You may have wondered how
a car tire with 30 pounds per square inch (psi)
of pressure can support a car. This is
an interesting question, and it is related
to several other issues, such as how much
force it takes to push a tire down the road
and why tires get hot when you drive (and
how this can lead to problems).
The next time you get in your car, take a
close look at the tires. You will notice
that they are not really round. There is a
flat spot on the bottom where the tire meets
the road. This flat spot is called the contact
patch, as illustrated here.
If you were looking up at a car through a
glass road, you could measure the size of
the contact patch. You could also make a
pretty good estimate of the weight of your
car, if you measured the area of the contact
patches of each tire, added them together
and then multiplied the sum by the tire
pressure.
Since there is a certain amount of
pressure per square inch in the tire, say 30
psi, then you need quite a few square inches
of contact patch to carry the weight of the
car. If you add more weight or decrease the
pressure, then you need even more square
inches of contact patch, so the flat spot
gets bigger.
A properly inflated
tire and an underinflated or overloaded
tire
You can see that the underinflated/overloaded
tire is less round than the properly
inflated, properly loaded tire. When the
tire is spinning, the contact patch must
move around the tire to stay in contact with
the road. At the spot where the tire meets
the road, the rubber is bent out. It takes
force to bend that tire, and the more it has
to bend, the more force it takes. The tire
is not perfectly elastic, so when it returns
to its original shape, it does not return
all of the force that it took to bend it.
Some of that force is converted to heat in
the tire by the friction and work of bending
all of the rubber and steel in the tire.
Since an underinflated or overloaded tire
needs to bend more, it takes more force to
push it down the road, so it generates more
heat.
Tire manufacturers sometimes publish a coefficient
of rolling friction (CRF) for their
tires. You can use this number to calculate
how much force it takes to push a tire down
the road. The CRF has nothing to do with how
much traction the tire has; it is used to
calculate the amount of drag or rolling
resistance caused by the tires. The CRF is
just like any other coefficient
of friction: The force required to
overcome the friction is equal to the CRF
multiplied by the weight on the tire. This
table lists typical CRFs for several
different types of wheels.
| Tire Type |
Coefficient of Rolling
Friction |
| Low rolling resistance car tire |
0.006 - 0.01 |
| Ordinary car tire |
0.015 |
| Truck tire |
0.006 - 0.01 |
| Train wheel |
0.001 |
Let's figure out how much force
a typical car might use to push its tires
down the road. Let's say our car weighs
4,000 pounds (1814.369 kg), and the tires
have a CRF of 0.015. The force is equal to
4,000 x 0.015, which equals 60 pounds
(27.215 kg). Now let's figure out how much power
that is. If you've read the HowStuffWorks
article How
Force, Torque, Power and Energy Work,
you know that power is equal to force times
speed. So the amount of power used by the
tires depends on how fast the car is going.
At 75 mph (120.7 kph), the tires are using
12 horsepower,
and at 55 mph (88.513 kph) they use 8.8
horsepower. All of that power is turning
into heat. Most of it goes into the tires,
but some of it goes into the road (the road
actually bends a little when the car drives
over it).
From these calculations you can see that
the three things that affect how much force
it takes to push the tire down the road (and
therefore how much heat builds up in the
tires) are the weight on the tires, the
speed you drive and the CRF (which increases
if pressure is decreased).
If you drive on softer surfaces, such as
sand, more of the heat goes into the ground,
and less goes into the tires, but the CRF
goes way up.
