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Question

Difficulty: MediumSpace for Turns

Two commercial vehicles—a single-unit delivery truck and a long tractor-semitrailer combination—are negotiating the same tight street corner. Which factor explains why the combination vehicle requires significantly more turning space due to off-tracking?

  1. The greater distance between the front steering axle and the rear trailer axle causes the rear wheels to follow a much shorter turning path.Answer
  2. B
    The higher gross vehicle weight of the combination unit creates centrifugal force that pushes the trailer outward away from the curb.
  3. C
    The presence of a fifth-wheel pivot eliminates trailer path variance and transfers all lateral turning forces to the front bumper.
  4. D
    The dual-tire configuration on the drive axles prevents the steer axle from achieving a full wheel cut angle.

Answer

The greater distance between the front steering axle and the rear trailer axle causes the rear wheels to follow a much shorter turning path.
In commercial driving, off-tracking (often called 'cheating') occurs because the rear wheels of any turning vehicle follow a shorter, tighter path than the front steering wheels. The greater the distance between the front steer axle and the rear axles of the vehicle or combination unit, the greater the off-tracking effect. Because a tractor-semitrailer has a substantially longer distance between its steering axle and rear trailer tandem axles than a single-unit truck, its rear wheels cheat significantly further inward, demanding much more turning space.

Step-by-Step Solution

1
Identify the physical principle governing space requirements during vehicle turns.
Off-tracking (or 'cheating') describes the phenomenon where the rear wheels of a turning vehicle do not follow the exact path of the front wheels, instead cutting a shorter radius toward the inside of the curve.
Understanding off-tracking is essential for calculating the clearance required to clear curbs, signs, and adjacent traffic lanes.
2
Analyze how vehicle dimensions influence the magnitude of off-tracking.
The amount of off-tracking is directly proportional to the wheelbase—specifically the distance from the steering axle to the rearmost trailer axle assembly.
A longer vehicle wheelbase creates a greater angular deviation between the vehicle's heading and the trailer's tracking path, necessitating wider turn setups.

Key Concept

Off-Tracking Geometry and Axle Spacing in Turn Space Management
Estimated Time:1m 0s
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Topics

Space for Turns

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