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Question

Difficulty: HardWheels and Tires

A driver reports a distinct lateral steering wheel wobble (shimmy) that occurs only when cruising at highway speeds between 55 and 65 mph. The vehicle tracks straight without pulling, and the symptom is not related to braking. What is the most likely cause of this condition?

  1. A
    Excessive backlash between the differential ring and pinion gears causing driveline vibration
  2. Dynamic wheel imbalance creating alternating lateral forces across the centerline of the wheel assemblyAnswer
  3. C
    Static wheel imbalance causing vertical hopping forces against the road surface
  4. D
    Lateral runout on the front brake rotors causing periodic friction pad deflection

Answer

Dynamic wheel imbalance creating alternating lateral forces across the centerline of the wheel assembly
Dynamic wheel imbalance is caused by an unequal distribution of mass on opposite sides of the tire's vertical centerline. As the wheel rotates at highway speeds (typically 50 to 65 mph), centrifugal force generates an alternating side-to-side twisting moment around the steering knuckle pivot, felt directly as steering wheel shimmy.

Step-by-Step Solution

1
Analyze the operating conditions and directional nature of the symptom.
The symptom is a lateral side-to-side steering wobble occurring specifically in the 55-65 mph highway speed range without pulling or braking involvement.
Speed-sensitive vibration without directional pull points directly to wheel and tire assembly balance rather than alignment or braking defects.
2
Differentiate between static and dynamic wheel unbalance modes.
Static unbalance creates vertical forces (wheel tramp/hop), whereas dynamic unbalance creates an alternating force couple across the tire centerline (lateral shimmy).
Dynamic unbalance requires spin-balancing with weights placed on both the inner and outer rim planes to counteract side-to-side wobble.

Key Concept

Distinction between dynamic wheel imbalance (lateral wobble/shimmy) and static wheel imbalance (vertical tramp/hop)
Estimated Time:1m 30s
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Topics

Wheels and Tires

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