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Difficulty: MediumSpeed on Curves and Downgrades

If a commercial motor vehicle begins to gain excessive speed while descending a steep grade, the driver should depress the clutch and downshift into a lower gear to restore engine braking control.

Answer: Answer

Answer

False. Downshifting must always be completed before initiating a descent; attempting to downshift while already picking up speed down a steep grade can lock the transmission in neutral and eliminate all engine braking.
The statement is false because standard commercial driving procedures mandate selecting and shifting into the proper low gear before starting down any steep grade. Once momentum builds on a descent, depressing the clutch disengages engine resistance, causing rapid acceleration. The resulting speed mismatch prevents the driver from shifting into a lower gear or even returning to the original gear, leaving the transmission in neutral and causing a total loss of engine braking.

Step-by-Step Solution

1
Identify the proper timing for transmission gear selection on steep downgrades.
Safe driving rules require shifting into the correct low gear before cresting the hill and starting down the slope.
The primary control mechanism on a downgrade is engine braking, which requires mechanical connection between the engine and drive wheels.
2
Analyze the mechanical consequence of depressing the clutch while gaining momentum on a grade.
Depressing the clutch disconnects engine drag, allowing gravity to accelerate the vehicle rapidly.
Engine rotational speed and road wheel speed diverge rapidly, making gear synchronization impossible without severe transmission damage or missing the gear entirely.
3
Determine the safety hazard of attempting an in-motion downshift during runaway descent.
The shifter can become stuck in neutral, leaving the driver with zero engine retarding force and relying solely on service brakes.
Relying strictly on service brakes to overcome gravitational acceleration causes extreme thermal overload and catastrophic brake fade.

Key Concept

Pre-crest Gear Selection and Downgrade Control
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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