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When an automobile coasts along the
road, the resistive force of rolling friction on tires
slows down the motion. The rolling friction of the tire
is slightly affected by the static friction of the
rubber on the pavement. The adhesion effect of the
rubber adds a little more to the rolling friction. But
the major contribution to the rolling friction is the
deformation of the tire while rolling. The coefficient
of friction for the automobile tire can be determined
experimentally, but it only applies to the specific
configuration.
Questions you may have are:
-
How does the rolling coefficient of
friction compare with the others?
-
How does deformation of affect
rolling friction?
-
How does adhesion of affect rolling
friction?
Friction effects
Rolling friction for a hard wheel on a
hard surface is quite small and is a combination of
contributions of static friction and friction from
molecular adhesion. For example, the coefficient of
rolling friction for a train wheel on a steel rail is
only 0.001. That is less than the coefficient of sliding
friction on ice.
But an automobile tire is made of rubber
and is filled with air. It deforms under the weight of
the car, and that deformation contributes greatly to the
rolling friction. The result is that the coefficient of
rolling friction is about 15 times as great. A typical
automobile tire has an average coefficient of rolling
friction of μR = 0.015.
Rolling friction equation for tires
You can apply the standard friction
equation for rolling wheels to try to determine the
value of rolling friction. That equation is:
FR = μR*W
where
FR is the resistive force of
rolling friction,
μR is the coefficient of rolling
friction for the two surfaces (Greek letter "mu" sub R),
and
W is the weight of the wheel plus
the weight of the automobile.
This equation is not as straightforward
as with sliding friction for hard surfaces, since μR
varies with the radius, width, treads, amount of
inflation and temperature of the tire, as well as the
type of rubber and the value of W. The surface roughness
of the pavement is also a factor.
What this means is that μR can vary
considerably depending on the experimental conditions.
Measuring coefficient of rolling
friction
One way to measure the tire's
coefficient of rolling friction is to roll the tire at a
given velocity and then measure how long it takes to
come to a stop. The equation used is:
μR= v*g/t
where
-
v is the initial velocity (m/s or
ft/s),
-
g is the acceleration of gravity
(9.8 m/s2 or 32 ft/s2),
-
t is the time in seconds it takes to
stop.
The problem is that since you are just
rolling the tire by itself, the deformation effects from
the weight of the car are not included. Thus the
coefficient would not be accurate.
On the other hand, if you started a car
rolling at velocity v, the tires would be deformed as in
actual use. The problem now would be that there would be
an added friction from the wheels turning on their axles
that would also slow down the car. Thus the coefficient
again would not be accurate.
The only way around that problem would
be to take a separate measurement of the friction of the
axles and their bearings and then subtract that from the
previous measurement.
My head hurts from all this! But I guess
you have to go through all this trouble if you want to
find an accurate coefficient of rolling friction for a
tire. But then, if you would change the air pressure,
you'd have to repeat the test.
When an automobile coasts along the
road, the resistive force of rolling friction on tires
slows down the motion. The rolling friction of the tire
is slightly affected by the static friction of the
rubber on the pavement and the adhesion effect of the
rubber. The major contribution to the rolling friction
is the deformation of the tire while rolling. The
coefficient of friction for the automobile tire can be
determined experimentally, but it only applies to the
specific configuration |
