HomeMy WebLinkAboutThe Citizen, 2007-10-25, Page 20(MS) - Do you know that today
all-wheel-drive (AWD) and four-
wheel-drive (4WD) vehicles make
up about 25 per cent of the car and
truck market, with prediction for 50
per cent of the market in years to
come according to reports by
“Forbes”?
For some that is surprising. It
wasn’t very long ago that these
drivetrains were reserved for a select
number of autos, mainly Jeeps and
commercial trucks.
Despite the fact that the systems
are typically heavier, and in turn
make a vehicle less fuel efficient, the
AWD and 4WD boom is continuing
to make gains.
All of this talk about four-wheel
capabilities begs the questions: Just
what’s the difference between the
two? Aren’t they just one and the
same? Actually, no.
We’ll get to what makes these two
drivetrains unique, but first, this
lesson on torque, traction and wheel
slip, courtesy of
HowStuffWorks.com will be helpful.
Torque:This is the twisting force
that the engine produces and what
moves the vehicle. The gears in a
vehicle’s transmission and the
differential multiply the torque and
split it up between wheels. Lower
gears provide more torque. Torque is
also directly related to traction,
meaning the maximum amount of
torque that can be created is
determined by the amount of
traction, not by the engine. A car
engine can be very powerful, but if
the traction is not there, the car
won’t move very well.
Traction:A number of factors
influence traction, including the
weight placed on the tires. However,
weight shifts as the car maneuvers.
For example, when a car makes
turns, weight shifts to the outside
wheels. During acceleration, weight
shifts to the rear wheels. The
coefficient of traction also is
involved. This is the relationship
between the amount of traction
between the tires and the road to the
weight resting on each tire. The
coefficient of traction can be
changed based on the tire used for a
specific type of road. For example, a
large, knobby tire for off-roading in
the mud would have more traction
than the same tire on a dry, flat road.
Wheel slip:This occurs when the
force being applied to a tire exceeds
the traction available to the tire.
Force can be longitudinal - that
which comes from the torque
applied by the engine or the break,
or lateral - that which is the force it
takes for a car to change directions
when driving around a turn.
Here’s where 4WD and AWD
come into play. In cars that drive
four wheels you have the ability to
double the amount of longitudinal
force (what makes the car go) that
the tires apply to the ground -
improving traction in many
conditions and reducing wheel slip.
Scenarios where AWD and 4WD
have advantages include snow,
where most 2WD cars lose traction
quickly; off-roading, where one or
more tires may lose traction when
crossing a stream, etc.; or when a
vehicle needs to scale slippery hills.
PAGE 20. THE CITIZEN, THURSDAY, OCTOBER 25, 2007.
Sliding through
Today all-wheel and four-wheel drive vehicles make up for
about 25 per cent of the car and truck market. (MS photo)
Gearing up for slippery road conditionsOOnn tthhee RRooaadd
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