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Difficulty: HardSpeed on Curves and Downgrades

When preparing to descend a steep mountain downgrade, a commercial motor vehicle operator should select the exact same transmission gear that would be used to climb that same grade.

Answer: Answer

Answer

False. Modern commercial motor vehicles must typically be shifted into a lower gear for descending a downgrade than what would be used to climb the same hill.
The statement is false because modern commercial trucks are designed with powerful engines and low aerodynamic drag, allowing them to climb steep hills in higher gears. Descending in that same gear provides insufficient engine braking resistance against gravity, leading to brake overheating and fade. Safe operating procedure dictates selecting a lower gear to descend than to climb.

Step-by-Step Solution

1
Analyze vehicle mechanics and aerodynamic advances in modern commercial vehicles.
Modern trucks possess high-horsepower engines and streamlined profiles, allowing them to pull heavy loads uphill in relatively high gears.
Understanding why the historical 'same gear up, same gear down' guideline is no longer valid for modern equipment.
2
Evaluate the downhill braking requirement versus engine retarding capacity.
Descending in an uphill climbing gear results in insufficient engine retarding effect, requiring continuous service brake application against gravitational acceleration.
Engine braking must serve as the primary speed control on downgrades to prevent thermal overload of the brake linings.
3
Apply standard commercial driving safety regulations for downgrade gear selection.
The driver must downshift to a lower gear than needed for climbing before beginning the descent.
Choosing a lower gear maximizes engine compression retarding force and maintains the vehicle at its safe speed without excessive brake heating.

Key Concept

Downgrade Transmission Gear Selection and Engine Retarding Dynamics
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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