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Difficulty: MediumMountain Driving and Steep Downgrades

After establishing a safe maximum speed of 30 mph30\text{ mph} before starting down a long, steep mountain grade in low gear, the vehicle accelerates under gravity to 30 mph30\text{ mph}. What specific braking action must the driver take to control vehicle speed effectively while preventing brake fade?

  1. A
    Maintain continuous light pressure on the service brake pedal throughout the entire descent to hold the speed steadily at 30 mph30\text{ mph}.
  2. B
    Pump the brake pedal rapidly in light, repetitive taps to scrub off 1 to 2 mph1\text{ to }2\text{ mph} while maintaining partial shoe contact with the drums.
  3. Apply the brakes firmly for approximately 3 seconds until speed drops to 25 mph25\text{ mph}, then release the pedal completely until speed reaches 30 mph30\text{ mph} again.Answer
  4. D
    Engage engine retarders at maximum strength while applying continuous foot brake pressure to keep the vehicle from ever dropping below 30 mph30\text{ mph}.

Answer

Apply the brakes firmly to reduce speed to 5 mph5\text{ mph} below the designated safe speed (down to 25 mph25\text{ mph}), then release the pedal completely until speed rises back to 30 mph30\text{ mph}.
The snub braking method is the only FMCSA-approved service braking technique for descending steep mountain grades. When the vehicle reaches the designated safe speed, the driver must apply the brakes firmly enough to feel a distinct slowdown until the speed is reduced to 5 mph5\text{ mph} below the safe speed (taking approximately 3 seconds), then release the brakes completely until the speed builds back up to the safe speed.

Step-by-Step Solution

1
Determine the proper target reduction speed based on the established safe speed.
The target reduction speed is 25 mph25\text{ mph}, exactly 5 mph5\text{ mph} below the 30 mph30\text{ mph} safe speed.
The standard snub braking procedure mandates slowing the commercial motor vehicle to 5 mph5\text{ mph} below safe speed.
2
Apply the service brakes with firm, steady pressure.
Vehicle speed decreases from 30 mph30\text{ mph} to 25 mph25\text{ mph} in approximately 3 seconds.
Firm application provides definitive deceleration without dragging the brakes over extended distances.
3
Release the brake pedal completely once the vehicle reaches 25 mph25\text{ mph}.
The vehicle coasts in the selected low gear while gravity gradually increases speed back to 30 mph30\text{ mph}.
Releasing the pedal fully allows air circulation and heat dissipation from the brake drums and linings before the next application.

Key Concept

Proper execution of the snub braking technique on steep mountain downgrades.
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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