While most 4WD trucks
and utility vehicles are design variations of most basic rear-wheel-drive
vehicles, most passenger cars featuring 4WD were developed from a
front-wheel-drive base model.
This feature a
transaxle and differential that drive the front wheels, plus some type of
mechanism for connecting the transaxle to a rear driveline. In many cases this
is a simple clutch type engagement.
4WD passenger cars
normally differ from heavier-duty 4WD trucks and vehicles in several other ways.
First, there is no separate transfer case; any gearing needed to transfer power
to the rear driveline is usually contained in the transaxle housing or small
bolt on extension housing. Four-wheel-drive passenger cars do not have lockout
hubs, traction is not quite as good in the worst conditions and tire wear not so
severe should the driver forget to take the car out of four-wheel drive.
Interaxle differentials are also very uncommon.
Most passenger cars
with 4WD are not designed for the rigors of off-road driving, in which clearance
over rocks and debris is needed. For example road driving there is no clearance
or durability problems. As with 4WD trucks, the passenger car 4WD mode should be
used on dry pavement due to increased tire and drivetrain wear.
Limited Slip and
To help compensate
for the lack of locking hubs many 4WD passenger cars use limited slip or open
differential designs for the front and rear differentials. These systems allow
enough differential slippage to substantially reduce wheel scrubbing and
A limited slip
differential (LSD) uses a friction clutch to join the right and left rear axle
shafts to the differential case. This system ensures that most power is supplied
where the traction is greatest. It permits the inner and outer wheels to rotate
at different speeds when turns are made, just as in general differentials.
In addition, the LSD
provides these features. When one front and one rear wheel, which are diagonal
to each other, slip on snow-covered roads and driving force cannot be delivered,
or when one rear wheel is caught in a ditch and runs idle, the LSD delivers
strong torque to the other wheel so the vehicle can run normally. Even if the
vehicle bumps with one rear wheel off the road, such as when driving on rough
terrain, gravel, or snow-covered roads, the LSD ensures easy straightforward
driveability by its limited differential function.
Other systems use an
open differential. Here the greatest power is supplied where there is the least
resistance. This is an excellent system for fuel economy and gear wear in
full-time, four-wheel drive, but it does not provide the true traction
advantages of four-wheel drive. For example, the greatest power would be
supplied to the slipping wheels (least resistance) on icy road conditions when
it is actually needed at the wheels facing the most resistance (for example,
where the greatest traction is).
Systems using an open
differential have a drive mode on the selector, which permits the differential
to be locked up in such conditions. This causes the drivetrain to function the
same as a part-time four-wheel-drive system. The power is distributed equally to
all the wheels. Vehicles should not be driven in this mode, except under poor
road conditions, for the same reasons that part-time systems should be used only