How do thirty pounds of air in your tires hold up two tons of car?

Cars are heavy machines, so how can a few small, pressurized tires support their weight? Marshall Brain explains how tire pressure and contact patches work in this episode.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff works dot com
where smart happens. Hi. I'm Marshall Brain with today's question,
how do thirty pounds of air in your tires hold
up two tons of car? The next time you get

(00:21):
in your car, take a close look at the tires.
You'll notice that they are not really round. They're squashed
at the bottom. That flat spot on the bottom where
the tire meets the road is called the contact patch
for the tire. If you were looking up at the
car through a glass road, you can measure the size
of this contact patch. You would multiply the length of

(00:43):
the contact patch by its width to get the area,
then add up the area for all four tires to
get the total area of the contact patch. For your
two ton car, or your four thousand pound car, or
your two thousand kilogram car, you will find that the
air of the contact patch is about equal to the
weight of the car divided by the tire pressure. In

(01:05):
this case, four thousand pounds divided by thirty pounds per
square inch equals add thirty three square inches. That may
seem like a lot, but your car's tires are probably
about seven inches wide. That means that the contact patch
for each tire will be only about four and a
half inches long. If you go outside and measure the

(01:26):
size of the contact patch on your car, you'll probably
find that it's 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 the tire until they won't go any further. Now,
measure the distance between the two pieces of paper. The

(01:48):
reason that the contact patches 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 way it is supported.
As you move towards the center of the contact patch,
more and more weight is supported. Now, let's say that

(02:10):
you drop the pressure in your tires to seven p
s i 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 side while 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 you see

(02:32):
in sports cars have short, stiff side walls, so these
tires will tend to squish less than the big tires
on an suv or a pickup truck. In fact, there
are some tires that are so stiff they can run
with no pressure in them at all. These are called
run flat tires. Be sure to check out our new
video podcast, Stuff from the Future. Join how Stuff Work

(02:54):
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