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.
Torque & Traction
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.
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
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.
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.
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.
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 gearing.
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 gear set. 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 drive train.
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 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.
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 rear axle.
Diagram of basic system.
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. The animation below shows how a system
like this reacts under various conditions.
Animation of a basic system encountering various combinations
of terrain. This vehicle gets stuck when two of its wheels are
on the ice.
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.
We give our thanks to
HOW STUFF WORKS for allowing us to use their info and
graphics!
For more information on how engines
or anything else in this world works, visit
www.howstuffworks.com.
We Now Offer
FINANCING
Here at
Total Car Care Centers we have all
the services your
vehicle needs to keep running smoothly.
Oil changes, transmission flushes, cooling system flushes, air
filters, & fuel filters are just a few of the many services & products we
offer!.
We can repair most makes and models,
foreign & domestic!
We
install quality rebuilt and/or used engines. We also perform minor
repairs that don't require engine removal.
