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How Four-Wheel Drive WorksThere are almost as many different types of four-wheel-d

systems as there are four-wheel-drive vehicles. It seems

every manufacturer has several different schemes for

providing power to all of the wheels. The language used

the different carmakers can sometimes be a little confusi

so before we get started explaining how they work, let's c

up some terminology:

Four-wheel drive - Usually, when carmakers say that a 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 f

time four-wheel drive. All-wheel-drive systems are desi

to function on all types of surfaces, both on- and off-road

ost of them cannot be switched off.

art-time and full-time four-wheel-drive systems can be evaluated using the same criteria. The best system will send exactly thght amount of torque to each wheel, which is the maximum torque that won't cause that tire to slip.

this article, we'll explain the fundamentals of four-wheel drive, starting with some background on traction, and look at the

omponents that make up a four-wheel-drive system. Then we'll take a look at a couple of different systems, including the one

und on the Hummer, manufactured for GM by AM General.

We need to know a little about torque, traction and wheel slip before we can understand the different four-wheel-drive system

und on cars.

orque, Traction and Wheel Slip

orqueis the twisting force that theengineproduces. The torque from the engine is what moves your car. The various gearsin

etransmissionanddifferentialmultiply the torque and split it up between the wheels. More torque can be sent to the wheels in

st gear than in fifth gear because first gear has a largergear-ratioby which to multiply the torque.

he bar graph below indicates the amount of torque that the engine is producing. The mark on the graph indicates the amount

rque 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

akes a bad start exceeds this torque, so the tires slip. As soon as they start to slip, the torque drops down to almost zero.

he interesting thing about torque is that in low-traction situations, the maximum amount of torque that can be created is

etermined by the amount of traction, not by the engine. Even if you have a NASCARengine in your car, if the tires won't stick

e ground there is simply no way to harness that power.

or the sake of this article, we'll define traction as the maximum amount of force thetirecan apply against the ground (or that t

round can apply against the tire -- they're the same thing). These are the factors that affect traction:

he weight on the tire -- The more weight on a tire, the more traction it has. Weight can shift as a car drives. For instance, wh

ar makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels. (SeeHow Brakes

Workfor more details.)

he coefficient of friction -- This factor relates the amount of friction force between two surfaces to the force holding the two

urfaces together. In our case, it relates the amount of traction between the tires and the road to the weight resting on each tire

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oefficient 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

stance, aNASCAR tirehas a very high coefficient of friction when it is driving on a dry, concrete track. That is one of the reaso

hyNASCAR race carscan corner at such high speeds. The coefficient of friction for that same tire in mud would be almost ze

y contrast, huge, knobby, off-roadtireswouldn't have as high a coefficient of friction on a dry track, but in the mud, their coeffi

friction is extremely high.

Wheel slip -- There are two kinds of contact that tires can make with the road: static and dynamic.

atic contact -- The tire and the road (or ground) are not slipping relative to each other. The coefficient of friction for static con

higher than for dynamic contact, so static contact provides better traction.

ynamic contact -- The tire is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have

action.

uite simply, wheel slip occurs when the force applied to a tire exceeds the traction available to that tire. Force is applied to the

two ways:

ongitudinally -- Longitudinal force comes from the torque applied to the tire by the engine or by the brakes. It tends to either

ccelerate or decelerate the car.

aterally -- Lateral force is created when the car drives around a curve. It takes force to make a car change direction -- ultimate

e tires and the ground provide lateral force.

et'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 plen

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

edal in the middle of the turn (don't do this!) -- your engine sends a lot more torque to the wheels, producing a large amount o

ngitudinal force. If you add the longitudinal force (produced by the engine) and the lateral force created in the turn, and the su

xceeds the traction available, you just created wheel slip.

ost people don't even come close to exceeding the available traction on dry pavement, or even on flat, wet pavement. Four-w

nd all-wheel-drive systems are most useful in low-traction situations, such as in snow and on slippery hills.

he benefit of four-wheel drive is easy to understand: If you are driving four wheels instead of two, you've got the potential to do

e amount of longitudinal force (the force that makes you go) that the tires apply to the ground.

his can help in a variety of situations. For instance:

snow -- It takes a lot of force to push a car through the snow. The amount of force available is limited by the available tractio

ost 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 ha

nly a small amount of traction. A four-wheel-drive car can utilize the traction of all four tires.

ff 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

ossing 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 trac

A four-wheel-drive car can utilize the traction of all four ti

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 notab

four-wheel-drive systems won't help you stop on slippery

surfaces. It's all up to thebrakesand theanti-lock braking

system(ABS).

Now let's take a look at the parts that make up a four-wh

drive system.

The most common type of differential -- the open differential

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omponents of a Four-wheel-drive System

he main parts of any four-wheel-drive system are the two differentials (front and rear) and the transfer case. In addition, part-ti

ystems have locking hubs, and both types of systems may have advanced electronics that help them make even better use of

vailable traction.

ifferentials A car has twodifferentials, one located between the two front wheels and one between the two rear wheels. They

e torque from the driveshaft or transmission to the drive wheels. They also allow the left and right wheels to spin at different

peeds 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 diffe

ath than the rear wheels, so each of the wheels is spinning at a different speed. The differentials enable the speed difference

etween the inside and outside wheels. (In all-wheel drive, the speed difference between the front and rear wheels is handled b

e transfer case -- we'll discuss this next.)

here are several different kinds of differentials used in cars and trucks. The types of differentials used can have a significant e

n how well the vehicle utilizes available traction. See How Differentials Workfor more details.

A typical part time four-wheel drive transfer case: The plangear reduction can be engaged to provide the low-range

gearing.

Transfer Case

This is the device that splits the power between the fro

and rear axles on a four-wheel-drive car.

Back to our corner-turning example: While the differen

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 co

be aviscous coupling, center differential or other type

earset. These devices allow an all-wheel-drive system to function properly on any surface.

he transfer case on a part-time four-wheel-drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the whee

re forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like th

hould 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

es to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain

ome transfer cases, more commonly those in part-time systems, also contain an additional set of gears that give the vehicle a

ange. This extra gear ratio gives the vehicle extra torque and a super-slow output speed. In first gear in low range, the vehicle

ight have a top speed of about 5 mph (8 kph), but incredible torque is produced at the wheels. This allows drivers to slowly an

moothly creep up very steep hills.

ocking Hubs

ach wheel in a car is bolted to a hub. Part-time four-wheel-drive trucks usually have locking hubs on the front wheels. When

heel drive is not engaged, the locking hubs are used to disconnect the front wheels from the front differential, half-shafts (the

hafts that connect the differential to the hub) and driveshaft. This allows the differential, half-shafts and driveshaft to stop spinn

hen the car is in two-wheel drive, saving wear and tear on those parts and improving fuel-economy.

anual locking hubs used to be quite common. To engage four-wheel drive, the driver actually had to get out of the truck and t

nob on the front wheels until the hubs locked. Newer systems have automatic locking hubs that engage when the driver switch

to four-wheel drive. This type of system can usually be engaged while the vehicle is moving.

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Whether manual or automatic, these systems generally use a sliding collar that locks the front half-shafts to the hub.

dvanced Electronics

n many modern four-wheel and all-wheel-drive vehicles, advanced electronics play a key role. Some cars use the ABS system

electively apply the brakes to wheels that start to skid -- this is called brake-traction control.

thers have sophisticated, electronically-controlled clutches that can better control the torque transfer between wheels. We'll ta

ok at one such advanced system later in the article.

rst, let's see how the most basic part-time four-wheel-drive system works.

Diagram of basic system

Four-wheel Drive Differential

The type of part-time system typically found on four-whee

drive pickups and older SUVs works like this: The vehicle

usually rear-wheel drive. The transmission hooks up dire

to a transfer case. From there, one driveshaft turns the fr

axle, and another turns the rear axle.

When four-wheel drive is engaged, the transfer case lock

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 anopen differential.

though this system provides much better traction than a two-wheel-drive vehicle, it has two main drawbacks. We've already

scussed one of them: It cannot be used on-road because of the locked transfer case.

he second problem comes from the type of differentials used: An open differential splits the torque evenly between each of the

heels it is connected to (seeHow Differentials Workfor more details). If one of those two wheels comes off the ground, or is oery slippery surface, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the oth

heel also receives zero torque. So even if the other wheel has plenty of traction, no torque is transferred to it. The animation b

hows how a system like this reacts under various conditions.

nimation of a basic system encountering various combinations of terrain. This vehicle gets stuck when two of its wheels are o

e.

reviously, we said that the best four-wheel-drive system will send exactly the right amount of torque to each wheel, the right

mount being the maximum torque that won't cause that tire to slip. This system rates fairly poorly by that criterion. It sends to b

heels the amount of torque that won't cause the tire with the least traction to slip.

here are some ways to make improvements to a system like this. Replacing the open differential with alimited-slip rear

fferentialis one of the most common ones -- this makes sure that both rear wheels are able to apply some torque no matter w

nother option is alocking differential, which locks the rear wheels together to ensure that each one has access to all of the tor

oming into the axle, even if one wheel is off the ground -- this improves performance in off-road conditions.

the next section, we'll take a look at what could be the ultimate four-wheel-drive system: the one on the Hummer.

he Four-wheel Drive Hummer

he AM General Hummer military vehicle combines some advanced mechanical technology with sophisticated electronics to cr

hat is arguably the best four-wheel-drive system available.

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he Hummer has a full-time system with additional features that can be engaged for enhanced off-road performance. In this

ystem, just as in our basic system, the transmission is hooked up to the transfer case. From the transfer case, one driveshaft

onnects to the front axle and one to the rear axle. However, the transfer case on the Hummer does not automatically lock the

nd rear axles together. Instead, it contains a set ofopen-differentialgears that can be locked by the driver. In open mode (not

cked), the front and rear axles can move at different speeds, so the vehicle can drive on dry roads with no problem. When the

fferential is locked, the front and rear axles each have access to the engine's torque. If the front wheels are in quicksand, the

wheels get all of the torque they can handle.

Diagram of Hummer system, one cool feature of the Hummer i

geared hubs it uses at each wheel. These raise the entire drivegiving the Hummer 16 inches (40.64 cm) of ground clearance, than double what most four-wheel drives have.

The front and rear differentials are bothTorsen different

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

the other wheel. Torsen differentials can transfer from tw

four times the torque from one wheel to the other. This is

big improvement over open differentials. But if one whee

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 t

p, the brake traction control applies the brakes to that wheel. This accomplishes two things:

keeps that tire from slipping, allowing it to make maximum use of its available traction.

allows the other wheel to apply more torque.

he Hummer system encountering various combinations of terrain: For the Hummer to get stuck, all four wheels would have to

action.

he brake traction control system applies significant torque to the wheel that wants to slip, allowing the Torsen differential to ap

wo to four times that increased torque to the other wheel.

et's put the Hummer to the test.

he system on the Hummer is capable of sending a large amount of torque to whichever tires have traction, even if this means

ending it all to a single tire. This brings the Hummer pretty close to our definition of an ideal four-wheel-drive system: one that

upplies each tire with the maximum amount of torque it can handle.

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