Does Weight on a Tire Affect Pressure?

I am having an interesting discussion with one of my fellow workers on how to air up a flat tire on an all terrain forklift. This is a 6000lb lift with extendable reach forks. 2 air in the tire I extended the forks and raised the front tires up off the ground so that we could more easily fill the flat tire. My associate argued with me that that was not necessary. I agreed it wasn't necessary but it was faster because we weren't just filling the tire.... we were also trying to pick the vehicle up off the ground while filling the tire. His argument was.... pressure is equal. He said if you have 50 pounds of air in a truck tire.... and you put a ton of weight on the bed of the truck, the tire still only has 50 pounds of pressure in it. I agreed with him. But I argued that this wasn't the case with filling the tire with air. I said to him so let's say all you want to do is lift up that tire and wheel. Is it easier for you to lift the tire and wheel why what is connected to the forklift.... or easier for you if if was disconnected from the lug nuts. He insists that I am incorrect. I am certain that by removing the weight of the forklift from the tire that we are trying to fill..... I am making it easier to put air into the tire. Which one of us are right?
- Al Cella (age 54)
Roseburg, OR USA

Hi Al,

Let's begin with your friend's example. If you have 50 psi air pressure in a truck tire, and then you put a massive weight in the truck bed, the tires will be squished downward. This will decrease the volume of the air in the tire, and the pressure will increase (by Boyle's law).

This effect might be small, since the tire can squish out sideways, and almost maintain its volume. Depending on how rigid the rubber is, the volume will still decrease somewhat. If you want to test how large the effect is, get your pressure gauge and forklift, do the experiment, and let us know!

Now, for your question. Because the pressure increases as you put weight on the tire, the tire becomes more difficult to pump up. Somehow when you take into account that increased air pressure, the extra work required must be enough to lift the truck.

So, your intuition is correct. 

Best,
David Schmid

p.s. Actually, I thought of a much simpler way to prove my argument. If you pump up the tires on a vehicle which is suspended, then the center of mass of the vehicle doesn't move, so all the work you have to do is to force the air into the tire. If the vehicle isn't suspended, then as the tires expand, the vehicle is lifted slightly higher into the air, raising its center of mass against the force of gravity. This increase in potential energy could only have come from the work you did in pumping the tire, so you clearly had to do additional work.

(published on 05/08/2013)