Differentials
A car has two differentials, one located between the two front wheels and one between the two rear wheels. They send the torque from the driveshaft or transmission to the drive wheels. They also allow the left and right wheels to spin at different speeds when you go around a turn.

When you go around a turn, the inside wheels follow a different path than the outside wheels, and the front wheels follow a different path than the rear wheels, so each of the wheels is spinning at a different speed. The differentials enable the speed difference between the inside and outside wheels. (In all-wheel drive, the speed difference between the front and rear wheels is handled by the transfer case -- we'll discuss this next.)

The most common type of differential - the open differential

There are several different kinds of differentials used in cars and trucks. The types of differentials used can have a significant effect on how well the vehicle utilizes available traction. See Differentials for more details.

The Transfer Case
This is the device that splits the power between the front and rear axles on a four-wheel-drive car.

A typical part time four-wheel drive transfer case. The planetary gear reduction can be engaged to provide the low-range gearing.

Back to our corner-turning example: While the differentials handle the speed difference between the inside and outside wheels, the transfer case in an all-wheel-drive system contains a device that allows for a speed difference between the front and rear wheels. This could be a viscous coupling, center differential or other type of gearset. These devices allow an all-wheel-drive system to function properly on any surface.

The transfer case on a part-time four-wheel-drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like this should only be used in low -traction situations in which it is relatively easy for the tires to slip. On dry concrete, it is not easy for the tires to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain.

Some transfer cases, more commonly those in part-time systems, also contain an additional set of gears that give the vehicle a low range. This extra gear ratio gives the vehicle extra torque and a super-slow output speed. In first gear in low range, the vehicle might have a top speed of about 5 mph (8 kph), but incredible torque is produced at the wheels. This allows drivers to slowly and smoothly creep up very steep hills.

Locking Hubs
Each wheel in a car is bolted to a hub. Part-time four-wheel-drive trucks usually have locking hubs on the front wheels. When four-wheel drive is not engaged, the locking hubs are used to disconnect the front wheels from the front differential, half-shafts (the shafts that connect the differential to the hub) and driveshaft. This allows the differential, half-shafts and driveshaft to stop spinning when the car is in two-wheel drive, saving wear and tear on those parts and improving fuel-economy.

Manual locking hubs used to be quite common. To engage four-wheel drive, the driver actually had to get out of the truck and turn a knob on the front wheels until the hubs locked. Newer systems have automatic locking hubs that engage when the driver switches into four-wheel drive. This type of system can usually be engaged while the vehicle is moving.

Whether manual or automatic, these systems generally use a sliding collar that locks the front half-shafts to the hub.

Advanced Electronics
On many modern four-wheel and all-wheel-drive vehicles, advanced electronics play a key role. Some cars use the ABS system to selectively apply the brakes to wheels that start to skid -- this is called brake-traction control.

Others have sophisticated, electronically-controlled clutches that can better control the torque transfer between wheels. We'll take a look at one such advanced system later in the article.

First, let's see how the most basic part-time four-wheel-drive system works.

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