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Question

Difficulty: MediumSpeed on Curves and Downgrades

A heavy commercial vehicle is approaching the crest of a steep, winding mountain descent. Arrange the following actions in the correct chronological sequence to maintain proper speed control and prevent brake fade throughout the descent.

  1. 1Assess the grade steepness, road curvature, and total vehicle weight to determine a safe maximum operating speed.
  2. 2Shift the transmission into a low gear that provides adequate engine retarding power before initiating the descent.
  3. 3Permit vehicle momentum to increase on the slope until speed reaches the predetermined safe operating speed.
  4. 4Apply firm, steady service brake pressure for roughly three seconds to reduce speed to 5 mph5\text{ mph} below the safe operating speed.
  5. 5Release the brake pedal completely to allow brake cooling while engine braking holds the vehicle until speed builds back to the safe limit.

Answer

The correct operational sequence begins with evaluating conditions to determine a safe maximum speed, downshifting into the proper low gear before starting down the slope, allowing speed to build to the target safe speed, applying firm braking to drop speed 5 mph5\text{ mph} below the safe speed, and finally releasing the brakes completely to allow drum cooling until the cycle repeats.
Proper heavy vehicle downgrade management requires establishing a safe speed based on vehicle weight and grade steepness, followed by selecting the proper gear prior to cresting the hill. During the descent, the snub braking technique is used: letting momentum reach the safe speed, applying firm brake pressure to drop 5 mph5\text{ mph} below the safe speed, and completely releasing the brakes to allow thermal dissipation.

Step-by-Step Solution

1
Evaluate road and vehicle factors to set a safe speed.
Driver establishes a concrete safe maximum speed before entering the descent.
Safe descending speed must account for total vehicle weight, length of grade, road curvature, and weather conditions.
2
Select and engage the appropriate low gear before the slope begins.
Engine compression provides primary retarding force without risk of missing a gear on the downgrade.
Attempting to downshift once vehicle momentum has built on a steep descent can result in getting locked in neutral.
3
Allow the vehicle to roll under engine retarding until reaching the target safe speed.
The vehicle accelerates naturally to the threshold where active braking is needed.
Engine braking slows acceleration, establishing the upper bound of the snub braking envelope.
4
Execute a snub brake application to reduce speed by 5 mph5\text{ mph}.
Vehicle speed decreases from the safe speed to 5 mph5\text{ mph} below it within approximately three seconds.
Short, firm brake applications generate less cumulative friction heat than continuous light pressure.
5
Fully release the service brakes until speed rebuilds to the target safe speed.
Brake components cool down as vehicle momentum slowly builds back to the safe threshold.
Complete release allows air circulation and heat dissipation across drums and linings, preventing thermal brake fade.

Key Concept

Proper Downgrade Speed Control and Snub Braking Sequence
Estimated Time:1m 15s
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Topics

Speed on Curves and Downgrades

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