Tires Supporting Cars
How do tires support the weight of a car?
The weight of the car is distributed over the 4 tires. The air pressure
provides a force sufficient to support the weight of the car. Here is a
"reality check" i.e. does this explanation "make sense" Typically, the tire
pressure is ~ 30 lb/in^2. Assume each tire has a contact area of 5x6 in^2.
That is probably OK for a "back-of-the-envelope" estimation. So the total
contact area is 4 x 5 x 6 in^2 = 120 in^2. At a pressure of 30 lb/in^2 that
is ~ 3600 lb. which is the order of magnitude of the weight of a car.
If the tires are under-inflated the contact surface area increases to
compensate for the weight of the car. If the car is heavily loaded (or the
vehicle is a 16 wheeler semi) the number of tires is increased by a factor
of 4X compared to a car, and the tire pressure is also increased.
Tires support a car's weight by distributing the weight across the ground.
Most car tires are inflated to around 30 PSI. (Pounds per square inch.) If
you look at where tires touch the road, you'll see they do not stay perfectly
circular, and instead 'spread out' just a bit. On my car, each tire has
about an 6 inch by 8 inch "footprint". That's 48 square inches per tire,
192 square inches for all 4, and 30 times that, or 5760 pounds.
One aspect of today's tires is they are filled
with air. Take a step back to wagon train wheels,
they were solid, or early cars had solid tires. A pencil is solid, it is
wood, with a thin sliver of lead
in the center. Now take a book, lay it on the table, flat, put one pencil
under the top part and another, under the bottom part. The book is
supported by the pencils. There is a space under the book. Air does not
support the book, so the weight of the book, can be consider to shift, over
to what is contacting it. The pencils. The pencil
is contacting the table. The weight of the book is less than the weight, or
force that would crush the pencil, so , in a way, the weight is transmitted
through the pencil to the table. If, instead of a pencil, you tried
something softer, like say a cooked noodle, of the same diameter as the
weight of the book would crush, or flatten the noodle, and the book would
drop, and be supported by the table. Remember those pictures of the
Flintstones in their car.
This one shows a stone for the wheels, they had another one with a tree. We
talked about a book, instead of a car with people, and a pencil instead of a
tree for wheels.
The modern cars made refinements, instead of a continuous solid wheel, they
use four wheels at the corners. Then, instead of solid the tires are filled
with air. Remember the noodle, the pasta is weak ( weaker than rubber) and
the air in the center of the noodle is not under pressure, so it collapses
under the weight. Tires are designed so the shell or rubber, can hold air
under pressure, so the combination is strong enough to let the weight, of the
car pass through to the road. The width of the tire is also involved. If
the car was like the Flintstone's, a big wheel is a waste, so they have
calculated the right width, rubber and air pressure to carry the weight.
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Update: June 2012