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Difficulty: MediumDriveshafts and Differentials

A rear-wheel-drive vehicle develops a noticeable driveline vibration at cruising speeds after a suspension alteration changed the rear axle pinion angle. A technician determines that the front and rear universal joints are operating at unequal working angles. Why do mismatched universal joint angles cause this vibration?

  1. The cyclical speed fluctuations created by the front universal joint are not cancelled out by the rear universal joint.Answer
  2. B
    The differential ring and pinion gears are forced out of mesh, causing gear teeth to skip under load.
  3. C
    The differential spider gears are forced to rotate continuously while the vehicle travels in a straight line.
  4. D
    The driveshaft slip yoke binds completely and prevents the driveshaft from rotating at the transmission output.

Answer

The cyclical speed fluctuations created by the front universal joint are not cancelled out by the rear universal joint.
A standard Cardan-style universal joint operating at an angle does not transfer rotational motion at a constant velocity; instead, the driven side speeds up and slows down twice per revolution. In a properly designed driveline, the front and rear universal joints operate at equal but opposite angles so the velocity fluctuations cancel each other out. When the operating angles are unequal, the fluctuations do not cancel, creating a torsional vibration that is felt throughout the vehicle at speed.

Step-by-Step Solution

1
Analyze the kinematics of a standard Cardan universal joint operating at an angle.
When a single Cardan universal joint operates at an angle, the driven shaft speeds up and slows down twice during each full revolution, resulting in non-uniform rotational velocity.
The geometry of the cross-and-yoke design causes angular velocity variations proportional to the operating angle.
2
Determine how automotive drivelines eliminate non-uniform rotational velocity.
Driveshafts use two universal joints phased in series at equal operating angles so the second joint's fluctuation is equal and opposite to the first joint's fluctuation.
Matching operating angles allows the driven pinion shaft to rotate at a constant velocity matching the transmission output shaft.
3
Evaluate the effect of unequal joint angles caused by the changed pinion angle.
The velocity variations fail to cancel, causing torsional acceleration and deceleration cycles that shake the drivetrain as a rhythmic vibration.
The remaining net velocity fluctuation produces torsional vibrations transmitted through the chassis.

Key Concept

Universal Joint Operating Angles and Velocity Fluctuation Cancellation
Estimated Time:1m 0s
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Topics

Driveshafts and Differentials

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