Four Wheel Drive Systems


Four Wheel Drive Systems

The typical truck or utility 4WD vehicle contains the following components: a front-mounted, longitudinally positioned engine; the transmission (either manual or automatic); two driveline shafts (front and rear); front and rear axle assemblies; and the transfer case.

The heart of most conventional four-wheel-drive vehicles is the transfer case, which is usually mounted to the side or the back of the transmission. A chain or gear drive within the case receives the power flow from the transmission and transfers it to two separate drive shafts leading to the front and rear axles.

A selector switch or shifter located in the driving compartment controls the transfer case so power is directed to the axles as the driver desires. Power can be directed to all four wheels, two wheels, or no wheels (neutral). On many vehicles, the driver is also given the option of low four-wheel-drive range for extra traction in especially rough conditions such as deep snow or mud.

The driveline from the transfer case shafts run to differentials at the front and rear axles. As on two-wheel-drive vehicles, these axle differentials are used to compensate for road and driving conditions by adjusting the rpm to opposing wheels. For example, the outer wheel must roll faster in a turn than the inner wheel during a turn because it has more ground to cover. To permit this action, the differential cuts back the power delivered to the inner wheel and boosts the amount of power delivered to the outer wheel.

U-joints are used to couple the driveline shafts with the differentials and transfer cases on all these vehicles. U-joints can also be used on some vehicles to connect the rear axle and wheels. Normally, however, rear axles are simply bolted to the wheel hubs.

The coupling between front wheels and axles is normally done with U-joints on heavy-duty vehicles and with CV (constant velocity) joints on lightweight vehicles. Generally, half axles or half shafts with CV joints are found on four-wheel-drive passenger cars. They can also be found on a number of passenger vans and on mini pickups and trucks.

On 4WD systems adapted from front-wheel-drive systems, a separate front differential and driveline are not needed. The front wheels are driven by the transaxle differential of the base model. A power takeoff is added to the transaxle to transmit power to the rear wheels in four-wheel-drive. This takeoff gearing is housed in a transfer case mounted to the transaxle housing. The gearing connects to a rear driveline and rear axle assembly that includes the rear differential.

More Four And All Wheel Drive

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