A tire, as shown in the photograph below, is inflated to its suggested value, in this case about 30 pounds per square inch.


Gwen will now sit on the platform, placing her weight on the tire.  When she does so, the bottom of the tire becomes flattened, as most people have observed when riding a bicycle.  The question this week involves what happens to the air pressure in the tire when the tire is "loaded" in this manner.

When the tire is loaded, the pressure in the tire:

  • (a) increases.
  • (b) decreases.
  • (c) remains the same.

To see the answer, click Read More on 1/2/15


The answer is (d): the pressure in the tire remains the same after the tire is loaded, as seen in an mpeg video by clicking your mouse on the photograph below.  Well, it actually increases a very small  amount, as seen on the video.


When a load is placed on the tire, the bottom of the tire flattens so that the force upward supporting the load is equal to the pressure times the contact area between the road and the tire.  It is tempting to conclude that this would lead to a small decrease in the volume of the tire and, according to the ideal gas law, an increase in the tire pressure.  However, the flattening of the bottom of the tire is accompanied by a slight bulging of the walls of the tire all around its perimeter, so there is no net decrease in the volume of the tire.  The pressure in the tire therefore remains the same. Here the volume of the  tire, and thus the pressure, changes by an insignificant amount.

When you put your spare tire in the trunk you put in the air pressure that you intend to drive with when the tire is loaded.  When you put the tire on the car the air pressure in the tire does not increase  significantly.

If you were to put so much air in the spare tire that the walls could  not expand whe the tire is installed, then the pressure would increase. However, tires are not correctly used at such high pressures.