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This came up somewhere else. I think I know the answer. But I'm not a thermodynamics expert, so I could be wrong.

The question is, do you have to change air pressure according to the altitude you're driving at? I'm thinking, no.

When you measure the pressure in your tires, what are you actually measuring? If the gauge reads "0", is there actually 0 psi in the tire? No. "0" would be a vacume, no pressure at all. You are mesuring the differance between ambient air pressure, and tire pressure. At standard temp and pressure, 41 psi actually means 58.7psi. (or something close to that).

Lets say you are in death valley (-200MSL), and fill your tire to 32 psi. Then you ride to my town (+6500MSL). What happens to your tire pressure? It should go up, Lets say to 37psi (let a physics whiz figure out he exact number). Now you have to let air out to compensate and get back to 32psi. So you let out 5 psi. Does it make sense to let out another 5psi, and set the pressure at 27?

So if the tire manufacturer recommends 32psi, to me that means 32psi greater than ambient air pressure for the tire to operate correctly. So it should be 32psi regardless of altitude, weather, or ambient air temperature.

Of course, that's just my opinion. I could be wrong. What do you think?
 

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It's my opinion that you should inflate to the recommended pressure, regardless of altitude. I believe your analysis of readings being relative to the ambient pressure is correct. A zero reading indicates a flat with pressure equal to the surroundings, not a vacuum in the tire, as you point out.

It's my suspicion that tire pressure changes much more dramatically with ambient temperature (say 100 in the valley and 40 in the mountains) than it would between sea level and 5000 feet. I doubt you'd ever change enough altitude to warrant stopping to adjust your tires on the way up or down, but you might check after arrival. (Adjusting your carb is another matter -- and the reason I chose to go with EFI!)

Hope we get some more posts -- spending time in Houston and in Colorado, this is interesting and important to me as well.

Cheers, John
 

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A gauge reads the pressure difference between atmospheric pressure and the pressure in the tire. Inflate the tire to the recommended pressure regardless of altitude, temperature, etc.

Nate
 

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Tire pressure is gage pressure or the pressure over 1 atmosphere, roughly 14.7PSI. The pressure in the tire over the ambient is what flattens out the tire contact patch.

The pressure in the tire does change with changes in the barometric pressure and altitude as you suspect. (This is how barometers work.) For weather systems, the changes are pretty small because the changes in our air pressure are pretty small. (In the range of 1" of mercury out of 30") That works out to about 3% or 1/2 PSI.

At altitude, things change more. Check out this link. Altitude vs Pressure . At 6000', you've gained about 3PSI. Typically, it gets cooler at altitude and the temp drop will lower pressure. this will counteract the increased pressure.

With those two factors coming into play, You should reset your tire pressure (to the same pressure) to suit the new altitude and temperature after your tires have cooled to the ambient temp.

Sorry for the long reply, HTH
 

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Originally posted by Jerry:
.... The pressure in the tire over the ambient is what flattens out the tire contact patch.....
I'm certainly not a rocket scientist, but I'm pretty sure that the flexibility of the tire and the weight of the car is what flattens out the contact patch. The pressure in the tire keeps the rim off the ground.
 

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I think that a "0" PSI reading is not a "vacume". I believe a "-0" reading would be a vacume. A true vacume is the absence of the earths gravitiational pull and atmospheric conditions.

Now some who knows needs to chime in because I am curious to know how close I got.

Somebody!!!
Anybody!!!
 

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Let's see if I can type thoughts this morning.

Pressure in a tire is measured in PSI (pounds per square inch). Think about it this way...

It measures the amount of force (in pounds) that is applied to one square inch of the inside of the tire.

That one square inch of tire has pressure from the outside too and it's actually the difference between the two that you're measuring. (in PSI.)

Simplified, Trapped air on the inside pushing on the inside of the square inch of the tire and ambient air pressure on the outside pushing on the same square inch of tire.)

Ambient air pressure is measured in inHg (inches of Mercury). It's the number of inches that the air pushes mercury up a tube in a Mercury barometer.

In case you were wondering,

PSI X 2.036 = Inches of mercury

The atmosphere changes all around but we accept that the 'standard pressure' is: 29.92 inHg. (at the standard temperature)

You would have vacuum if you measured 'pressure' but found it to be less than outside air pressure in inHg or a negative PSI.

Tire pressure goes up as you gain altitude to a very small degree.

Tire pressure goes up as the air cools to a much larger degree.

Did the fingers type something intelligible? It's early! :D

-R

[ November 23, 2004, 08:01 AM: Message edited by: ChevyCobra - Randy J. ]
 

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Here is a little technical data from the national atmoshperic tables, standard day for comparison
Altitude ft Pressure(psia) temperature degF
-1000 15.23 62.5
0 S.L. 14.7 59.0
+6000 11.78 37.6

Bob Mac
FFR3981
 

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Quote from bob:
“So if the tire manufacturer recommends 32psi, to me that means 32psi greater than ambient air pressure for the tire to operate correctly. So it should be 32psi regardless of altitude, weather, or ambient air temperature.”

You are correct, but I would like to add that the tire pressure should be checked when the tire is at ambient temperature.

Gage pressure + atmospheric pressure = absolute pressure.
0 absolute pressure = vacuum
0 gage pressure = atmospheric pressure

Tire pressure:
At sea level, the absolute pressure is approximately 14.7 psi as previously stated which is also called atmospheric pressure. As you go up into the mountains, the air is thinner and this drops. If you go down into a valley, the atmospheric pressure increases.

At sea level, when a tire is inflated to 32 psi, the pressure inside the tire is 32 psi greater than the pressure outside the tire (atmospheric). The gage pressure is 32 psi. The absolute pressure is 32 psi + 14.7 psi = 46.7 psi.

When you take this tire into the mountains, the atmospheric pressure drops, let’s say to 12.5 psi. The absolute pressure inside the tire does not change because we have not inflated the tire or allowed air out.
Gage pressure + 12.5 psi = 46.7 psi
Gage pressure = 34.2 psi

Now to operate this tire at the manufacturer’s recommendation, you need to let out 2.2 psi while you are in the mountains.

Quote from bob:
“Lets say you are in death valley (-200MSL), and fill your tire to 32 psi. Then you ride to my town (+6500MSL). What happens to your tire pressure? It should go up, Lets say to 37psi (let a physics whiz figure out he exact number). Now you have to let air out to compensate and get back to 32psi. So you let out 5 psi. Does it make sense to let out another 5psi, and set the pressure at 27?”

No
 

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ok ok ok now the answer............ a pressure gauge works with a bellows, one side will have atmospheric pressure on it, so it does change for temps and altitude. So what ever the gauge reads when you check tire pressure, it is the correct pressure in the tire. If you wanted the pressure to stay the most consistent, use nitrogen in them. That will not change as much with temp changes.


Added: "Tire pressure goes up as the air cools to a much larger degree." Tire pressure will decrease with a drop of temp, as the air in the tire gets hotter, it expands, that is why they run bleeders on tires that run circle track. Keeps the stagger the same as the pressure builds up in the tires and releaves the excess pressure.

[ November 23, 2004, 09:31 AM: Message edited by: Eric the Red ]
 

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Slaga is correct. In the engineering world the equation you learn is:

Pabsolute = Pgage + Patmospheric

Pabsolute and Patmospheric are read as psia (pounds per square inch absolute). Pgage is read as psig (pounds per square inch gage). Absolute is referenced to zero psia, and gage is referenceed to atmospheric (normally 14.7 psia at sea level).

Remember that if you have 32 psig of tire pressure (at sea level) and take the car to a higher elevation where there is, say, 13.7 psia of atmospheric pressure, the tire pressure will now read 33 psig. This assumes the tire is non-flexible. But since tires are flexible, as soon as the outside pressure starts dropping, the tire will start expanding to try to better equalize the increasing pressure differential, so the pressure won't actually increase the full pound. This is the difference between the 'real' and 'ideal' case.

No need to talk about inches Hg or inches of H2O (unless you just want to talk in different units for some reason - they are all just different units for pressure), psi is the norm.

Eric, let's remember that the air we breath is 70-80% nitrogen, so I don't know that the difference would be noticable.
 

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Eric also makes a good point of the temperature effect on tire pressure. The equation there is:

Pv = nRT

where
P = pressure
v = volume
T = temperature
n & R are constants

So:

P is directly proportional to T/v, meaning pressure goes up as ambient temperature goes up, and vice versa.

So boB, if you go from the heat of Death Valley to the mountains of Colorado, the pressure drop would tend to inflate your tires, and the temperature drop would tend to deflate your tires...
 

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I think it is more complicated than just pressure equations. I quote from the Spring 2001 issue of Invention & Technology:

"Tires of any kind are remarkable things. Drivers tend to think of them, when they think of them at all, as balloons to keep inflated. Engineers think of them as highly sophisticated structures that perform a variety of distnct tasks. Robert Snyder, a former tire design chief for Uniroyal, notes that they present a paradox. Take a set of tires inflated to 32 pounds per square inch, load a 2,000 pound car onto them, and their internal air pressure doesn't increase. It remains at 32 psi. That's because a tire is not a simple bladder of air; it's an engineered structure, as complex as a suspension bridge. The air inside is a tensioning mechanism, not a load-bearing mechanism. The thousands of individual fibers that circle the tire, kept in tension by the air, are what actually carry the load."
 

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The reason they use Dry nitrogen in aircraft tires is because of the differences in tempatures for when an aircraft is at flight altitude, nice cold air, and it is landing. The change in pressure is not as dramatic from 70 degree day 100psi to 20,000 feet, kinda cold, back down for a landing, and tires heated up fast from the landing.
 

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One other thing to ponder. Most of the pressure in a tire is the resistance of the tire to expansion and not from atmospheric prressure on the tire itself. If tire pressure is measure at my home in Lubbock, Texas, altitude 3,200 feet and compared at the same temperature to my mountain home in Red River, New Mexico, elevation 8,750 feet, there is very little difference in tire pressures.
Even so I would regularly check my tire pressure when traveling to a new altitude.
Hope this helps,
Joe
 

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OK, I understand that a GAUGE would read higher at a higher altitude, because it measures the difference. However, would not the absolute pressure in the tire remain the same? If not, how would it GAIN pressure. Now, if we were going the other way around - filling the tire at 100,000 ft - then the absolute pressure in the tire would increase at sea level, due to the outside pressure increase. Right?

td

td

td
 

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If you assume that the volume and the temperature remain constant, then Pabsolute in the tire remains constant at all elevations. I know this is not correct in the real word, but it is a close approximation. I made this assumption in my previous explanation. Temperature changes are easy to account for but volume is extremely difficult. The shape of the tire (low profile or swamper off-road tires) and tire composition (Modulus of elasticity) alone could be the basis for a college thesis.

[ November 23, 2004, 02:04 PM: Message edited by: slaga ]
 

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my brain hurts.
 

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Originally posted by Oldguy668
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quote:
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Originally posted by Jerry:
.... The pressure in the tire over the ambient is what flattens out the tire contact patch.....
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I'm certainly not a rocket scientist, but I'm pretty sure that the flexibility of the tire and the weight of the car is what flattens out the contact patch. The pressure in the tire keeps the rim off the ground.
Joe,

Poor choice of words on my part. It was very early ... What I was trying to say is that the pressure difference between the inside and outside of the tire (gage pressure) effects the size and shape of the contact patch. What I meant by 'flat' was that the patch would be even from side to side.

Sorry if I offended you.
 
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