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Difficulty: MediumMountain Driving and Steep Downgrades

Prior to starting down an unfamiliar 5-mile5\text{-mile}, 7%7\% mountain grade while hauling 68000 lbs68{}000\text{ lbs} in wet weather conditions, what combination of operational procedures should a driver execute to safely manage vehicle speed and prevent foundation brake failure?

  1. A
    Downshift the transmission only after the vehicle gains excessive speed halfway down the slope, keeping the service brakes continuously applied to hold the vehicle at exactly the safe speed threshold.
  2. B
    Maintain normal highway cruising gear to conserve momentum, engage the auxiliary retarder at maximum capacity to supplement braking on the wet surface, and apply continuous light pressure to the service brake pedal.
  3. Select a transmission gear low enough to provide effective engine braking before starting down, switch off auxiliary retarders to avoid a drive-wheel skid on wet pavement, and utilize snub braking if road speed reaches the safe speed.Answer
  4. D
    Shift the transmission into neutral while descending straight segments to cool the drivetrain, relying on periodic aggressive brake pedal applications only before entering curves.

Answer

Select a low transmission gear before beginning the descent to utilize engine braking, disengage auxiliary retarders due to wet road conditions, and apply snub braking to control speed as necessary.
Selecting the proper low gear before cresting the hill establishes engine compression braking as the principal retarding force. Turning off auxiliary retarders is mandatory on wet surfaces to eliminate the danger of drive-wheel lockup. Using snub braking (applying firm pressure to drop speed 5 mph5\text{ mph} below safe speed, then releasing) provides essential cooling intervals for foundation brakes.

Step-by-Step Solution

1
Select the appropriate low gear before cresting or starting the descent.
The engine compression provides primary speed control, minimizing reliance on mechanical service brakes.
Attempting to downshift once descending can result in missing a gear, leaving the transmission in neutral.
2
Turn off or disengage all engine retarders on wet or slick road surfaces.
The drive wheels maintain traction without being locked up by auxiliary retarding force.
Retarders apply retarding force only to the drive axles, which can break traction and cause a sudden jackknife on slippery pavement.
3
Apply the proper snub braking technique when road speed exceeds the established safe speed.
The vehicle slows by approximately 5 mph5\text{ mph} below the safe speed in about 3 seconds3\text{ seconds}, after which brakes are released.
Releasing the brake pedal allows brake drums and linings to dissipate heat, preventing thermal expansion and catastrophic brake fade.

Key Concept

Steep downgrade speed management, pre-crest gear selection, retarder safety in adverse weather, and snub braking technique.
Estimated Time:1m 15s
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Topics

Mountain Driving and Steep Downgrades

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