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Question

Difficulty: EasyMountain Driving and Steep Downgrades

Commercial motor vehicles descending steep mountain grades generate significant heat within their foundation brakes. What condition occurs when excessive heat causes brake drums to expand and brake linings to lose their friction, resulting in severely diminished stopping power?

  1. A
    Drive-wheel skidding resulting from automatic retarder lockup
  2. Brake fade and a dangerous reduction in braking effectivenessAnswer
  3. C
    Normal brake cooling that allows continuous light pedal application without downshifting
  4. D
    Instant tire pressure loss requiring an immediate hard emergency stop

Answer

Brake fade and a dangerous reduction in braking effectiveness
Brake fade is the direct result of excessive heat generated during long downgrades. When heat builds up, brake drums expand away from the shoes and the friction coefficient of the linings drops drastically, requiring more pedal force for less stopping power until braking ability is lost.

Step-by-Step Solution

1
Identify the physical effects of repeated or continuous braking on steep downgrades.
Excessive friction generates extreme heat, causing brake drums to expand outward and altering the chemical friction properties of brake linings.
Understanding the thermal limits of commercial vehicle brake drums is critical for mountain descent safety.
2
Determine the mechanical consequence of drum expansion and lining friction loss.
Brake shoes must travel farther to contact expanded drums, contact pressure decreases, and overall stopping capability diminishes.
This condition is defined in commercial vehicle operation as brake fade.
3
Identify the primary operational countermeasure.
Select the proper low gear before descent and use intermittent snub braking to allow cooling.
Engine braking must serve as the primary speed retardant to prevent thermal brake failure.

Key Concept

Brake fade occurs on steep downgrades when extreme heat expands brake drums and reduces lining friction.
Estimated Time:45s
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Topics

Mountain Driving and Steep Downgrades

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