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Difficulty: MediumMountain Driving and Steep Downgrades

A commercial motor vehicle driver is preparing to descend a steep 6%6\% mountain downgrade with a fully loaded vehicle. In what chronological sequence should the driver execute the proper procedures to maintain vehicle control and prevent brake fade throughout the descent?

  1. 1Shift the transmission into the proper low gear before reaching the crest and beginning the downgrade.
  2. 2Allow the vehicle to accelerate under engine braking until road speed reaches the designated safe speed.
  3. 3Apply the service brakes with firm, steady pressure until speed drops to 5 mph5\text{ mph} below the safe speed.
  4. 4Release the service brakes completely once vehicle speed has been reduced.
  5. 5Repeat the snub braking cycle each time vehicle speed climbs back up to the designated safe speed.

Answer

The correct procedural order begins with selecting and shifting into the proper low gear before cresting the hill, followed by letting the vehicle accelerate to its safe speed, applying firm brake pressure until speed is 5 mph5\text{ mph} below safe speed, completely releasing the brakes to allow cooling, and finally repeating this snub braking cycle whenever the vehicle accelerates back to the safe speed.
Safe mountain descent requires pre-descent gear selection to harness engine compression braking, followed by strict adherence to the snub braking technique: allowing speed to reach safe speed, braking firmly to 5 mph5\text{ mph} below safe speed, fully releasing the brakes for cooling, and repeating this sequence as needed.

Step-by-Step Solution

1
Select and shift the transmission into the appropriate low gear before starting down the grade.
Maximum engine braking retarding force is engaged before gravity takes effect.
Modern transmissions cannot reliably be shifted into a lower gear once vehicle momentum and downhill gravity have built up.
2
Monitor speed as the vehicle descends until it reaches the pre-determined safe speed.
Identifies the exact threshold where supplemental friction braking is required.
Operating at or below the safe speed ensures the driver maintains full steering and directional control.
3
Apply the service brakes with steady, firm pressure to drop speed to 5 mph5\text{ mph} below safe speed.
Speed drops by 5 mph5\text{ mph} in approximately three seconds.
Snub braking produces short, effective deceleration cycles rather than constant heat-generating friction.
4
Release the service brakes completely once the target reduced speed is reached.
Brake linings disengage from drums or rotors, allowing heat dissipation.
Continuous light pressure creates continuous friction and causes brake fade or total failure.
5
Allow speed to rise back to the safe speed and repeat the cycle for the remainder of the downgrade.
Maintains an alternating sawtooth speed profile within a safe 5 mph5\text{ mph} range.
Repeating the cycle balances vehicle speed control with necessary brake cooling periods.

Key Concept

Steep Downgrade Speed Management and Snub Braking Sequence
Estimated Time:1m 30s
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Topics

Mountain Driving and Steep Downgrades

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