Front-wheel drive
vs.
rear-wheel drive vs. all-wheel drive
Automotive history is adept at repeating itself and while virtually
all technological trends have resurfaced many times over the course
of the last 100 years, there are significant milestones for each
ground-breaking innovation.
The same is true for drive trains and driven wheels.
Notwithstanding the odd exception, rear-wheel drive was the staple
for the majority of passenger and commercial vehicles as far back as
the birth of the car as we know it today.
In the late 1950s, and after a number of niche models, it was the
Mini that established the now-commonplace transverse engine,
front-wheel drive layout as a genuinely practical proposition.
Then, in 1980, Audi’s Quattro was the pioneer that brought
four-wheel drive from the off-road arena into the world of on-road
high-performance cars.
These three radically different drivetrain approaches have now
proliferated into the model ranges of virtually all brands, offering
a huge amount of choice for the consumer.
Here are the key differences to consider when choosing what’s right
for you.
REAR-WHEEL DRIVE
Advantages
Many regard rear-wheel drive as the purist’s solution and the only
layout for optimum driving enjoyment. The theory goes that by
driving the rear wheels, the front wheels are allowed to get on with
steering, and a majority of the braking, for complete and balanced
driving manners.
Generally speaking, driving the rear wheels also promotes a more
inherent tendency for oversteer on the limit, which is considered
more fun and manageable in the right hands than the understeer often
associated with front-drivers.
The engine in a rear-wheel drive is typically oriented
longitudinally (if located at the front), which allows the
installation of a larger engine provided the engine bay is long
enough to accommodate it.
Disadvantages
The natural tendency to oversteer can be unsafe for inexperienced
drivers (although it is up to the driver to maintain control of any
vehicle given the circumstances), but modern electronic stability
systems are very effective at keeping cars on the straight and
narrow.
Sending power from a front-mounted engine to the rear wheels
requires a propshaft and rear differential, which take up space and
add more weight. Most rear-drives have a ‘transmission tunnel’ which
robs some cabin room along the middle of the car.
FRONT-WHEEL DRIVE
Advantages
By confining the bulky engine, gearbox, driveshafts and steering to
the front end of the car, the rest of the vehicle’s space is freed
up for occupants and luggage.
The overall vehicle package can also be made smaller and virtually
all vehicles in the compact segment use a front-engine, front-wheel
drive layout.
Disadvantages
For high-performance applications, getting power down to the road
through the front wheels causes more problems than a rear-driven
car, and wheelspin is more likely, particularly on slippery surfaces
such as mud and ice.
Another typical front-drive trait in more powerful vehicles is an
effect called ‘torque steer’ which is when the car pulls to one side
with heavy throttle application. Many manufacturers have attempted
to counter the effect with a number of solutions, and with varying
levels of success.
Front-drive cars use a device called a ‘constant velocity’ joint or
CV joint which allows the drive shaft to turn with the steering
wheels but the limited dexterity of the CV joint also has a
limitation on maximum steering angle and turning circle.
With the entire drivetrain over the front axle, front-wheel drives
have a natural tendency towards understeer, which is exacerbated
with the application of too much power, just as excessive power
encourages oversteer in rear-drives. It is also harder for engineers
to distribute the vehicle weight equally over the axles, which
effects handling.
ALL-WHEEL DRIVE
Advantages
In a word – traction. With all four wheels sending torque to the
surface, all-paw cars offer a safety and performance advantage
compared with two-wheel drive cars, particularly in adverse weather
conditions and on loose surfaces.
Acceleration from a standing start is generally faster in
high-powered vehicles with the maximum power of the engine shared
between a larger tyre contact patch.
The traction advantage also makes four-wheel drive cars more
suitable for towing larger loads or on slippery surfaces where the
extra ground-clearance of an SUV is not necessary.
Disadvantages
With an extra axle to drive, the additional differential and pair of
half-shafts makes a four-wheel drive system heavier than two-wheel
drive systems and this will have an effect on fuel consumption and
possibly handling.
There are systems that disconnect one pair of wheels when they are
not required which reduces rotational losses but they cannot reduce
the consumption caused by carrying around extra weight.
The extra differential and drive shafts can also increase the amount
of service items that require attention when in for maintenance
work.
Myth...
All-wheel drive systems do not increase the ‘grip’ of a tyre.
Four-wheel drives have a traction advantage over two wheel drives
because all four tyres can contribute but, ultimately, the grip of a
vehicle is dictated by the performance of the rubber on the road.
This particularly applies to during cornering where the speed of the
car is governed, not by the driven wheels, but more by the
weight-distribution of the car, centre of gravity, tyre performance
and suspension system.
There are exceptions to all of the examples given, and the above is
intended only as a general guide to help you start thinking about
which layout best fits your requirements, but as always, the best
approach is to get out there and start test driving.
