A drum brake slows a vehicle by pressing curved brake shoes outward against the inside of a rotating metal drum. The drum is attached to the wheel, so friction inside the drum reduces the wheel’s rotational motion. Drum brakes matter because they are common on rear wheels, parking brakes, trailers, and heavy vehicles.
Understanding them helps students connect force, friction, heat, and mechanical advantage in a real vehicle system.
When the driver presses the brake pedal, hydraulic pressure pushes pistons in the wheel cylinder outward. The pistons spread the brake shoes until their friction lining rubs the inner surface of the drum. This contact creates a braking torque opposite the wheel’s rotation and converts kinetic energy into thermal energy.
Springs pull the shoes back when the pedal is released, and adjusters help keep the shoe to drum gap small as the linings wear.
Key Facts
- Braking force comes from friction: Ff = μN, where μ is the coefficient of friction and N is the normal force between shoe and drum.
- Braking torque is torque from friction: τ = rF, where r is the drum radius and F is the tangential friction force.
- The wheel cylinder changes hydraulic pressure into force: F = PA, where P is brake fluid pressure and A is piston area.
- Energy is not destroyed during braking. The vehicle’s kinetic energy is mostly converted into heat in the drum and shoes.
- Return springs pull the brake shoes away from the drum when hydraulic pressure drops.
- An adjuster keeps the shoes close to the drum so the pedal does not travel too far before braking begins.
Vocabulary
- Brake drum
- A rotating metal cylinder attached to the wheel that provides the inner surface for the brake shoes to rub against.
- Brake shoe
- A curved metal part with friction lining that presses outward against the inside of the drum.
- Wheel cylinder
- A small hydraulic cylinder that uses brake fluid pressure to push the brake shoes apart.
- Friction lining
- The high friction material on the brake shoe that grips the drum and converts motion into heat.
- Return spring
- A spring that pulls the brake shoes back inward after the driver releases the brake pedal.
Common Mistakes to Avoid
- Thinking the drum squeezes inward on the shoes is wrong because in a drum brake the shoes press outward against the inside surface of the rotating drum.
- Ignoring heat buildup is wrong because braking converts kinetic energy into thermal energy, and too much heat can reduce friction and cause brake fade.
- Using only force and forgetting radius is wrong because braking effect depends on torque, so the same friction force produces more stopping torque at a larger drum radius.
- Assuming worn shoes only make brakes quieter or louder is wrong because worn linings increase the shoe to drum gap and can increase pedal travel or reduce braking performance.
Practice Questions
- 1 A wheel cylinder has a piston area of 3.0 cm^2 and the brake fluid pressure is 2.0 MPa. What force does the piston exert on one brake shoe? Convert cm^2 to m^2 before calculating.
- 2 A drum brake produces a tangential friction force of 900 N at an effective drum radius of 0.14 m. What braking torque does it apply to the wheel?
- 3 Explain why a drum brake can feel weaker after repeated hard stops down a hill, even if the pedal force from the driver stays the same.