the braking force, slowing the rotation of the drum and wheel until they come to a stop.
An important aspect of drum brakes is the self-energizing effect. As the drum attempts to rotate the brake shoes forward, the friction force also pushes the shoes outward and increases the braking force. This automatic self-energization gives drum brakes excellent stopping power.
Drum Brake Advantages and Disadvantages
Advantages of Drum Brakes
- Low cost and simple design
- The self-energizing effect increases braking power
- Less prone to fading at high temperatures compared to discs
- Protected from elements inside a sealed drum
Disadvantages of Drum Brakes
- Not as effective in wet conditions due to moisture buildup
- More complex servicing due to internal components
- Prone to brake shudder if drums or shoes are warped
- Slower heat dissipation compared to disc brakes
Types of Drum Brakes
Basic Drum Brakes
The basic drum brake design relies solely on hydraulic pressure pushing the brake shoes out against the drum when the pedal is pressed. These are common in older vehicle designs.
Servo Assisted Drum Brakes
For heavier vehicles that require more braking power, servo-assisted drum brakes use a vacuum servo to provide mechanical assistance to the hydraulic pressure. This amplifies the braking force.
Self-Adjusting Drum Brakes
Self-adjusting drum brakes have mechanisms that automatically adjust the shoe position over time as the brake linings wear down. This maintains the proper shoe-to-drum clearance.
Drum Brake Maintenance and Safety
Inspecting Drum Brakes
It’s important to routinely inspect the condition of drum brake components. Look for signs of overheating, uneven wear, cracks, leaks, or loose parts. The drums should also be checked for signs of scoring or wearing too thin.
Replacing Drum Brake Components
The brake shoes, drums, wheel cylinders, springs, and other hardware will eventually wear out and need replacement. Always replace components in axle sets and with high-quality parts.
Drum Brake Safety Tips
- Don’t drive on worn drums or shoes, have them serviced promptly.
- Ensure hardware is assembled correctly to avoid brake failure.
- Adjust brakes properly so shoes don’t drag and overheat.
- Confirm self-adjusters are working on vehicles equipped with them.
While they have been around for decades, drum brakes continue to be the go-to rear brake option on many modern vehicles. Understanding how the components work together hydraulically to slow the vehicle provides insight into proper drum brake maintenance and service. Keeping drum brake systems in excellent operating condition is key to safe braking.
What are the main parts of a drum brake system?
The main parts are the brake drum, brake shoes with friction lining, wheel cylinder, return springs, adjusters, and various small hardware like pins and clips.
Why can drum brakes fade under repeated heavy braking?
Heat buildup in the drum can create a brake fade. The shoes and drum absorb high heat and expand, reducing braking effectiveness until they cool down.
How often should drum brakes be inspected?
Drum brakes should be inspected at least twice per year along with routine maintenance. Check for wear, adjustments, leaks, etc.
Can warped drums cause problems?
Yes, warped drums can lead to brake pulsation and vibration when slowing down. The drums need to be machined or replaced if excessively warped.
What maintenance is required on drum brakes?
Frequent inspection for wear and proper adjustment. Replacement of shoes, drums, cylinders, and springs once they are excessively worn. Proper lubrication of contact points.