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Difficulty: HardMountain Driving and Steep Downgrades

A commercial vehicle combination grossing 78,000 lbs78,000\text{ lbs} is approaching the summit of a 5-mile5\text{-mile}, 7%7\% downgrade during a steady rainstorm. Which combination of pre-descent configuration and speed control techniques should the driver execute to safely navigate the downgrade without causing brake fade or traction loss?

  1. A
    Keep the auxiliary engine retarder on its highest setting to spare foundation brakes, and shift to higher gears as engine RPM approaches the maximum threshold.
  2. B
    Proceed over the summit in cruising gear to evaluate downhill momentum, and double-clutch into a lower gear only after road speed exceeds the posted truck advisory limit.
  3. Shift into the proper low gear before cresting the summit, turn off the engine retarder due to wet pavement, and utilize snub braking to keep speed within the safe envelope.Answer
  4. D
    Maintain cruising gear while applying continuous, light brake pedal pressure throughout the descent to hold a steady speed without cycling the brakes.

Answer

Shift into the proper low gear before cresting the summit, turn off the engine retarder due to wet pavement, and utilize snub braking to keep speed within the safe envelope.
Safe mountain descent requires pre-selecting a low transmission gear before beginning the downgrade so that engine compression serves as the principal braking mechanism. Auxiliary retarders must be switched off on wet surfaces to avoid drive-wheel skids, and foundation brakes must be used intermittently via snub braking to manage speed without overheating the brake components.

Step-by-Step Solution

1
Select and engage the appropriate low gear before the vehicle crests the summit and starts down the grade.
The engine compression provides the primary retarding force necessary to control downhill speed.
Gravity pulls a heavy load rapidly down steep grades. Downshifting must occur prior to the descent because high momentum prevents matching engine RPM once speed builds, creating a high risk of getting stuck in neutral.
2
Switch off auxiliary retarders (such as engine brakes, exhaust brakes, or hydraulic retarders) because the road surface is wet.
The drive wheels are protected from sudden loss of lateral traction.
Retarder braking force is applied exclusively to drive axles; on wet, icy, or snow-covered roads, this sudden drag can instantly break tire adhesion and induce a drive-wheel skid or jackknife.
3
Apply snub braking (apply firm pressure to slow approximately 5 mph5\text{ mph} below the safe speed, then fully release the pedal) whenever road speed reaches the designated safe speed.
Vehicle speed remains controlled while allowing the brake drums and linings intermittent cooling intervals.
Continuous light braking builds extreme heat that causes mechanical brake fade, whereas snub braking allows cooling periods between cycles.

Key Concept

Steep Downgrade Speed Management and Wet Weather Braking
Estimated Time:2m 0s
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Topics

Mountain Driving and Steep Downgrades

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