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A big argument by the nitrogen-in-the-tire crowd is that:

Nitrogen atoms are bigger and thus less likely to escape the tire, bringing stability to your tire pressure.

If Earth's atmosphere is %78.084 percent nitrogen, then the non-Nitrogen composition is ~22%. If this 22% is more likely to seep out of the tire, and Nitrogen doesn't seep out, then simply filling the tire up with air will maintain the original 78% N, plus the added nitrogen which would be 78% of the remaining 22% (assuming total seepage).

Simply, won't a tire that's been filled up 10 times in the course of its normal life already be disproportionately Nitrogen if this argument is true?

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The idea that nitrogen stays in the tire longer seems crazy to me. Particularly if the mechanism of leakage is 'small' leaks or valve defects, on the scale of atoms these are still gigantic features. See also: straightdope.com/columns/read/2694/… –  Greg P Dec 30 '10 at 16:51
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More general question: How does air escape from tires or balloons? –  endolith Dec 30 '10 at 17:06
    
@endolith: it diffuses out. The question now is how porous is the rubber? Cracks and other such imperfections will of course accelerate the seeping out. –  user172 Dec 30 '10 at 17:15
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@J.M. The question is really what is meant by "diffuses." Is it a molecular diffusion process (like air diffusing through water) or a process that can be modelled by tiny currents of air moving through 'cracks'? These are very different possibilities. Note that even rubber that is 'perfect' from an engineering standpoint will have 'defects' from a physicist's standpoint (i.e. rubber is nothing at all like a perfect crystal). –  Greg P Dec 30 '10 at 17:21
    
@Greg: exactly. Rubber only becomes "crystalline" if you stretch it (how much of a stretch depends on the rubber); at the normal conditions of a tire, you'll have "windows" for air to seep in/out. –  user172 Dec 30 '10 at 17:25

3 Answers 3

This probably depends on the tire structure; in general:

  • for hydrodynamic size hole there will be no difference in move of oxygen and nitrogen
  • for a very small holes there will be some sorting due to effusion, but this is pretty inefficient process since atomic masses of oxygen and nitrogen are similar; what's worse, it will work in the opposite direction -- oxygen molecules are a bit heavier than nitrogen ones, so due to the Graham's law the nitrogen will escape about 1.07 times faster than oxygen.
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This says it all –  Ebenezer Sklivvze Jan 1 '11 at 21:38

I thought the big attraction of nitrogen was its low reactivity. The oxygen can chemically combine with rubber molecules, sort of like conbustion, but far below ignition temperature, and the claim is that that effects the tire lifetime. These oxidation reactions will rates will be much faster at higher temperature, but tires do heat up while driving. I was talked into N2, the last time I got new tires. I don't know if it is was really worth the approx $8 per tire it cost me however (i.e. how much the tire lifetime improves and the slight improvement in vehicle milage is actually worth). So if the O2 molecules are diffusing out faster, but more importantly chemically combining with the rubber, I would expect the O2 concentration to decrease with time.

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Yeah, the more serious webpages suggests that it is only about corrosion -- yet it even seems the most gain is from removing water and oil (air is pumped from contaminated atmosphere at the station, while nitrogen is from bottle and was dried and cleaned during production). –  mbq Dec 30 '10 at 20:59
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I don't understand this argument. Oxidation still occurs on the outside where all the wear and tear occurs. I've never had a tire fail because the inside wore out or corroded. Besides, I think going down the interstate, slamming rubber down on pavement under a 5,000 lb car 1,065 times per second is going to to drive the relative benefits of preserving the inside of the tire to near-zero. –  Robert Cartaino Dec 30 '10 at 23:30
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Robert, The pressure on the inside is 3 to 4 atmospheres, and I would expect the reaction rate to be proportional to the partial pressure of the O2. If it requires two O2 molecules (or OH radicals, or whatever) if would scale as pressure squared, but I suspect oxidation is controlled by the first power. Then the air temperature inside the tire will be higher as well, so I think oxidation rate on the insude will be several times higher than outside. Of course you get mechanical wear on the outside, but a lot of energy is deposited in the tire body. –  Omega Centauri Dec 31 '10 at 1:32
    
Oxidation of the outside probably doesn't matter. Sure, oxidation would break some bonds near the surface, but that layer is removed by the wear and tear, exposing new and un-oxidized rubber. –  MSalters Jan 4 '11 at 14:47
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The N2 fill is originally an aircraft requirement, and was mainly to remove water which would build up in the tire and freeze at high altitudes. Since you were carrying a pressurised N2 cylinder out to the plane anyway to fill various pneumatics it made sense to use the same for the tires rather than having a separate dry-air cylinder. –  Martin Beckett Aug 3 '11 at 14:57

The nitrogen in a tire does not stay in the tire longer than oxygen.

The reason nitrogen is used in high-altitude aircraft tires is that it is a dry, non-reactive, cheap gas; dry being the operative word. Condensation in aircraft tires can freeze and cause small leaks in the tire valves during flight, which can lead to catastrophic landings.

Using a dry gas such as nitrogen in auto tires can result in a more consistent tire pressure, because water condensation and evaporation can change the pressure. This can be important in high performance situations such as race cars.

However, the reason nitrogen is most often used in auto tires is marketing. It's cool because airplanes and race cars use it.

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