Using Nitrogen Versus Air for Race Tires

[tom91ita]

the oxidation rate can be an issue. the rate is proportional to the partial pressure of the gas. for the outside of the rim, this is typically 3 psia for oxygen (21% of 14.7 psia). or if you want to compensate for your elevation, go to http://www-personal.usyd.edu.au/~gerhard/pressure.html

in tires that are about 35 psig, the O2 partial pressure is about 10 psia (21% of (35 + 14.7)). so the corrosion rate inside the tires would be about 3 times typical. this is one of the reasons that alloy rims are more prone to losing air than the old trusty steel rims. the aluminum oxidation products interfere with the seating surface, etc.

the increased oxidation due to partial pressures is one of the contributing factors to the deaths due to fire in the Apollo 1 capsule. they had pure oxygen inside and were also pressure testing the capsule to ~15 psig to simulate the pressure difference due to the vacuum of space. they now had the partial pressue of oxygen at 30 psia or ~10 times normal. some metals will burn under these conditions. and burning is just a very nasty form of oxidation.

arguably not an issue with the race tires since we are checking pressures so often and changing tires more frequently(?).

with regards to moisture, it is not the fact that there are a few drops present as much as it that any moisture in the tire will evaporate when it gets hot and add more pressure. the PV=nRT holds true but we are adding more "n" when the drops turn to vapor. there is less moisture in your compressed air since the water in a vapor form is related to the partial pressure related to the temperature. cold winter air will help but during a hot humid day, more water should be condensing in your compressor's tank (and why we need to drain them).

and since the molecular weight of N2 is 28 vs. O2 being 32, i am assuming that with a larger electron cloud, the O2 is fatter but at this point, i really don't care. but if we are actually talking about the relative difference between the specific heat capacity of the gases, this small difference is likely dwarfed by the mass of the rim and the tire.

i personally use my practice & qualifying sessions as a time to estimate my heat/pressure gain for that track on that day at those ambient conditions. i then go with three columns for the air pressure (start, projected, actual). if the actual ends up higher/lower than i had wanted/projected, i adjust the starting pressure.

and then there is the whole thing regarding tire temperatures being the better guide, etc. and the fact that i don't want to bother putting nitrogen cylinders in and out of my hatchback daily driver or haul the rig to the gas supplier to keep it in the DOT required upright position.

p.s., i do have a cordless impact.

[Spinnetti]

the oxidation rate can be an issue. the rate is proportional to the partial pressure of the gas. for the outside of the rim, this is typically 3 psia for oxygen (21% of 14.7 psia). or if you want to compensate for your elevation, go to http://www-personal.usyd.edu.au/~gerhard/pressure.html

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Ok, that was more interesting, but the reality is in the greater scheme of the normal material world of cheap race cars and so-so drivers, it doesn't make a bit of difference. Why not just run some of those pressure setting valve caps, or as you mentioned learn how much pressure your tires typically gain from temperature rise and set them accordingly?

Anyway, more power to those that want to do it. I'd have it done when I have them mounted at the shop simply because they will do it for me, but I sure won't be worrying about it at the track!