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Question

Difficulty: MediumStopping and Braking Procedures

A tractor-trailer unit is traveling without cargo on a dry asphalt roadway when traffic suddenly slows ahead. Which of the following statements correctly explains how the unloaded condition influences the vehicle's stopping behavior and braking requirements compared to a fully loaded vehicle?

  1. A
    The unloaded vehicle stops in a significantly shorter distance because lower gross mass minimizes kinetic energy without altering tire traction.
  2. The unloaded vehicle requires a greater stopping distance because reduced axle weight provides less tire traction, making wheels bounce and lock up more readily.Answer
  3. C
    The unloaded vehicle requires activating the engine retarder at maximum setting during all deceleration to simulate cargo weight on the drive axle.
  4. D
    The unloaded vehicle maintains the identical stopping distance because brake chamber pushrod travel determines mechanical braking force regardless of payload.

Answer

The unloaded vehicle requires a greater stopping distance because reduced axle weight provides less tire traction, making wheels bounce and lock up more readily.
According to official CDL guidelines, empty combination vehicles require greater stopping distances than fully loaded vehicles. When a trailer is empty, there is insufficient weight over the axles to maintain optimal tire traction against the road surface. Consequently, the powerful commercial braking system easily causes the tires to bounce and lock up, leading to sliding and significantly extended stopping distances.

Step-by-Step Solution

1
Analyze how vehicle payload affects tire-to-road friction and normal force on commercial vehicle axles.
An empty combination vehicle exerts substantially less downward force on its drive and trailer tires, resulting in reduced contact patch grip.
Braking force is limited by the maximum frictional traction available between the tire tread and the road surface.
2
Evaluate the dynamic behavior of an empty commercial vehicle under brake application.
The brakes easily overpower the available traction, causing wheels to bounce, slide, and lock up more readily.
Standard air brake systems are engineered to stop fully loaded combinations, so unladen wheels experience rapid lockup and wheel hop.
3
Determine the net effect on stopping distance and the required braking procedure.
Stopping distance increases, requiring earlier brake application and smoother brake modulation to avoid skidding.
Locked and sliding tires provide lower dynamic friction than rolling tires at the threshold of adhesion.

Key Concept

Effect of Vehicle Loading on Commercial Vehicle Stopping Distance and Traction
Estimated Time:1m 15s
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Topics

Stopping and Braking Procedures

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