Pressure question - is inside pressure the same as road pressure? 1

[SirJohnAMacDonald]

I am reading on another forum a discussion that the pressure inside the tire is directly related to the pressure exerted on the road.

http://forums.vwvortex.com/zerothread?id=3360620&page=2

It's a bit of a circus.

Im not sold on it, but the hump might be onto something. Rock crawlers use very low tire pressures. I still don't believe it.

Now Im not a tire engineer, but maybe some of you are, Is there any truth to this? Or are tires really strong enough to hold themselves together that it isn't true? Is there an ENGINEERING resource to look up?

??

 

[CapriRacer]

Just an update from The Tire Society Meeting in Akron, OH September 25 and 26, 2007.

There were a few papers that dealt with footprint pressure distribution. One in particular described the effect different footprint pressure distribtions would have on predicting, using computer modeling, the forces generated by a tire - especially cornering forces. It would have simplified the analysis if the footprint pressure could have been assumed to be equal to the inflation pressure, but this PhD didn't do that. That's because he knows this not to be true.

 

[Compositepro]

Yes, and don't forget other critical factors, like the fact that the air pressure inside the tire is greater at the bottom than the top do to the effect of gravity.

 

[NormPeterson]

I'm not sure what you're trying to say, but maybe I'm just a little slow today.

Anyway, I suspect that a reasonable model for a real-world tire carcass lies somewhere between an idealized zero bending stiffness carcass model (like a soap bubble) and an infinitely rigid carcass model (a thick-wall pipe or a compressed gas torus or cylinder might be real-world approximations). Neither extreme represents a pneumatic tire very well, so it seems pointless to use either of those limiting-case models as the basis for analytical approach here.

Somehow I don't think a zero bending stiffness tire carcass would offer satisfactory directional stability or control . . .


Norm

 

[crysta1c1ear]

Yes, and don't forget other critical factors, like the fact that the air pressure inside the tire is greater at the bottom than the top do to the effect of gravity.

LOL! [highlight]Critical?[/highlight]
More like insignificant!

There is a 'well known' formula - well known at least by people that are interested in such things - for how pressure drops with altitude due to gravity. The formula predicts exponential decay,
exp(-mgh/2T)
where m is the molecular mass of the gas, eg 29 atomic mass units for air, g is gravity, h is the height, and T is the temperature. (The gas constant 2 is modified by a conversion factor if inconsistent units are used for measuring energy, eg k/2 if one uses joules for potential energy m*g*h and kelvin for thermal energy T. The 2 then becomes the Bolzmann constant k in joules per kelvin. If molecular mass is replaced by molar mass, then a further conversion constant of a kmol is intoroduced and the constant k become R, the gas constant in joules per kmol kelvin.)

Using that, I get a 0.006% drop in pressure at an altitude of 1 metre.

I would say air pressure changes with height in a tire due to gravity are effectively nil for all practical purposes.

For a reference for the atmospheric lapse rate formula, see

http://en.wikipedia.org/wiki/Atmospheric_pressure

about half way down, where it is called Equation 2.

Incidentally, I disagree with the formula. If you throw a ball in the air, it loses kinetic energy as it gains potential energy. Air molecules do the same, that it they will get colder as they go higher. However the formula is derived assuming constant temperature as one goes higher, instead of conservation of energy. But that doesn't really affect the fact that pressure drop due to altitude within a tire is completely negligable in my opinion.

PRINT (1-EXP(29*amu*g*1*m/(2*300*kelvin)))/percent) "%"

Adding this line of BASIC to a program that already has atomic mass units, gravity, metres, kelvin and percent already defined prints
-0.00570569177%

So the pressure drop in a metre should be about 0.006%, not a lot.

 

[CapriRacer]

OK, guys,

It's Round 2:

The folks are CarTalk p[osted a question.

Here it is:

"..... This week on Car Talk we talked with Amy in Louisville. She and her dad have been discussing a profound automotive physics conundrum. Specifically: Why doesn't a car's tire pressure change when it's lifted off the ground? Amy's dad thinks it should because you're taking 3,000 pounds of weight off the tires, and "un-scrunching" them. Yet, his trusty tire gauge isn't measuring any change. We weren't afraid to wade in with an answer, and got a little help from our pal Wolfgang. His credentials? A lot more than ours. He's a physicist at the World's Greatest University, just down the street from Car Talk Plaza in Our Fair City.

But what do you think? Did Wolfgang (and we) have it right.... or would you like to go mano-a-mano with someone in a white lab coat at a lectern?

Share your thoughts here. And, trust us, we'll love watching this little discussion develop!

Yours in hot air, no matter what the pressure,



Tom and Ray
Click and Clack, the Tappet Brothers

Posted by: Tom and Ray


The discussion has been quite lengthy - probably the only thing that has generated as much interest has been naming the new cartoon show - Honestly, the CarTalk are just so entertaining, I can't believe this new show can't be a hit!!

But I digress:

So if bfaull shows up, look at the attachment - especially page :

 

[SirJohnAMacDonald]

That same web site I referenced at the top of course has a similar discussion.

I was able to observe a change in pressure.

Are you trying to get to that web site's discussion? It was quite entertaining, test results notwithstanding.

 

[SirJohnAMacDonald]

Well why don't I link it:

http://forums.vwvortex.com/zerothread?id=1705601

 

[CapriRacer]

Sir John:

I am humbled by your promptness:

Do you have no life?

(OK, I have an excuse!! What's yours?)

(BTW, do I have to be a British subject to get knighthood?)