Torque, Traction and
Wheel Slip
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.
The bar graph below
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.
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.
·
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.
Quite simply, wheel slip
occurs when the force applied to a tire exceeds the traction available to that
tire. Force is applied to the tire in two ways:
·
Longitudinally -- Longitudinal force comes
from the torque applied to the tire by the engine or by the brakes. It tends to
either accelerate or decelerate the car.
·
Laterally -- Lateral force is created
when the car drives around a curve. It takes force to make a car change
direction -- ultimately, the tires and the ground provide lateral force.
Let's say you have a fairly
powerful rear-wheel-drive car, and you are driving around a curve on a wet
road. Your tires have plenty of traction to apply the lateral force needed to
keep your car on the road as it goes around the curve. Let's say you floor the
gas pedal in the middle of the turn (don't do this!) -- your engine sends a
lot more torque to the wheels, producing a large amount of longitudinal force.
If you add the longitudinal force (produced by the engine) and the lateral
force created in the turn, and the sum exceeds the traction available, you just
created wheel slip.
Most people don't even come
close to exceeding the available traction on dry pavement, or even on flat, wet
pavement. Four-wheel and all-wheel-drive systems are most useful in
low-traction situations, such as in snow and on slippery hills.
The benefit of four-wheel
drive is easy to understand: If you are driving four wheels instead of two,
you've got the potential to double the amount of longitudinal force (the force
that makes you go) that the tires apply to the ground.
This can help in a variety
of situations. For instance:
·
In snow -- It takes a lot of force
to push a car through the snow. The amount of force available is limited by the
available traction. Most two-wheel-drive cars can't move if there is more than
a few inches of snow on the road, because in the snow, each tire has only a
small amount of traction. A four-wheel-drive car can utilize the traction of
all four tires.
·
Off road -- In off-road conditions,
it is fairly common for at least one set of tires to be in a low-traction
situation, such as when crossing a stream or mud puddle. With four-wheel drive,
the other set of tires still has traction, so they can pull you out.
·
Climbing slippery hills -- This task requires a lot
of traction. A four-wheel-drive car can utilize the traction of all four tires
to pull the car up the hill.
There are also some
situations in which four-wheel drive provides no advantage over two-wheel
drive. Most notably, four-wheel-drive systems won't help you stop on slippery
surfaces. It's all up to the brakes and theanti-lock braking system (ABS).
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