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Question

Difficulty: MediumMountain Driving and Steep Downgrades

During an extended descent on a steep downgrade, excessive reliance on the foundation service brakes can cause severe brake fade. Which mechanical condition directly causes drum brakes to fade and lose stopping power under excessive heat?

  1. A
    Continuous friction generates heat that welds the brake linings directly to the drums, locking the wheels into a skid.
  2. Brake drums expand outward from extreme heat, requiring shoes to move farther and reducing contact friction.Answer
  3. C
    Pressurized air in the brake lines overheats and vents through governor check valves, resulting in zero application pressure.
  4. D
    Excess heat triggers the auxiliary retarder to mechanically disconnect the s-cam rollers from the foundation brake shoes.

Answer

Brake drums expand outward from extreme heat, requiring shoes to move farther and reducing contact friction.
Brake fade occurs because excessive heat causes the brake drums to expand outward. As the drums expand, the brake shoes must travel farther to make contact, requiring more pushrod stroke than is available and drastically reducing the friction force between the linings and drums.

Step-by-Step Solution

1
Identify the primary source of brake fade during continuous downgrade braking.
Excessive friction between the brake shoes and drum generates intense thermal energy.
Foundation brakes convert vehicle kinetic energy into heat energy, which accumulates rapidly during prolonged use on downgrades.
2
Analyze the physical effect of intense heat on the brake drum metal.
The cylindrical brake drum experiences thermal expansion, widening its internal diameter outward.
Metals expand when heated, increasing the clearance between the friction surfaces.
3
Determine the operational consequence on braking capability.
The brake shoes must travel farther than normal pushrod stroke allows, resulting in glazed linings and reduced friction force.
When pushrod travel reaches its mechanical limit or linings glaze, the system cannot apply sufficient clamping force to slow the vehicle.

Key Concept

Thermal expansion of brake drums and friction lining glazing under excessive downgrade braking
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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