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Tire Pressure

It is impossible for the tire pressure to change (at constant temperature of course) unless you add or remove air from the tire. You guys are losing it, or what is left of it, because I know you studied this in Thermo 101.


The “guys” don’t visit here. There’s only us lowlifes.

Not having heard the show you’re alluding to, I have no comment to offer. Other than the word “permeation”.

Let’s see, remove the wheel, check the pressure, return the wheel and check the pressure. Not a lot of change, but it will be higher.

I’d bet you can now guess the why.

Ok, aside from any leakage. I guess also you need to assume that the tire doesn’t stretch, which is not that unreasonable for most high quality tires under normal pressure.

When you load a tire, a flat spot on the bottom of the tire is created where it is contact with the ground. The weight on the tire is equal to the area of the flat spot (in square inches) times the tire pressure (in psi). Add more weight - the flat spot increases to equalize the weight. But the pressure does not increase.

That’s correct, but does not mean that the pressure in the tire would not also increase.

Take the thought experiment to the extreme. If you were to put a loaded Peterbuilt rig totalling 80,000 pounds on Honda Civic sized tires, the tires would probably explode. Why would they explode? Because the pressure created would exceed the amount the tires could hold.

To apply this to your example, if the tires were at 30 psi and were loaded with 600 pounds each, the “flat spot” would be 20 square inches. But adding increasing the weight to 1200 pounds might increase the air pressure to 35 psi. The "flat spot would increase to 34.28 square inches, certainly larger but not by a factor of 2.

Air does compress. Adding load to air in a tire can in fact increase the pressure. Increasing the “flat spot” and increasing the pressure are not mutually exclusive.

Let’s not forget that altitude will also affect pressure in the tire. I don’t have the whole story here so I don’t know if altitude even enters into the equasion or not. There is always more than one way to skin a cat.

Yep. Well I guess, like a lot of scientific concepts, it is all in the assumptions, which is why I stated “high quality tires under normal pressure.”

Loading a car until the tires explode is not really normal. Doubling the weight would be like putting a whole other car on the roof. Again, not normal.

The basic equilibrium relationship, as I’m sure you know, includes T, P, and V. So I thought that with T and V constant (stipulated in the assumptions), it is reasonable to assume P is constant.

But my original post is oversimplified, for sure.

The caller was wondering whether the tire pressure would read differently with and without a person in the car. That means a change closer to 150 lbs, which would suggest no detectible change, which is what the guys said. Probably there would be no change at all, because of the change in contact patch, and I think that small distinction-- no change vs no measurable change is the point of disagreement here.

The point is that you stated (if I understood correctly) that the weight of the car does not increase the pressure in the tires. It does. The amount is small, but the same principle that caused the tires in my truck illustration to explode also applies to adding lesser weight.

Thanks for clarifying the question.

Adding a person will change the pressure, but not enough to be measurable with any tire gage. You’d need a manometer, a liquid-column manometer.

Um, unless perhaps the person was one of the “Biggest Loser” contestants.

“The weight on the tire is equal to the area of the flat spot (in square inches) times the tire pressure (in psi).”

Sorry, but this old wive’s tale is not true.