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
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
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
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.