---
title: "Disc brakes and drum brakes"
subject: "Automotive Technology"
chapter: 7
key: "semester_5/automotive_technology:disc-brake-vs-drum-brake"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/disc-brake-vs-drum-brake
site: BCT Engineering
---

# Disc brakes and drum brakes

Automotive Technology · Chapter 7: Braking Systems

- Asked in: year not given, 6 marks
- Video: [Disc vs Drum Brakes: Which is Better & how do they work?](https://www.youtube.com/watch?v=ixNTRIYt0LI) — Simmo's Home Workshop
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/automotive_technology/disc-brake-vs-drum-brake
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-7

## Question

Describe the construction of a disc brake and a drum brake (leading-trailing shoe type) and differentiate between them.

## Short answer

A disc brake uses a caliper with pads clamping a rotating disc, offering good cooling and fade resistance but no self-energising. A drum brake uses a wheel cylinder to expand shoes against a rotating drum; the leading shoe self-energises, reducing pedal effort, but heat dissipation is poor and wet performance suffers.

## Answer

#### Disc brake

```text
   caliper (fixed to knuckle)
   |  [pad] |disc| [pad]  |
   |<- piston pushes pads against the rotating disc ->|
```

- A cast-iron **disc (rotor)** rotates with the wheel hub (vented in front brakes).
- A **caliper**, mounted on the steering knuckle, holds two friction pads and one or two hydraulic pistons. Fluid pressure pushes the pads against both faces of the disc.
- Floating calipers have a piston on one side only; fixed calipers have pistons on both sides.

#### Drum brake

```text
   brake drum (rotates with wheel)
     shoe A (leading)   shoe B (trailing)
        \  wheel cylinder  /
   anchor pin  +  return springs
```

- A cast-iron **drum** rotates with the wheel. Two curved **shoes** with friction linings are on a fixed backing plate.
- The wheel cylinder pushes the shoes outward against the inside of the drum. In a leading-trailing design, the leading shoe is self-energised by drum rotation.
- Return springs retract the shoes; an adjuster compensates for wear; a handbrake link is easy to add.

#### Comparison

| Point | Disc brake | Drum brake |
|---|---|---|
| Heat dissipation | Good (open to air) | Poor (enclosed) |
| Fade resistance | High | Low (drum expands) |
| Wet performance | Good (water thrown off) | Poor |
| Self-energising | None; needs higher force or booster | Present; less force needed |
| Wear check / service | Easy | Harder |
| Weight and cost | Light for performance, costlier | Heavy, cheaper |
| Parking brake | Difficult | Easy |
| Use | Front wheels of nearly all cars, all wheels in sports cars | Rear wheels of small cars, trucks |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where is the caliper of a disc brake mounted and what does it hold?
   A. On the steering knuckle, holding two friction pads and hydraulic piston(s)
   B. On the axle, holding the wheel cylinder only
   C. On the backing plate, holding two curved shoes
   D. On the drum, holding a return spring
   Answer: A. The caliper on the knuckle carries two pads and one or two pistons that clamp the rotating disc. (Key point: Disc brake construction)

2. Which brake is self-energising and what does this mean?
   A. Disc brake; the booster is not needed
   B. Drum brake; the leading shoe is dragged on by drum rotation, so less force is needed
   C. Both brakes equally
   D. Disc brake; pads press harder as the disc heats
   Answer: B. In a leading-trailing drum brake the leading shoe is pulled into the drum by rotation. Disc brakes have no self-energising and need more force. (Key point: Self-energising)

3. Why does a disc brake resist fade better than a drum brake?
   A. It uses lighter friction linings
   B. It has return springs
   C. It is open to the air so it dissipates heat well, while a drum expands away from the shoes
   D. It is enclosed, so heat is trapped
   Answer: C. Open discs cool well, while the enclosed drum heats and expands, which reduces braking (fade). (Key point: Fade and heat)

4. Where are disc and drum brakes typically used on cars?
   A. Disc only in trucks
   B. Drum everywhere in sports cars
   C. Disc at front wheels of nearly all cars; drum at rear of small cars and trucks
   D. Disc at the rear of small cars; drum at the front
   Answer: C. Discs are used at the front (most braking), while drums, cheaper and easy for a parking brake, are used at the rear of small cars and trucks. (Key point: Use of each)

## Related in Automotive Technology

- [Stopping distance and brake energy](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abraking-distance-energy-numerical) — Automotive Technology, chapter 7
- [Tyre construction, types and factors affecting life](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atyre-construction-bias-radial-life) — Automotive Technology, chapter 8

## Related in other subjects

- [Internal expanding shoe brake analysis](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Ainternal-expanding-shoe-brake) — Machine Design II, chapter 7
- [Brake energy and drum temperature rise](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Abrake-energy-absorbed-temperature-rise) — Machine Design II, chapter 7
- [Clutch versus brake and self-energising](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Aclutch-brake-differences-self-energizing) — Machine Design II, chapter 7
- [Dynamometers and rope brake power](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Adynamometer-types-rope-brake-numerical) — Instrumentation and Measurement, chapter 7