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Difficulty: HardDriveshafts and Differentials

When a passenger vehicle equipped with an open differential makes a sharp right turn on dry pavement, how do the internal differential gears interact to allow smooth cornering?

  1. A
    The spider gears lock against the differential carrier case, forcing both the left and right side gears to rotate at identical speeds.
  2. B
    The spider gears disengage mechanically from the left side gear, allowing the outside wheel to freewheel without receiving drive torque.
  3. The spider gears rotate on their shaft as they walk around the slower right side gear, driving the left side gear faster than carrier speed.Answer
  4. D
    The spider gears hold both axle side gears stationary relative to the axle housing while power bypasses them through an internal viscous coupling.

Answer

The spider gears rotate on their shaft as they walk around the slower right side gear, driving the left side gear faster than carrier speed.
In an open differential, the differential carrier rotates at ring gear speed. When negotiating a right turn, the inner right wheel meets higher rolling resistance and slows down. This forces the bevel spider gears to rotate on their differential cross pin as they walk around the slower right side gear. This rotational motion directly increases the speed of the outer left side gear, making it turn faster than the carrier while maintaining equal torque distribution across both drive axles.

Step-by-Step Solution

1
Determine the relative wheel speeds required during a right-hand turn.
The inside (right) wheel travels along a smaller arc and slows down relative to the differential carrier, while the outside (left) wheel must cover a greater distance and rotate faster.
Geometry dictates that tires on the outer radius of a corner must travel farther in the same amount of time than tires on the inner radius.
2
Analyze the mechanical response of the differential spider gears on their cross shaft.
The slower-turning right side gear causes the spider gears to pivot and walk around it on their differential pinion shaft rather than remaining stationary relative to the carrier.
When both wheels turn at the same speed (straight line), the spider gears do not rotate on their pin; a speed difference between side gears forces them to turn.
3
Identify the kinematic effect on the opposite (outer/left) side gear.
The rotation of the spider gears adds speed to the left side gear, increasing its rotational velocity above carrier speed by the exact amount the right side gear decreased below carrier speed.
The differential carrier speed equals the average rotational speed of both axle side gears.

Key Concept

Kinematic operation of open differential spider and side gears during vehicle cornering
Estimated Time:1m 15s
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Topics

Driveshafts and Differentials

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