Where is the caliper of a disc brake mounted and what does it hold?

On the steering knuckle, holding two friction pads and hydraulic piston(s)

On the axle, holding the wheel cylinder only

On the backing plate, holding two curved shoes

On the drum, holding a return spring

Which brake is self-energising and what does this mean?

Drum brake; the leading shoe is dragged on by drum rotation, so less force is needed

Disc brake; the booster is not needed

Disc brake; pads press harder as the disc heats

Why does a disc brake resist fade better than a drum brake?

It is open to the air so it dissipates heat well, while a drum expands away from the shoes

It uses lighter friction linings

It is enclosed, so heat is trapped

Where are disc and drum brakes typically used on cars?

Disc at front wheels of nearly all cars; drum at rear of small cars and trucks

Disc at the rear of small cars; drum at the front

Practice · 6 marks

Disc brakes and drum brakes

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.

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

Disc brake

   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

   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

✓ Answer checked by Claude Opus 5.5 (high effort)

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