Four-wheel-drive (4WD) and
all-wheel-drive (AWD) systems can dramatically increase a vehicle's
traction and handling ability in rain, snow, and off-road driving.
Considering that the vehicle's only contact with the road is the small
area of the trees, driving and handling is vastly improved if the work
load is spread out evenly among four wheels rather than two.
Factors such as the side
forces created by cornering and wind gusts have less effect on vehicles
with four driving wheels. The increased traction also makes it possible
to apply greater amounts of energy through the drive system. Vehicles
with 4WD and AWD can maintain control while transmitting levels of power
that would cause two wheels to spin either on take off or while rounding
a curve. The improved traction of 4WD and AWD systems allows the use of
tires narrower than those used on similar 2WD vehicles. These narrow
tires are less expensive. They also tend to cut through snow and water
rather than hydroplane over it.
can transfer large amounts of horsepower through the drivelines without
fear of drivetrain damage. This is not necessarily true of
front-wheel-drive vehicles and all-wheel-drive vehicles (both of which
Both 4WD and AWD systems
add initial cost and weight. With most passenger cars, the weight
problem is minor. A typical 4WD system adds approximately 170 pounds to
a passenger car. An AWD system adds even less weight. The additional
weight in larger 4WD trucks can be as much as 400 pounds or more.
The systems also add
initial cost to the vehicle. Vehicles equipped with 4WD and AWD require
special service and maintenance not performed on 2WD vehicles. However,
the slight disadvantages of 4WD and AWD are heavily outweighed by the
traction and performance these systems offer. Their popularity is
increasing at a rapid rate, and technicians must be prepared to diagnose
and repair these systems.
4WD versus AWD
Due to the many names
manufacturers give their drive systems, it is often difficult to clearly
define the difference between 4WD and AWD.
In this text, 4WD systems
are those having a separate transfer case. They also give the driver the
choice of operating in either 2WD or 4WD through the use of a shift
lever or shift button. The locking hubs used in 4WD systems can be
either manual or automatic locking. Finally, a 4WD system may or may not
use an interaxle differential or viscous clutch to distribute driving
power to its front and rear drivelines.
differ in several major ways. They do not have a separate transfer case.
All-wheel-drive systems use a front-wheel-drive transaxle equipped with
a viscous clutch, center differential, or transfer clutch. The viscous
clutch, center differential, or transfer clutch transfer power from the
transaxle to a rear driveline and rear axle assembly. All-wheel-drive
systems do not give the driver the option of selecting 2WD or 4WD modes.
The system operates in continuous 4WD.