There are almost as many different types of
four-wheel-drive systems as there are four-wheel-drive
vehicles. It seems that every manufacturer has several
different schemes for providing power to all of the wheels.
The language used by the different carmakers can sometimes be
a little confusing, so before we get started explaining how
they work, let's clear up some terminology:
Four-wheel drive - Usually, when carmakers say
that a car has four-wheel drive, they are referring
to a part-time system. For reasons we'll explore
later in this article, these systems are meant only for use
in low-traction conditions, such as off-road or on snow or
All-wheel drive - These systems are sometimes
called full-time four-wheel drive. All-wheel-drive
systems are designed to function on all types of surfaces,
both on- and off-road, and most of them cannot be switched
Part-time and full-time four-wheel-drive
systems can be evaluated using the same criteria. The best
system will send exactly the right amount of torque to each
wheel, which is the maximum torque that won't cause that tire
In this edition of HowStuffWorks,
we'll explain the fundamentals of four-wheel drive, starting
with some background on traction, and look at the components
that make up a four-wheel-drive system. Then we'll take a look
at a couple of different systems, including the one found on
the AM General Hummer.
Torque, Traction and Wheel Slip We need to
know a little about torque, traction and
wheel slip before we can understand the different
four-wheel-drive systems found on cars.
Torque Torque is
the twisting force that the engine
produces. The torque from the engine is what moves your car.
The various gears in the
multiply the torque and split it up between the wheels. More
torque can be sent to the wheels in first gear than in fifth
gear because first gear has a larger gear-ratio
by which to multiply the torque.
This bar graph indicates the
amount of torque that the engine is producing. The mark on the
graph indicates the amount of torque that will cause wheel
slip. The car that makes a good start never exceeds this
torque, so the tires don't slip; the car that makes a bad
start exceeds this torque, so the tires slip. As soon as they
start to slip, the torque drops down to almost
The interesting thing about torque is that in low-traction
situations, the maximum amount of torque that can be created
is determined by the amount of traction, not by the engine.
Even if you have a NASCAR
engine in your car, if the tires won't stick to the ground
there is simply no way to harness that power.
Traction For the sake of
this article, we'll define traction as the maximum
amount of force the tire can
apply against the ground (or that the ground can apply against
the tire -- they're the same thing). These are the factors
that affect traction:
The weight on the tire - The more weight on a
tire, the more traction it has. Weight can shift as a car
drives. For instance, when a car makes a turn, weight shifts
to the outside wheels. When it accelerates, weight shifts to
the rear wheels. (See How Brakes
Work for more details.)
The coefficient of friction - This factor relates
the amount of friction force between two surfaces to the
force holding the two surfaces together. In our case, it
relates the amount of traction between the tires and the
road to the weight resting on each tire. The coefficient of
friction is mostly a function of the kind of tires on the
vehicle and the type of surface the vehicle is driving on.
For instance, a NASCAR
tire has a very high coefficient of friction when it is
driving on a dry, concrete track. That is one of the reasons
race cars can corner at such high speeds. The
coefficient of friction for that same tire in mud would be
almost zero. By contrast, huge, knobby, off-road tires
wouldn't have as high a coefficient of friction on a dry
track, but in the mud, their coefficient of friction is
Wheel slip - There are two kinds of contact that
tires can make with the road: static and dynamic.
static contact - The tire and the road (or
ground) are not slipping relative to each other. The
coefficient of friction for static contact is higher than
for dynamic contact, so static contact provides better
dynamic contact - The tire is slipping relative
to the road. The coefficient of friction for dynamic
contact is lower, so you have less traction.
Why Does Wheel Slip
Happen? Quite simply, wheel slip occurs when the
force applied to a tire exceeds the traction available to that
tire. Force is applied to the tire in two ways:
Longitudinally - Longitudinal force comes from
the torque applied to the tire by the engine or by the
brakes. It tends to either accelerate or decelerate the car.
Laterally - Lateral force is created when the car
drives around a curve. It takes force to make a car change
direction -- ultimately, the tires and the ground provide
Let's say you have a fairly powerful
rear-wheel-drive car, and you are driving around a curve on a
wet road. Your tires have plenty of traction to apply the
lateral force needed to keep your car on the road as it goes
around the curve. Let's say you floor the gas pedal in the
middle of the turn (don't do this!) -- your engine
sends a lot more torque to the wheels, producing a large
amount of longitudinal force. If you add the longitudinal
force (produced by the engine) and the lateral force created
in the turn, and the sum exceeds the traction available, you
just created wheel slip.
Most people don't even come close to exceeding the
available traction on dry pavement, or even on flat, wet
pavement. Four-wheel and all-wheel-drive systems are most
useful in low-traction situations, such as in snow and on
slippery hills. In the next section, we'll see how
four-wheel-drive systems can help in these situations.
Four-wheel Drive and Low Traction The
benefit of four-wheel drive is easy to understand: If you are
driving four wheels instead of two, you've got the potential
to double the amount of longitudinal force (the force that
makes you go) that the tires apply to the ground.
This can help in a variety of situations. For instance:
In snow - It takes a lot of force to push a car
through the snow. The amount of force available is limited
by the available traction. Most two-wheel-drive cars can't
move if there is more than a few inches of snow on the road,
because in the snow, each tire has only a small amount of
traction. A four-wheel-drive car can utilize the traction of
all four tires.
Off road - In off-road conditions, it is fairly
common for at least one set of tires to be in a low-traction
situation, such as when crossing a stream or mud puddle.
With four-wheel drive, the other set of tires still have
traction, so they can pull you out.
Climbing slippery hills - This task requires a
lot of traction. A four-wheel-drive car can utilize the
traction of all four tires to pull the car up the hill.
There are also some situations in which four-wheel drive
provides no advantage over two-wheel drive. Most notably,
four-wheel-drive systems won't help you stop on slippery
surfaces. It's all up to the brakes and
braking system (ABS).
In the next section, we'll take a look at the parts that
make up a four-wheel-drive system.
The Parts The main parts of any
four-wheel-drive system are the two differentials (front and
rear) and the transfer case. In addition, part-time systems
have locking hubs, and both types of systems may have advanced
electronics that help them make even better use of the
Differentials A car has
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
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
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 How
Differentials Work 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
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
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
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
First, let's see how the most basic part-time
four-wheel-drive system works.
A Basic System The type of part-time system
typically found on four-wheel-drive pickups and older SUVs
works like this: The vehicle is usually rear-wheel drive. The
transmission hooks up directly to a transfer case. From there,
one driveshaft turns the front axle, and another turns the
Diagram of basic
When four-wheel drive is engaged, the transfer case locks
the front driveshaft to the rear driveshaft, so each axle
receives half of the torque coming from the engine. At the
same time, the front hubs lock.
The front and rear axles each have an open
differential. Although this system provides much better
traction than a two-wheel-drive vehicle, it has two main
drawbacks. We've already discussed one of them: It cannot be
used on-road because of the locked transfer case.
The second problem comes from the type of differentials
used: An open differential splits the torque evenly between
each of the two wheels it is connected to (see this
page of How Differentials Work for more details). If one
of those two wheels comes off the ground, or is on a very
slippery surface, the torque applied to that wheel drops to
zero. Because the torque is split evenly, this means that the
other wheel also receives zero torque. So even if the other
wheel has plenty of traction, no torque is transferred to it.
The animation below shows how a system like this reacts under
Animation of a basic system
encountering various combinations of terrain. This vehicle
gets stuck when two of its wheels are on the
Previously, we said that the best four-wheel-drive system
will send exactly the right amount of torque to each wheel,
the right amount being the maximum torque that won't cause
that tire to slip. This system rates fairly poorly by that
criterion. It sends to both wheels the amount of torque that
won't cause the tire with the least traction to slip.
There are some ways to make improvements to a system like
this. Replacing the open differential with a limited-slip
rear differential is one of the most common ones -- this
makes sure that both rear wheels are able to apply some torque
no matter what. Another option is a locking
differential, which locks the rear wheels together to
ensure that each one has access to all of the torque coming
into the axle, even if one wheel is off the ground -- this
improves performance in off-road conditions.
In the next section, we'll take a look at what could be the
ultimate four-wheel-drive system: the one on the Hummer.
The Ultimate System
AM General Hummer military vehicle combines some advanced
mechanical technology with sophisticated electronics to create
what is arguably the best four-wheel-drive system available.
The Hummer has a full-time system with additional features
that can be engaged for enhanced off-road performance. In this
system, just as in our basic system, the transmission is
hooked up to the transfer case. From the transfer case, one
driveshaft connects to the front axle and one to the rear
axle. However, the transfer case on the Hummer does not
automatically lock the front and rear axles together. Instead,
it contains a set of open-differential
gears that can be locked by the driver. In open mode (not
locked), the front and rear axles can move at different
speeds, so the vehicle can drive on dry roads with no problem.
When the differential is locked, the front and rear axles each
have access to the engine's torque. If the front wheels are in
quicksand, the rear wheels get all of the torque they can
Diagram of Hummer system, one cool feature of
the Hummer is the geared hubs it uses at each wheel.
These raise the entire driveline, giving the Hummer 16
inches (40.64 cm) of ground clearance, more than double
what most four-wheel drives
The front and rear differentials are both Torsen® differentials. These
differentials have a unique gearset: As soon as it senses a
decrease in torque to one wheel (which occurs when a tire is
about to slip), the gearset transfers torque to the other
wheel. Torsen differentials can transfer from two to four
times the torque from one wheel to the other. This is a big
improvement over open differentials. But if one wheel is off
the ground, the other wheel still gets no torque.
To handle this problem, the Hummer is equipped with a
brake traction control system. When one tire starts to
slip, the brake traction control applies the brakes to that
wheel. This accomplishes two things:
It keeps that tire from slipping, allowing it to make
maximum use of its available traction.
It allows the other wheel to apply more torque.
The brake traction control system applies
significant torque to the wheel that wants to slip, allowing
the Torsen differential to apply two to four times that
increased torque to the other wheel.
Let's put the Hummer to the test.
The Hummer system
encountering various combinations of terrain: For the Hummer
to get stuck, all four wheels would have to lose
The system on the Hummer is capable of sending a large
amount of torque to whichever tires have traction, even if
this means sending it all to a single tire. This brings the
Hummer pretty close to our definition of an ideal
four-wheel-drive system: one that supplies each tire with the
maximum amount of torque it can handle.
For more information on four-wheel drive and related
topics, check out the links on the next page.