Tired Weight

Calculate the weight of a car by using air pressure

By measuring the amount of surface each tire presses onto the ground and the pressure inside each tire, you can calculate the weight of a car.

(This activity is not in metric since most common tire pressure gauges are in PSI or Pounds per Square Inch. A metric conversion can me made at the end if necessary.)

A car

  Tire gauge in PSI (pound per square inch)

  Ruler (inches)

  A clean flat dry piece of ground

  Thin cardboard or manila folders

You need to measure the surface area of the bottom of each tire. This "footprint" can be found by shoving pieces of cardboard against the base of each wheel. Remember to set the parking brake while taking these measurements.

1) Most tire footprints will be roughly rectangular in shape. To find the length and width of this footprint, use thin cardboard to define each tire's footprint boundry. Shove the cardboard under the tire as tightly as possible to border each edge of the footprint.

Shove the cardboard under the tire in all four directions.

2) Roll the car off from the cardboard and measure the area bounded by the cardboard. Take measurements in inches.

3) Use a tire gauge to measure the internal air pressure of the tire.

4) Find the amount of tire surface area touching the ground. To find this, multiply the length and width of the "footprint." Your answer should be in square inches.

5) To find the amount of weight the tire holds, multiply the surface area by the psi in that tire.
When you multiply square inches by pounds per square inch, the square inches cancel and you're left with pounds.


6) Repeat these steps for all other tires.

7) Add the weight together for all four tires - that's the total weight of the car.

8) To see how close you came to the real weight of the car, check the owner's manual or look at the specification plate on the inside of the driver's side door.

All of a car's weight is spread among each of the four tires. The weight on each tire is spread among it's footprint.

No accounting for tread.
Don't worry about the gaps in the tire where the tread isn't. The air inside the tire presses down on the smooth interior wall of the tire, the uneven exterior tread is irrelevant.

Original idea from Paul Hewitt.

Eric Muller




©1997 The Exploratorium, 3601 Lyon Street, San Francisco, CA 94123