There are almost as many different types of four-wheel-drive systems
as there are four-wheel-drive vehicles. The language used by the different car makers 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 car makers 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.
For this 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, Traction and Wheel Slip
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
bar graph below 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 zero.
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.
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.
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.
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.
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).
how four wheel drive works??
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 (see 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.
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.
