The rear wheels of a car do not always turn at the same speed. When the car isturning or when tire diameters differ slightly，the rear wheels must rotate at differentspeeds.
If there were a solid connection between each rear axle and the differential case，thetires would tend to slide, squeal，and wear whenever the vehicle turned. A differential isdesigned to prevent this problem.
Driving straight ahead
When driving straight，both rear wheels turn at the same speed. The spinning caseand pinion shaft rotate the differential pinion gears. The teeth on the pinion gears applytorque to the axle gears and axles. Balanced forces make the differential appear to belocked.
Fig. 9.11 illustrates the action of a differential when the vehicle is turning a corner.Note how the outer wheel is turning faster than the inner wheel. The out wheel musttravel farther than the inner wheel.
The action of the spider gears allows each axle to change speed while still transferringtorque to propel the car. Without a differential，you could break an axle or wear out tiresbecause of the different turning speeds of the rear wheels.