If drum brakes overheat, the vehicle's stopping distance will increase. What is the reason for this?
- The drums will expand. ✓
- Air pressure will drop.
- The wheels will lock up.
- Friction between the tire and the pavement surface will decrease.
Answer: A. The drums will expand.
The drums expand as they overheat, so the brake shoes may lose contact with them and braking weakens.
Why this is the answer
The handbook says that when brake shoes press on drums, friction makes heat, and too much heat can cause brake fade or failure. It specifically states that as brake drums overheat they expand, to the point where they may not even touch the shoe. With less or no contact between shoe and drum, the brakes cannot produce the same stopping force, so the vehicle takes longer to stop. That is why the correct reason is drum expansion, not a change in tire grip or air pressure.
Why the other answers are wrong
- B. Air pressure will drop. — Air pressure dropping is not the reason given for overheated drum brakes; the passage ties the loss of braking to the drum expanding away from the shoe.
- C. The wheels will lock up. — The passages do not say overheating makes the wheels lock up; they describe brake fade and the drum losing contact with the shoe.
- D. Friction between the tire and the pavement surface will decrease. — The passage says heat is produced at the brake drum or rotor, and it does not say overheating reduces friction between the tire and the pavement; the loss of contact is between the shoe and the drum.
Remember it
Think of heat making the drum grow away from the shoes.