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What Wheel Drive?
By Karl
Brauer
Last updated:
2001-04-17

How many driven wheels does
a vehicle really need? The proliferation of SUVs on America's
highways and byways over the last ten years would suggest that two
simply isn't enough. But what exactly are the advantages of
four-wheel drive (or all-wheel drive) versus front-wheel drive and
rear-wheel drive?

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First of all, before we're
"driven" to confusion by all of this terminology, let's lay a few
ground rules. In most automotive circles (and for the purposes of
this discussion) the following acronyms will hold true:

AWD = all-wheel
drive
4WD = four-wheel drive
RWD = rear-wheel drive
FWD
= front-wheel drive

Each of these drive systems
has its own advantages and disadvantages which I'll cover in greater
detail.

Since the majority of
vehicles sold today are front-wheel drive, we'll start here. As the
name suggests, FWD vehicles use only the front wheels to deliver
power from the engine to the pavement. While relatively rare on the
streets of America during the first half of the 20th century, FWD
cars now account for approximately 70 percent of new car offerings.
Why the recent switch from RWD to FWD in new cars? Well, since most
vehicles carry their engines up front, it is a simpler task to get
that engine power to the driven wheels when they are both on the
same end of the car. Essentially, you turn the engine sideways and
connect a short driveshaft between either side of the engine and the
two front wheels. Less distance and fewer parts between the engine
and the driven wheels means less horsepower loss through mechanical
inefficiency. Additionally, fewer parts means reduced production
costs and, ultimately, a lower sticker price. One final advantage of
FWD is that it puts the engine weight directly over the driven
wheels which can improve traction on slippery or snow-packed
roads.

Despite its practical
advantages, FWD has several disadvantages when related to vehicle
performance. First of all, since vehicle weight shifts to the rear
of a car during hard acceleration, FWD cars will always be fighting
a losing battle when it comes to straight-line acceleration.
Handling performance also suffers on FWD platforms since the option
of using "throttle-induced oversteer" or sliding the rear end
doesn't exist (unless you get really creative with the emergency
brake).

In the end FWD is a very
practical way to power a vehicle but its inherent design is
contradictory to performance. Look at it this way: Your car came
with four wheels, why put the stress of steering, braking, and
acceleration on only two of them?

Take everything I just said
about FWD cars and reverse it for RWD cars. Performance goes up
because now the two rear wheels handle the duties of acceleration,
leaving the front wheels to focus solely on directing the vehicle
(plus the majority of braking). This means that when you punch it,
vehicle weight transfers to the rear end, just where you want it
during acceleration. You can also "steer" a RWD car with the gas
pedal by applying power and sliding the rear end while in a corner.
If you don't think this is an advantage, try driving a Mazda Miata
and a Chevy Cavalier convertible through the same set of backroad
twisties. Even though they're both top-down, four-cylinder compact
cars, the experience will be very different with a clear victory in
"fun factor" going to the Miata.

This doesn't necessarily
make rear-wheel drive the "better" configuration since it has
disadvantages too. As mentioned earlier, RWD cars require a
driveshaft (and dreaded interior "hump") and a differential to get
power from the engine to the rear wheels. These components add extra
cost and weight to a vehicle while robbing horsepower and making RWD
cars generally less efficient than FWD vehicles. This layout is also
more challenging to drive in inclement weather where, without the
aid of traction control, a RWD car can more easily end up on
somebody's front lawn or stuck in a ditch.

I personally have an
affinity for RWD because its biggest disadvantage is also its
biggest advantage; it's less practical and, consequently, more
fun.

Both of the previous
drivetrain layouts are based on using only half of a vehicle's
wheels to deliver power to the ground. But those other wheels are
just sitting there, why not use them too? Actually, the concept of
using all-wheel drive is not new. A Dutch manufacturer named Spyker
had one of the first full-time 4WD vehicles on display at the Paris
Motor Show in 1903.

While we're here let's go
over what the supposed differences are between four-wheel drive and
all-wheel drive. 4WD was basically the original term used to
describe all four wheels being driven on early Jeeps and trucks.
These systems were not very advanced in that they simply put power
to all four wheels, usually after an interior shift lever was moved
and/or the vehicle's hubs were manually "locked" by the operator.
During this period 4WD was thought of as a purely off-road utility.
Today 4WD still refers to vehicles that can opt for two- or
four-wheel drive. These vehicles often have a 4WD "low" and "high"
setting that can be switched using an interior lever. The "low"
setting provides even greater torque for pulling or climbing in an
off-road environment. The "high" is useful for slippery on-road
situations like packed snow or ice. 4WD also uses a locking center
differential to avoid unnecessary slippage between the right and
left wheels while off-road.

AWD generally operates all
the time with no "low" or "high" gearing options. While 4WD is
basically an SUV term, associated with off-road use, AWD is a
car/wagon/minivan term that means traction improvement for
performance or bad weather reasons. Some vehicles, like the Mercury
Mountaineer or the Lexus RX300, are tougher to classify because they
are SUVs in appearance but use AWD instead of 4WD.

The "all" part of all-wheel
drive is a bit misleading since the majority of AWD vehicles use the
front wheels primarily and only direct power to the rear wheels when
a sensor detects front-wheel slip. Other cars, like the Porsche 911
Turbo, use AWD for performance purposes like getting from
zero-to-sixty in 3.7 seconds. Even with sufficient horsepower, a
Porsche with only two-wheel drive would be traction-limited to high
4- or low 5-second zero-to-sixty times. AWD also helps keep Subaru
wagons and Chrysler minivans from sliding on wet or snowy roads by
redirecting power from "the wheels that slip to the wheels that
grip."

So, it seems like 4WD/AWD is
the best way to go, right? Well, not exactly. Both of these systems
add substantial weight, complexity and cost to a vehicle. They also
reduce gas mileage because of increased drag on the drivetrain. When
you consider that traction control is becoming more and more common
on front- and rear-wheel drive cars, the supposed advantages of
4WD/AWD start to wane.

By now it should be obvious
that there is no "best" drivetrain layout; just different designs
with different characteristics. Hopefully, after reading this
column, you'll have a better idea which one is right for your
circumstances and not get suckered into buying an all-wheel drive
Subaru for your daily commuter in Phoenix or a BMW M3 for exploring
the Yukon.

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