# Air in Tire Question

#### JohnDubYa

Suppose you have the air inside a tire travelling down the length of the tire (that is, in circular motion inside the tire). Now as the air slows down inside the tire, what happens to the tire pressure?

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#### ArmoSkater87

Why would the air be circulating in the first place? But if it were, and it slowed down...then the pressure would increase. This is because traveling air has lower pressure, which is called the venturi effect (sp?).

#### LURCH

I wonder if the effect would be measurable. It is my understanding that certain vehicles have equipment to report air pressure in the tires to a gauge in the dashboard. If the car is brought to a stop, the air inside the tires should continue to circulate in the direction tire was spinning. But this continued motion should gradually slow down over time. I wonder if a person with an air pressure gauge in the dashboard can watch the air pressure in the tire drop rather suddenly as they bring the car to a stop, and then gradually rise as the air inside the tire slows to a stop, applying more lateral pressure against the gauge.

#### jamie

as the air moves faster the pressure increases this is because the air molecules bounce of the tyre walls more frequently generating more heat

#### NateTG

Homework Helper
JohnDubYa said:
Suppose you have the air inside a tire travelling down the length of the tire (that is, in circular motion inside the tire). Now as the air slows down inside the tire, what happens to the tire pressure?
Is this like a rolling tire stopping, or is the tire stationary the entire time? Also, where on the tire are you measuring pressure? Do you want to account for Bernouli effects, or deformation of the tire due to it resting on the ground?

#### JohnDubYa

The tire is resting flat on the ground. It does not matter how the air was made to move initially. Yes, I think Bernoulli effects are important. The deformation of the tire is not an issue.

So far we have one saying the pressure increases, and another that says the pressure decreases.

#### jamie

hello jondubya
try this simple experiment
check the tyres pressures on your car, then take your car for a drive.
recheck your tyre pressures noting that the temperature or the tyre has increased,you can feel this with your hand.
let it cool down again and recheck.
If you have seen top fuel drag cars there rear tyres are virtually flat until they wheel spin and heat up (which increases the pressure). when they cool down they diflate again.
P.S I used to be responsible for keeping the fastest street legal bettle in the world in a straight line and this is one of the considerations i had too account for
I hope this helps

#### Moose352

Hmmm, would the centripedal force exerted on the air particles by the tire increase pressure?

#### jamie

yes, but this would make the particles move faster thus heat them up.
also this does not only apply to round objects. any movement would have this effect.
think of a microwave oven.
put a raw egg in it and laugh when it explodes then think about what caused this?
increase in heat which increases pressure.

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#### ArmoSkater87

There are several factors involved in this situation...so its hard to tell now. You have bernouli affects, centripital acceleration, friction against the ground. Lets think about this, the car is moving, since air has very little friction, it takes a long time for it to start circulating. When it circulates, you have the air pressure drop from bernouli affects. The centripital force makes the air molecules pile up on the bottom of the tire, therefore, having a higher pressure. The friction of the tire against the ground heats up the rubber, but i think we can rule this one out because rubber is a bad thermal conductor. But there is another source of friction, which is the friction of the air against the bottom and sides of the tire (more against the botton since thats the area of higher pressure). SO, now we have higher pressure and temperature at the bottom of the tire, while a little lower pressure towars the center, but at the same time we have all this air circulating, which causes the overall pressure to go down (this is alll while the car is in motion). So, when the car slows down and stops, the air takes a little while to stop circulating (which makes it increase in pressure) because of the small amount of friction it has. At the same time you have the air (hot in the biginning) cooling down slowly, which causes a decrease in pressure. From my understanding of it, there is an equilibrium effect going on here. I dont think you can qualitatively conclude whether it will increase or decrease in pressure.

#### JohnDubYa

There are several factors involved in this situation...so its hard to tell now. You have bernouli affects, centripital acceleration, friction against the ground.
There is no friction against the ground. The tire is laying flat on the ground, with the air moving inside the tire. Over time, the air slows down.

Lets think about this, the car is moving, since air has very little friction, it takes a long time for it to start circulating. When it circulates, you have the air pressure drop from bernouli affects.
Does the pressure drop? Bernoulli's effect is easy to explain in terms of flat surfaces, but curved surfaces are another matter.

The centripital force makes the air molecules pile up on the bottom of the tire, therefore, having a higher pressure.
Again, the tire is lying flat on the ground, not standing vertically.

#### JohnDubYa

If you have seen top fuel drag cars there rear tyres are virtually flat until they wheel spin and heat up (which increases the pressure). when they cool down they diflate again.
This effect has little to do with temperature. It takes quite awhile to heat the interior of a tire, yet the inflation occurs immediately once they begin spinning at high speed. (This effect is due to the inertia of the tread.)

#### NateTG

Homework Helper
If Bernouli effects are important, then the cross section of the tire may well be an issue. Even relatively sane looking fluid dynamics situations can lead to problems where there are sponaeous jumps in pressure. It's a deep and difficult field that I don't know much about.

That said, I believe that the pressure on the exterior wall woud decrease, and the pressure on the interior wall would increase. Since the value is typically on the inside of the tire, that would indicate that the 'tire pressure' would increase while the air stops. However, if you're measuring the pressure by checking the tire diameter, you might find that it decreases.

#### JohnDubYa

Assume a very large cross-section.

This should be easy to verify. Every tire shop has a tire balancer. Simply spin the tire on the balancer until the air inside is reliably moving with the tire. Stop the tire and measure the temperature. Then wait a few hours and measure the temperature again.

Good physics experiment. :)

#### ArmoSkater87

JohnDubYa, did u even read everything in the post i had? In that post i was using the tire on a moving car.

#### JohnDubYa

Yes, but *I* originated the post. Having the tire spinning on a tire balance is not going to heat the tire by any significant factor, which was my objection to your placing the tire on a moving car. You specifically mentioned "friction on the ground," which complicates the issue for no good reason.

#### Cyrus

Why would the air go round with the tire? It would seem that a gas will have random disorder to it. There is nothing pertruding out of the tire, ie. vanes to cause the flow to stirr as it goes round. Only the particles that are along the sides of the wall would be carried around. It would seem that as the tire moves forward, more of the air would strike the back of the tire as it moves forward in time, so the back of the tire would heat up the most. If this were to continue for a long period, the pressure would drop, as a result of tranfering energy to the rear inner tire wall. After a while, I would think the pressure in the tire should go back up as heat is put back into the air that gave it up to friction.

#### NateTG

Homework Helper
cyrusabdollahi said:
Why would the air go round with the tire?
There is friction between the interior of the tire and the gas in it, and gas-gas friction like in a viscous fluid. It might take a while, and warm up the air a little, but the air will at, least in net, be moving with the tire after a while spinning at the same speed.

#### Cyrus

I agree, but if I were to look at the case of a car, the air in the car would have friction with the car moving forward. But clearly the air remains stagnant inside my car. Because in order for the air to stay piled up in the back of my car, the friction that it would encounter when it hits the interior walls would have to be so great that it can only deflect backwards, thus no air moving to the front part of the car. Same as with the tire, wouldent it not seem logical that the friction of the air and the tire would have to be as great too? I would think the air inside the tire remains stagnant as well provided that it is moving with some constant angular velocity. If we consider the amount of air in the tire, which might be alot if its got some good pressure, then if the air were to swirll, the tire would have to give up angular momentum as it emparts it on the air molecules. This would mean that a tire with no friction on its bearings would slow down with time, since the air is being bounced around and thus given momentum in order to keep it swrilling with the wheel. I donnol, im just throwing some things out there to consider, i might be wrong, but at least ill learn something new.

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#### Cyrus

I will reply to myself! Ha :-). Cyrus I diagree you are wrong, based on your same analogy, the air in the car would move with the car, likewise the air with the tire would move forward with the tire, and around with the tire. In that way, the air would seem stationary to a person inside the tire. Now I have a question for you all. If the air is moving inside the tire, I would think that the static pressure would want to drop, and be convereted to dynamic pressure. But somehow I think this is wrong. If im inside my car and the air is moving with me inside the car, I dont experience a drop in air pressure. Likewise, I would expect the tire to remain at the same pressure as if it were not moving, since it is in an enclosed and isolated system. Maybe im just traveling too slow in my car to detect this change in pressure, I dont know, can anyone anwser my question?

#### john6.4

Well I am not a studied pro. in physics or anything but my theory on the air in the tire to add to thought

The air in the tire is an a sealed chamber and for this to change pressure in any way besides being heated from traveling down the road is that it is not possible for pressure to go down when stopping if anything it will increase due to the compression of the tire but since the tire does not change size becoming smaller or larger it will not change tire pressure much.

My question : Does the air in the tire move? due to being in a sealed chamber i believe that the air travels with the car but does not move as much as one would think you see unlike in a vaccume there is no force to move the air this sealed unit cannot mix air it has no stirring agents. there r ribs on the inside of the tire this will cause some air disturbance but i am thinking that there is a stationary Column of air that only travels with the vehicle but not in a circle around the tire. The only way I think it is possible for air to move in a tire is A: you are airing it up B: your have a hole in the tire and the air is escaping...

#### john6.4

Cyrus you r right the air inside your car only travels with you the pressure change in the car from speed is only gradual unless you plan on breaking the sound barrier but with that the air pressure will change if you open a window or increase or decrease in elevation however the tire is the same in that if you go up or down in elevation the pressure would increase or decrease as if you got a hole in your tire like opening the window of your car then the pressure of the tire would change..

There is I bvelieve some friction in the tire but not as much as you would hope for because the inside air stays fairly stationary and I say this becazuse if you opperate a vehicle several thousand miles after you got new tires and then you let the air out of the tire it smells stagnent kinda dead so to speak so it is as if it has not ever moved. Because there is no recirrulation Process only air in and air out.

I would love to find a way to figure out what goes on inside a tire I have a new vehicle that reads tire pressure constantly and it will not tell me a tire is low unless it is 5 psi. low on air so stopping and atmospheric pressure and travel do not affect the tire much heatted highways in the summer make tire pressure increase this is why on the tire it suggests the tire pressure be checked when cold.

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