A commercial motor vehicle driver has calculated a safe speed of 40\text{ mph} for an upco… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial motor vehicle driver has calculated a safe speed of $40\text{ mph}$ for an upcoming steep mountain descent. Arrange the following steps in the correct chronological sequence to properly execute downgrade speed control and snub braking.

1Select and shift into a transmission gear low enough to provide effective engine braking before starting down the grade.
2Allow the vehicle speed to build up to the predetermined safe speed of $40\text{ mph}$ .
3Apply the foundation service brakes firmly enough to feel a definite slowdown until speed drops to $35\text{ mph}$ .
4Release the service brakes completely to allow the brake drums and linings to cool while speed gradually increases back to $40\text{ mph}$ .

Answer

The correct chronological sequence is: (1) Select and shift into a proper low gear before starting down the grade, (2) Allow vehicle speed to reach the predetermined safe speed of

40\text{ mph}

, (3) Apply the service brakes firmly until speed drops to

35\text{ mph}

(

5\text{ mph}

below safe speed), and (4) Release the service brakes completely to allow cooling until speed reaches

40\text{ mph}

again.

The standard snub braking method requires shifting into the correct low gear before cresting the hill, letting vehicle speed reach the safe speed (

40\text{ mph}

), applying service brakes firmly to reduce speed by

5\text{ mph}

(to

35\text{ mph}

), and then completely releasing the brakes until speed increases back to the safe threshold.

Step-by-Step Solution

Pre-select the appropriate low transmission gear prior to descending the crest.

Engine compression provides primary retarding force, preventing excessive gravity-induced acceleration.

Shifting on a downgrade is extremely hazardous because gear synchronization may fail, leaving the vehicle in neutral.

Monitor speed as gravity accelerates the combination to the established safe speed.

The vehicle reaches

40\text{ mph}

without premature friction brake application.

Braking too early or riding the brakes causes heat buildup without allowing cooling intervals.

Apply the foundation brakes with steady pressure for roughly 3 seconds to reduce speed by

5\text{ mph}

Vehicle speed drops from

40\text{ mph}

down to

35\text{ mph}

Firm, brief brake application achieves required deceleration quickly while minimizing continuous friction heating.

Fully release the brake pedal once speed drops to

35\text{ mph}

Brake linings and drums disengage and cool as speed slowly climbs back to

40\text{ mph}

The cooling phase is essential to prevent thermal brake fade and total brake failure on long downgrades.

Key Concept

Snub braking procedure and pre-downgrade gear selection for steep descents

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic