Brake Drum

The brake drum is the primary rotating component of a drum brake system and was once the standard across automotive braking setups. While modern passenger cars have predominantly adopted front disc brakes, brake drums remain widely utilised on the rear axles of certain passenger models and in heavy commercial vehicles due to their cost efficiency and inherent self-energising (self-servo) properties.

Definition and Key Role

A brake drum is a cylindrical component cast from iron that rotates synchronously with the wheel. Upon braking, the brake shoes are pushed outwards by the wheel cylinder to contact the drum's inner friction surface, generating the necessary stopping power. Its primary engineering advantage over disc brakes lies in its notable "self-servo" effect, which allows it to deliver substantial braking torque with relatively light actuation force.

Structural Components

Brake Drum: The rotating unit cast from grey iron, with a hardened friction surface engineered to resist wear and tear. 

Brake Shoes: Curved metal assemblies fitted with friction linings, housed inside the brake drum.

Wheel Cylinder: Utilises hydraulic fluid pressure to force the brake shoes outwards against the drum's inner wall. 

Return Springs: Ensure the brake shoes swiftly retract from the drum wall once hydraulic pressure is released, preventing brake drag. 

Self-Adjuster: Automatically compensates for lining wear to maintain proper shoe-to-drum clearance and ensure consistent brake pedal travel.

Working Principle

The self-servo principle delivers distinct operating characteristics for drum brakes:

Hydraulic Actuation: Depressing the brake pedal forces hydraulic fluid into the wheel cylinder, actuating its internal pistons.

Frictional Expansion: The wheel cylinder pistons push the brake shoes outwards toward the rotating drum.

Self-Servo Action: As the linings make contact with the rotating drum, frictional drag wedges the leading shoe further into the drum wall. This mechanical wedging action creates a self-amplifying braking effect, keeping initial pedal effort light.

Release and Retraction: When the pedal is released, the return springs pull the shoes back to their resting position, disengaging them from the drum wall. 

Common Faults and Diagnostics 

Brake Fade (Thermal Fade): Because the drum is an enclosed assembly, its heat dissipation is significantly inferior to that of a disc brake. Prolonged or heavy braking on steep downhill descents causes intense heat build-up, reducing the lining friction coefficient and leading to rapid brake fade.

Abnormal Brake Noise: Heavy dust accumulation inside the drum or worn-out linings exposing metal backing plates will produce high-pitched screeching or metallic grinding noises under braking. 

Brake Drag or Seizure: If the return springs weaken or the wheel cylinder pistons corrode and seize, the shoes will fail to fully retract. This causes rolling resistance, increases fuel consumption, and leads to severe drum overheating.

Excessive Brake Pedal Travel: Typically caused by an inoperative or seized self-adjuster resulting in excessive clearance between the shoes and drum, requiring manual cleaning and readjustment. 

Maintenance and Replacement Guidelines

Drum Skimming Limits: Much like brake disc skimming, a brake drum that has worn beyond its maximum allowable inside diameter (Max ID) must be replaced. Indiscriminate machining weakens the drum wall, compromising structural strength and risking catastrophic cracking. 

Dust Servicing: Because drum brakes are enclosed, friction dust inevitably accumulates inside. Thoroughly clearing this dust during shoe replacements or routine servicing is essential to restore responsiveness and eliminate brake squeal. 

Axle-Pair Replacement: Brake shoes must always be replaced in full axle sets. It is also recommended to inspect the return springs at the same time to prevent uneven braking caused by spring fatigue.

Care and Maintenance

Periodic Dust Removal: Given that drum brakes trap brake dust easily, removing the drums for cleaning at regular intervals (such as every 20,000 km) is crucial to maintaining crisp pedal feel.

Handbrake Adjustment: On most rear-drum applications, the drum mechanism also operates as the handbrake. If handbrake lever travel feels excessively long, it generally indicates that the internal shoe clearance requires adjustment.

Avoid Thermal Shock from Cold Water: Spraying cold water onto a scorching hot brake drum (such as during car washing right after hard driving) can warp the drum, causing brake shudder and pedal pulsation.

Industry Trends

While drum brakes are gradually being phased out of mainstream passenger cars, they remain irreplaceable in specific roles: 

Heavy Commercial Vehicles: Thanks to their high mechanical braking torque and rugged durability, they provide exceptional reliability on heavy lorries tackling demanding downhill routes.

Rear Braking in Compact Cars: On entry-level and budget-focused models where rear braking loads are minimal compared to the front, drum brakes deliver ample stopping power at exceptionally low maintenance costs.

Electrified Vehicle (NEV) Integration: Selected electrified models utilise a drum-in-disc setup, taking advantage of the drum mechanism's superior parking-lock holding force to secure heavier vehicle kerb weights safely on inclines.