If drum brakes overheat, the vehicle's stopping distance will increase. What is the reason for this?
- The drums will expand. ✓
- Friction between the tire and the pavement surface will decrease.
- The wheels will lock up.
- Air pressure will drop.
Answer: A. The drums will expand.
When drum brakes overheat, the brake drums expand, which is why stopping distance increases.
Why this is the answer
The handbook explains that cast iron brake drums expand when they get hot. As the drum grows, it moves away from the brake shoes, so the shoes no longer press against it the way they should. That loss of contact lowers braking efficiency, and if the brake chamber bottoms out, braking power drops sharply. Less braking power means the vehicle takes longer to stop, which is exactly why overheated drums increase stopping distance.
Why the other answers are wrong
- B. Friction between the tire and the pavement surface will decrease. — The passages describe the drum moving away from the shoes, not any change in grip between the tire and the road surface.
- C. The wheels will lock up. — The passages never say overheated drums make the wheels lock up; they say braking efficiency falls off.
- D. Air pressure will drop. — Air pressure dropping is not given as the reason in the passages; the cause described is the heated drum expanding.
Remember it
Picture the hot drum swelling and pulling away from the shoes, so the shoes have less to grab.