How do 30 pounds of air in your tires hold up 2 tons of
car?
Answer
The next time you
get in your car, take a close look at the tires. You
will notice that they are not really round. They are squished
at the bottom. The flat spot on the bottom where the tire
meets the road is called the contact patch.
If you were looking up at a car through a glass road, you
could measure the size of the contact patch. You would
multiply the length of the contact patch by its width to get
the area, then add up area for all four tires to get the total
area of the contact patch.
A tire showing
the side and bottom view of the contact patch.
For your 2-ton (4,000 lb) car, you will find that the area
of the contact patch is about equal to the weight of the car
divided by the tire pressure. In this case 4,000 pounds
divided by 30 pounds per square inch equals 133 square inches.
That may seem like a lot, but your car's tires are probably
about 7 inches wide. That means that the contact patch for
each tire will be about 4.75 inches long.
If you go outside and measure the size of the contact
patch, you will probably find that it is actually even bigger
than this. You can measure the width of the tread pattern
anywhere on the tire to get a pretty good idea of the width of
the contact patch. To get the length, take two sheets of paper
and slide them under the front and back of a tire until they
won't go any farther. Now measure the distance between the two
pieces of paper.
The reason that the contact patch is even bigger than this
calculation suggests is that, at the back and front edge of
the contact patch, the pressure exerted on the ground is not
very high. At the point where the tire is just barely touching
the ground, almost no weight is supported. As you move toward
the center of the contact patch, more and more weight is
supported.
Now let's say you dropped the pressure in your tires to 7.5
psi, a quarter of what it was. You would find that your
contact patch did not get four times bigger. This is where the
stiffness of the sidewall of the tire comes in. When the
pressure is this low the structure of the tire starts to bear
some of the weight of the car.
Low-profile tires like sports car tires have short, stiff
sidewalls, so these tires will tend to squish less than the
big tires on SUVs and pickup trucks. In fact there are some
tires that are so stiff they can run with no air pressure in
them. These are called run flat tires.
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