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Difficulty: HardSpeed on Curves and Downgrades

A commercial motor vehicle is traveling down a prolonged grade that contains several consecutive sharp curves. Which combination of operating techniques is essential for the driver to maintain directional stability and prevent brake overheating?

  1. A
    Remain in cruising gear to preserve driveline momentum, applying light, continuous service brake pressure throughout the entire downgrade and releasing the pedal only while navigating the curves.
  2. Select a low transmission gear before starting down the grade, decelerate to a safe speed before entering each curve, and apply intermittent snub braking rather than continuous pedal pressure.Answer
  3. C
    Maintain normal downhill speed until entering each curve, applying firm foundation braking mid-turn to load the front suspension and increase steer-axle tire grip.
  4. D
    Rely primarily on auxiliary retarders at maximum intensity during wet curve transitions and downshift across multiple gears midway down the grade if vehicle momentum builds.

Answer

Selecting a low transmission gear before starting down the grade, decelerating to a safe speed before entering each curve, and applying intermittent snub braking.
The correct procedure mandates downshifting into the proper low gear before cresting the grade so that engine compression acts as the main holding force. Speed must always be reduced before entering curves to prevent dangerous lateral forces on high center-of-gravity vehicles. On the downgrade, speed control must be maintained through snub braking (braking firmly to 5 mph5\text{ mph} below safe speed, then fully releasing) to allow brake components cooling time and prevent thermal brake fade.

Step-by-Step Solution

1
Select the correct low gear before the descent begins.
Engine compression provides natural retarding power, establishing the primary speed control mechanism.
Once descent momentum begins on a steep slope, attempting to downshift can result in getting stuck in neutral and complete loss of engine braking.
2
Decelerate to well below the maximum safe speed prior to entering curve radii.
Centrifugal force is kept within safe mechanical and tire-friction thresholds.
Commercial vehicles have high centers of gravity; braking inside the curve overloads tire traction and dramatically increases rollover or drive-wheel skid potential.
3
Execute proper snub braking on downhill segments.
Brakes are applied firmly until speed drops 5 mph5\text{ mph} below the established safe speed, then released completely.
Intermittent snub braking gives drums and linings periodic cooling cycles, preventing glaze formation and thermal brake fade.

Key Concept

Downgrade Speed Management and Curve Negotiation
Estimated Time:1m 30s
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Topics

Speed on Curves and Downgrades

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