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I now the A12 and sr 71 had nitro in tires, but thats extreme. what are the beginning factors that dictate what is put in the tires?

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    $\begingroup$ Related-- aviation.stackexchange.com/questions/64682/… --see several answers $\endgroup$ Feb 17 at 14:45
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    $\begingroup$ Why would that be "extreme"? You can get nitrogen put in your regular car tires at many mechanic shops. $\endgroup$ Feb 17 at 15:13
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    $\begingroup$ @HiddenWindshield -- I suspect the questioner meant not that using nitrogen was extreme, but that the A-12 and SR-71 represented extreme cases in terms of the demands imposed on tires. $\endgroup$ Feb 18 at 15:34
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    $\begingroup$ I use at least 78% nitrogen in my car's tires, and I think you should too. $\endgroup$ Feb 20 at 3:52

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Transport airplanes use dry (completely dehumidified) nitrogen from high pressure bottles for tire servicing. Shop air is "dehumidified" using water separators but will still have water vapour in it. Bottled dry nitrogen is completely moisture free.

Besides humidity, the main objective is removal of oxygen. Oxygen, the other major element in air at 21%, tends to oxidize the rubber, degrading it.

So using dry nitrogen eliminates humidity related problems and eliminates internal oxidation of the rubber (as well as making the inside atmosphere completely incombustible), so it's generally the standard for commercial aircraft where the tires run at extreme pressures and speeds.

Light airplanes will use dry nitrogen if it's available and the owner wants to go to the expense, or just regular shop air as you would a car.

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    $\begingroup$ Although oxygen molecules are smaller than nitrogen (0.299 vs 0.305nm) the difference is insignificant from a leakage perspective. That’s nanometers of course, not nautical miles $\endgroup$
    – Frog
    Feb 17 at 18:54
  • $\begingroup$ Thermal expansion coefficient of oxygen and nitrogen should be basically the same i.e. like the one of any other (ideal) gas. I suppose that the main reason to use pure nitrogen instead of air is still to avoid to feed any fire in case of incident. $\endgroup$
    – sophit
    Feb 17 at 19:31
  • $\begingroup$ @Frog even at 150-200 psi? $\endgroup$
    – John K
    Feb 17 at 20:24
  • $\begingroup$ @sophit I thought there were small differences at higher pressures no? $\endgroup$
    – John K
    Feb 17 at 20:26
  • $\begingroup$ @sophit Air does not behave as an ideal gas due to the presence of water vapor. Water has a strong dipole moment, which makes it deviate from an ideal gas, particularly as the tire is expected to operate within the relatively small range between freezing and boiling of water. $\endgroup$
    – user71659
    Feb 18 at 3:14
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Nitrogen has a lot of benefits when used in tyres, not just in aircraft but in cars, trucks, motorbikes and so on. First consider the alternative. If you fill the tyre with air you will get some water vapour, exactly how much depends on the humidity at the time. Water density varies between 0.6Kg/m3 to 997Kg/m3 over just 100 degrees C. That results in a big change of pressure from even a small amount of water. Air has many other things in it as well, including oxygen which supports combustion and hydrogen which leaks out reducing pressure. Nitrogen is cheap and it is boring. The molecules are all the same size so there are no leaks, it doesn't burn in an accident and the pressure remains linear with temperature. If you have the option, and most places that do a lot with tyres do have the option, then using nitrogen is a cheap and effective solution for managing tyres.

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    $\begingroup$ I'm not sure this answer presents a balanced viewpoint when it comes to road vehicles. Most objective, independent studies show no more than negligible benefits for cars, making the whole thing a marketing gimmick. It is correct that nitrogen is used in aircraft tires with benefits arising from the high temperatures reached. $\endgroup$
    – TypeIA
    Feb 17 at 17:22
  • $\begingroup$ @TypeIA The issue is how much O2 remains in tires. Car garages generate N2 on-site from molecular sieves, which still are 2-5% O2. Further, the tire isn't flushed and often topped off with air, making the ratio even worse. Airplanes use N2 from suppliers that are better than 99.9%. $\endgroup$
    – user71659
    Feb 17 at 18:48
  • $\begingroup$ "The molecules are all the same size so there are no leaks". I think that also the molecules of oxygen have all the same size. Why should it have an impact on leaking? $\endgroup$
    – sophit
    Feb 17 at 19:36
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    $\begingroup$ The leak rate has only quite remote connection with the molecule size. The important thing is how much the corresponding gas is soluble in rubber. And the oxygen is indeed much more soluble.. $\endgroup$
    – fraxinus
    Feb 19 at 16:02
  • $\begingroup$ @TypeIA - Road cars do not significantly benefit from nitrogen because road cars do not tend to experience extremes and a difference in pressure of 20% from optimum can rarely be detected by the majority of drivers. Race cars and motorbikes are much more sensitive to pressure and so the benefit is greater. $\endgroup$
    – Paul Smith
    Feb 19 at 21:42

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