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Difficulty: EasyMountain Driving and Steep Downgrades

You are operating a loaded commercial motor vehicle down a steep 6% mountain downgrade with a designated safe speed of 35 mph35\text{ mph}. Which of the following procedures describes the correct braking method (snub braking) to maintain vehicle control and prevent brake fade?

  1. A
    Apply light, continuous pressure on the service brake pedal throughout the entire descent to hold vehicle speed steadily at 35 mph35\text{ mph}.
  2. Apply the brakes firmly enough to reduce vehicle speed to 30 mph30\text{ mph} (5 mph5\text{ mph} below safe speed), then completely release the brakes until speed reaches 35 mph35\text{ mph} before repeating.Answer
  3. C
    Shift the transmission into neutral to coast downhill while pumping the brake pedal rapidly to keep the air tanks pressurized.
  4. D
    Remain in the highest gear and rely entirely on maximum retarder settings without using the service brakes, even on slippery road surfaces.

Answer

Apply the brakes firmly enough to slow the vehicle to 5 mph below the safe speed, then release the brakes completely until speed returns to the safe speed before repeating the cycle.
Proper snub braking consists of applying the service brakes firmly until speed drops 5 mph5\text{ mph} below the designated safe speed (from 35 mph35\text{ mph} to 30 mph30\text{ mph}), and then completely releasing the pedal. This allows the brake drums and linings adequate cooling intervals between applications, preventing thermal brake fade.

Step-by-Step Solution

1
Determine and establish the safe speed based on vehicle weight, length and steepness of grade, road conditions, and weather.
Safe downhill operating speed set at 35 mph35\text{ mph} in an appropriate low gear.
Setting a safe speed before the descent provides the baseline for vehicle control and engine braking.
2
Allow the vehicle in low gear to reach the safe speed of 35 mph35\text{ mph}, then apply the service brakes with moderate, firm pressure.
Speed decreases to 30 mph30\text{ mph} (5 mph5\text{ mph} below safe speed) in approximately 3 seconds.
Firm application slows the vehicle effectively without dragging the brake linings continuously.
3
Release the service brakes completely once the vehicle slows to 30 mph30\text{ mph}.
Vehicle slowly accelerates under gravity back toward 35 mph35\text{ mph}.
Releasing the brake shoes from the drums allows airflow to dissipate heat, preventing brake fade.
4
Repeat the firm application and release cycle whenever vehicle speed reaches 35 mph35\text{ mph}.
Vehicle maintains safe speed throughout the downgrade without overheating the brakes.
Cyclical snub braking is the proven technique mandated by FMCSA regulations for safe downgrade descent.

Key Concept

Proper Snub Braking Technique on Steep Downgrades
Estimated Time:45s
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Topics

Mountain Driving and Steep Downgrades

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