Examkin

Question

Difficulty: Very hardWheels and Tires

A vehicle exhibits a rhythmic vertical vibration, often described as "wheel hop," while cruising in a straight line at 70 mph. The technician verifies the rims are not bent and successfully zeroes out all tire and wheel assemblies on a standard unloaded dynamic spin balancer. However, the highway-speed vibration persists during a follow-up road test. Which of the following best explains the most likely cause of this persistent vibration?

  1. A
    The vehicle's coil springs have degraded and are failing to dampen standard road oscillations, allowing the wheel to bounce continuously.
  2. Excessive radial force variation within the tire's internal construction is creating an upward force against the vehicle when rolling under load.Answer
  3. C
    Worn brake shoes are dragging intermittently against the disc brake rotors, creating a rotational imbalance at high speeds.
  4. D
    The differential assembly is failing to allow the wheels to turn at different speeds during straight-line highway cruising, causing tire scrub.

Answer

The correct answer identifies that the tire has excessive radial force variation, creating an upward force when the stiff spot in the tire's internal construction rolls against the pavement under the weight of the vehicle.
The correct answer accurately identifies Radial Force Variation (RFV) as a structural defect inside the tire where a portion of the sidewall or tread is stiffer than the rest. A standard unloaded spin balancer only measures weight imbalance and cannot detect this stiff spot. However, when the tire is placed under the vehicle's weight on the road, the stiff spot creates an upward thrust each time it contacts the pavement, resulting in a rhythmic vertical vibration or wheel hop.

Step-by-Step Solution

1
Analyze the symptoms provided in the scenario.
The vehicle has a high-speed vertical vibration that remains even after a standard dynamic spin balance.
Establishing the baseline rules out standard weight imbalance as the root cause.
2
Evaluate the capabilities of the diagnostic equipment used.
A standard spin balancer rotates the tire freely in the air (unloaded) and only detects weight distribution discrepancies.
Understanding equipment limitations helps identify faults the equipment cannot detect, such as structural tire defects.
3
Identify tire faults that only manifest under vehicle weight.
Radial force variation (RFV) occurs when a tire has a stiff spot in its internal ply construction, acting like a stiffer spring that pushes up against the vehicle once per revolution under load.
This precisely matches the symptom of an unloaded balanced tire that still hops on the road.
4
Evaluate and eliminate the distractors based on mechanical principles.
Identify fundamental mechanical contradictions in the wrong options (e.g., confusing shocks with springs, or disc components with drum components).
Ensures the selected answer is both mechanically sound and the only plausible choice.

Key Concept

Advanced Tire Diagnostics and Radial Force Variation
Estimated Time:1m 30s
Rate this question

Topics

Wheels and Tires

More questions from this topic