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Overview
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 ice.
· 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 off.
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 to slip.
In this article, 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 basic system.
The Basics
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 transmission and differential
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.
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: |
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·
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.
· 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 why NASCAR 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 extremely
high.
· 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 traction.
· Dynamic contact - The tire is slipping
relative to the road. The coefficient of friction
for dynamic contact is lower, so you have less traction.
Wheel Slip
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 lateral force.
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 has 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 the anti-lock braking system (ABS).
Differentials - Transfercase
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 available traction.
Differentials - Axles
A car has two differentials, 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 turn.
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.)
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.
The
Transfer Case
This is the device that splits the power between the
front and rear axles on a four-wheel-drive car.
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
drivetrain.
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 and Advanced Electronics
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 fuel-economy.
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. 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 rear axle.
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.
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.
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.
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