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Difficulty: MediumSpeed on Curves and Downgrades

A driver is operating a loaded commercial combination vehicle on an undivided highway and approaches an unbanked curve. Why is it essential for the driver to reduce speed to a safe level before entering the curve rather than braking while navigating through the turn?

  1. A
    Braking mid-curve causes the air brake chambers to lose pressure rapidly, whereas straight-line braking eliminates perception and reaction distance requirements.
  2. Tires have a limited amount of traction that must be shared between steering forces and braking; braking inside the curve can exceed tire grip and trigger a skid, jackknife, or rollover.Answer
  3. C
    Entering a curve at cruising speed and braking sharply midway stabilizes the trailer by shifting the gross cargo weight over the drive axles.
  4. D
    Applying continuous light brake pedal pressure throughout the curve prevents brake fade while counteracting the vehicle's centrifugal force.

Answer

Tires have a limited amount of traction that must be shared between steering forces and braking; braking inside the curve can exceed tire grip and trigger a skid, jackknife, or rollover.
The correct answer correctly states that tire traction is finite and must be shared between steering and braking. Slowing down prior to curve entry ensures that maximum lateral grip remains available to steer through the curve, preventing dangerous skids, jackknifes, or rollovers.

Step-by-Step Solution

1
Analyze tire traction dynamics during cornering
Tires provide a finite amount of grip (friction) to manage longitudinal (braking/accelerating) and lateral (cornering/steering) forces simultaneously.
When turning, a substantial portion of available tire adhesion is required solely to keep the vehicle following the curve against centrifugal force.
2
Evaluate the effect of braking while already in the curve
Applying the brakes mid-curve demands additional longitudinal grip, which can overload the total available tire adhesion limit.
Exceeding available tire grip causes wheel lockup, loss of steering control, drive-wheel skids, jackknifing, or dynamic rollovers.
3
Identify the standard CDL safe procedure for curves
Slowing down to a safe speed in a straight line before entering the turn allows the vehicle to navigate the curve under light power without braking.
Entering at a controlled speed reserves the maximum available tire grip for steering and lateral stability.

Key Concept

Tire Traction Allocation and Pre-Curve Speed Reduction
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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