Pro and Con Features of Front Wheel Drive
Compared to Advantages
- Interior space: Since the powertrain is a
single unit contained in the engine compartment of the vehicle, there is
no need to devote interior space for a driveshaft tunnel or rear differential,
increasing the volume available for passengers and cargo.
- Instead, the tunnel may be used to route the
exhaust system pipes.
- Weight: Fewer components usually means lower
weight.
- Improved fuel efficiency due to less weight.
- Cost: Fewer material components and less
installation complexity overall. However, the considerable MSRP
differential between a FF and FR car cannot be attributed to layout alone.
The difference is more probably explained by production volume as most rear-wheel
cars are usually in the sports/performance/luxury categories (which tend
to be more upscale and/or have more powerful engines), while the FF
configuration is typically in mass-produced mainstream cars. Few modern
"family" cars have rear-wheel drive as of 2009, so a direct cost
comparison is not necessarily possible. A contrast could be somewhat drawn
between the Audi A4 FrontTrak (which has an FF layout and front-wheel
drive) and a rear-wheel-drive BMW 3-Series (which is FR), both which are
in the compact executive car classification.
- Improved drivetrain efficiency: the direct
connection between engine and transaxle reduce the mass and mechanical inertia
of the drivetrain compared to a rear-wheel-drive vehicle with a similar
engine and transmission, allowing greater fuel economy.
- Assembly efficiency: the powertrain can often
be assembled and installed as a unit, which allows more efficient
production.
- Placing the mass of the drivetrain over the
driven wheels moves the centre of gravity farther forward than a
comparable rear-wheel-drive layout, improving traction and directional
stability on wet, snowy, or icy surfaces.
- Predictable handling characteristics:
front-wheel-drive cars, with a front weight bias, tend to understeer at
the limit, which (according to SAAB engineer Gunnar Larsson) is easier
since it makes instinct correct in avoiding terminal oversteer, and less
prone to result in fishtailing or a spin.
- A skilled driver can control the movement of
the car even while skidding by steering, throttling and pulling the hand
brake (given that the hand brake operates the rear wheels as in most
cases, with some Citroen and Saab models being notable exceptions).
- It is easier to correct trailing-throttle or
trailing-brake oversteer.
- The wheelbase can be extended without
building a longer driveshaft (as with rear-wheel-driven cars).
- Front-engine front-wheel-drive layouts are
"nose heavy" with more weight distribution forward, which makes
them prone to understeer, especially in high horsepower applications.
- If a front-engine front-wheel-drive layouts
is fitted with a four-wheel-drive, plus enthusiast driver aids, such as
active front differential, active steering, and ultra-quick
electrically-adjustable shocks, this somewhat negate the understeer
problem and allow the car to perform as well as a front-engine
rear-wheel-drive car. These trick differentials, which are found on the Acura
TL SH-AWD and Audi S4 3.0 TFSI quattro, and Audi RS5 4.2 FSI quattro, are
heavy, complex, and expensive. While these aids do tame front end plow,
cars fitted with these systems are still at a disadvantage when track
tested against rear-wheel drive vehicles (including those with added
four-wheel drive).
- Torque steer is the tendency for some
front-wheel-drive cars to pull to the left or right under hard
acceleration. It is a result of the offset between the point about which
the wheel steers (it is aligned with the points where the wheel is
connected to the steering mechanisms) and the centroid of its contact patch.
The tractive force acts through the centroid of the contact patch, and the
offset of the steering point means that a turning moment about the axis of
steering is generated. In an ideal situation, the left and right wheels
would generate equal and opposite moments, canceling each other out;
however, in reality, this is less likely to happen. Torque steer can be
addressed by using a longitudinal layout, equal length drive shafts, half
shafts, a multilink suspension or centre-point steering geometry.
- In a vehicle, the weight shifts back during
acceleration, giving more traction to the rear wheels. This is one of the
main reasons nearly all racing cars are rear-wheel drive. However, since
front-wheel-drive cars have the weight of the engine over the driving wheels,
the problem only applies in extreme conditions such as attempting to
accelerate up a wet hill or attempting to beat another RWD car off the
line.
- In some towing situations, front-wheel-drive
cars can be at a traction disadvantage since there will be less weight on
the driving wheels. Because of this, the weight that the vehicle is rated
to safely tow is likely to be less than that of a rear-wheel-drive or
four-wheel-drive vehicle of the same size and power.
- Traction can be reduced while attempting to
climb a slope in slippery conditions such as snow- or ice-covered
roadways.
- Due to geometry and packaging constraints,
the CV joints (constant-velocity joints) attached to the wheel hub have a
tendency to wear out much earlier than the universal joints typically used
in their rear-wheel-drive counterparts (although rear-wheel-drive vehicles
with independent rear suspension also employ CV joints and half-shafts).
The significantly shorter drive axles on a front-wheel-drive car causes
the joint to flex through a much wider degree of motion, compounded by
additional stress and angles of steering, while the CV joints of a
rear-wheel-drive car regularly see angles and wear of less than half that
of front-wheel-drive vehicles.
- Turning circle — FF layouts almost always
use a Transverse engine ("east-west") installation, which limits
the amount by which the front wheels can turn, thus increasing the turning
circle of a front-wheel-drive car compared to a rear-wheel-drive one with
the same wheelbase. A notable example is the original Mini. It is widely
misconceived that this limitation is due to a limit on the angle at which
a CV joint can be operated, but this is easily disproved by considering
the turning circle of car models that use a longitudinal FF or F4
layout from Audi and (prior to 1992) Saab.
- The FF transverse engine layout (also known
as "east-west") restricts the size of the engine that can be
placed in modern engine compartments, so it is rarely adopted by powerful
luxury and sports cars. FF configurations can usually only accommodate Inline-4
and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8
will rarely fit, though there are exceptions. One way around this problem
is using a staggered engine.
- It makes heavier use of the front tyres
(i.e., accelerating, braking, and turning), causing more wear in the front
than in a rear-wheel-drive layout.
- Under
extreme braking (like for instance in a panic stop), the already front
heavy layout further reduces traction to the rear wheels. This results in
disproportionate gripping forces focused at the front while the rear does
not have enough weight to effectively use its brakes. Because the rear
tyres' capabilities in braking are not very high, a significant number of
cheaper front drive vehicles use drum brakes in the rear even today.
- The steering 'feel' is more numbed than a RWD
car. This is due to the extra weight of drive shafts and CV join
components that increase unsprung weight. Combined with torque steer,
determining how much lateral traction is actually available is more
difficult if not impossible especially during high performance driving.
No comments:
Post a Comment