The Differential 

The differential is a vital link in a car's drivetrain, whether it is a front wheel or rear wheel drive vehicle. It's importance can be illustrated by observing the path that each wheel travels when a car turns. When turning, the outside wheel on each axle must travel farther to reach the same angular position as the inner wheel on the same axle. In order to travel farther in the same amount of time, it must also travel somewhat faster than the inner wheel. But, if the axle were a solid shaft, each wheel would be forced to travel at the same speed and the car would have great difficulty turning. So, if a car is to be able to turn smoothly, there must be a mechanism that allows unequal speeds on two wheels on a given drive axle. This is where the differential gears are used.

From reading the drivetrain page (see drivetrain), one can see that the drive shaft in a rear wheel drive vehicle is mounted parallel to the direction that the car is pointing. In order to change the direction of the rotating shaft to match the direction that one wants the wheels to turn, a ring and pinion gear are used. The pinion gear is attached to the end of the drive shaft as it enters the differential case. As it turns, its gears cause the ring (crown wheel) gear to rotate. Since their shafts are mounted at a 90-degree angle to each other, this gear set transforms longitudal rotation to transverse rotation. 

Now that rotational direction has been addressed, differential action must be dealt with. Attached to the ring gear, then, is the differential casing. It rotates as the ring gear rotates. Sideways through the center of the differential case is a shaft on which two planet pinion (or spider) gears are mounted. These small gears are the at the heart of the differential action. Besides the spider gears, there are two side gears (sun gears), each attached to one axle shaft such that the right axle shaft is attached to one and the left to another.

When the vehicle is travelling straight, power is transmitted from the pinion gear to the ring (crown) gear which is attached to the differential case. The differential case rotates together with the ring gear, causing the spider or planet pinion gears to rotate with it. The spider gears do not spin under these circumstances, but remain in position relative to the differential case. Since they are meshed with the side gears, each side gear will rotate the same speed as the differential carrier, and will drive the car. 

When the car is turning, the planet pinion gears allow the wheels to turn at different speeds, by rotating themselves to permit the two side or sun gears to turn at different speeds.

A front wheel drive car has the same type of differential gears as a rear wheel drive does, but does not include the ring and pinion gears, since shaft rotation is already transversal (see drivetrain). Instead, one of three methods of transmitting power from the transmission to the differential is used:

  1. A belt can be used between the output shaft of the transmission and the differential.
  2. A chain can be used between the output shaft of the transmission and the differential.
  3. Direct gearing can be used between the output shaft of the transmission and the differential.

Regardless of the method used, the differential in a front wheel drive works exactly the same as that of a rear wheel drive vehicle.


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