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Difficulty: Very hardDriveshafts and Differentials

During a tight, low-speed U-turn on dry pavement, a driver feels a harsh mechanical binding and hears the rear tires aggressively scrubbing. However, the vehicle operates flawlessly and quietly when driven in a straight line at all speeds. The vehicle is equipped with a conventional open differential. Which internal drivetrain failure is the most direct cause of this condition?

  1. The differential spider gears are seized to their cross shaft, forcing the left and right axle shafts to rotate at identical speeds.Answer
  2. B
    The driveshaft slip yoke is mechanically bound on the output splines, preventing driveline length changes and causing lateral wheel drag.
  3. C
    The hypoid ring and pinion gear set has insufficient backlash, locking the entire differential carrier and stopping the outer wheel.
  4. D
    The universal joints are operating at severe angles, creating a cyclical speed fluctuation that forces the inner and outer wheels out of sync.

Answer

The differential spider gears are seized to their cross shaft, forcing the left and right axle shafts to rotate at identical speeds.
When cornering, the internal spider gears must rotate on the cross shaft to allow the axle side gears to spin independently. If they seize, the differential becomes fully locked. Because both wheels are then forced to spin at the identical speed of the ring gear, they will violently scrub against dry pavement during turns while operating perfectly normally in a straight line.

Step-by-Step Solution

1
Analyze the reported symptom and operating conditions.
The vehicle exhibits tire scrubbing and mechanical binding exclusively during tight turns on dry pavement. Straight-line driving is normal.
Identifying when the symptom occurs isolates the mechanical components that are active during that specific maneuver.
2
Relate the symptom to vehicle drivetrain kinematics.
Tire scrubbing in a turn indicates that the rear wheels are being forced to rotate at the same speed, failing to account for the longer path of the outer wheel.
Cornering naturally requires the outer wheel to rotate faster than the inner wheel to prevent traction loss or binding.
3
Identify the component responsible for differentiating wheel speeds.
Inside an open differential, the spider (pinion) gears rotate on the cross shaft to allow the side gears (connected to the axles) to spin at different speeds.
The ring/pinion and driveshaft deliver power to the carrier, but only the internal spider and side gears split that power unequally when turning.
4
Determine the failure mode that matches the symptoms.
Seized spider gears act like a locked differential (spool), causing the axles to rotate together. This has no effect on straight-line driving but causes severe binding in turns.
If the spider gears cannot rotate on their shaft, differentiation is mechanically impossible.

Key Concept

Open differential kinematics and the function of spider gears during cornering.
Estimated Time:2m 0s
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Topics

Driveshafts and Differentials

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