Reply to "Nitrogen in tires?"

I guess I’ve discovered I like web surfing while watching football playoffs and BCS games. I’m also a contrarian by nature and have throw the BS flag.

There is a lot of (mis)information and myths out there on the internet regarding the use of N2 in tires and most of what’s out there is complete rubbish perpetrated by those that would like you to believe it’s worth $10/tire to fill your passenger car tires with N2 as opposed to just maintaining their shop air like they should……but then would you pay $10/tire for that?

I would never have thought it possible sell ordinary consumers N2 for their tires until folks started selling bottled water for more than soda and beer….that’s marketing boys…not science but it’s still all business. So all in good fun I’m about to add to the internet content on the subject and I don’t mean to offend anyone but you can decide whether the following is fact or just more internet fiction.

The ideal gas law will very closely predict the behavior of O2 & N2 at these pressures and temperatures. Remember the old PV=nRT from chemistry class? When Volume is held constant it says the absolute pressure change is proportional to the change in absolute temperature.

At 20C, you add 273 to get an absolute temperature of 293 Kelvin. (Or if you’re not a metric kind of guy at 70F you add 460 to get 530 Rankin)

In any case, if it was a 70F day and your tire was at 30 psig (30 + 14.7=44.7 absolute pressure), and you had a 10% increase or decrease in temperature; mind you, this is +-10% of absolute temperature or 530R in this case, so we’re talking a 53F change up or down from 70F starting point, your tire pressure would change 10% absolute pressure or in this case 4.47 psi……so 30+-4.7psig for a 53F temp change from room temp.

That temperature change is realistic (and then some) for racing conditions and the corresponding pressure change is a big deal performance wise for people racing on slicks. The Pros plan for the increase in pressure because they know it doesn’t matter whether it’s O2, N2, or air as long as it behaves like an ideal gas, which these gases will if they’re (very) dry. Water does not behave ideally because it undergoes a phase change twice over the decrease in temperature in the range I mention above. If the gas in the tire is (near) saturated at 70F it will begin undergo phase change with some of the gas immediately condensing to liquid when you drop the temperature. By the time you reach 32F the water becomes solid (frost/ice) and at that point it ceases to exert any pressure at all on the tire wall. This will make the temperature change more drastic than ideal gas law predicts more so on temperature loss especially when you go through freezing.

If you search the subject most of the net info will say N2 is more stable than air. If you read on, some will go on to say it’s not actually because of O2 content, but the contribution of water vapor in shop air. Some even go onto to argue it’s because the oxygen is more reactive (which it is) and is causing corrosion and thus bead seal corrosion and associated leakage which is exacerbated by….you got it, water. Some of the articles even blather on about the solubility of O2 in rubber in conjunction with water. I’ve got a close friend that works at the local company that invented the vulcanizing process for recapping tires. They understand how tire rubber behaves. He smiles at me when I mention this to him. I’ve demounted some old dry-rotted tires that look like hell on the inside but I’ve also demounted many that look pristine on the inside. How’d that happen? It wasn’t because they were filled with N2.

A few of the internet articles will actually come out and say it’s not the N2 that is the benefit, but the absence of water.

Is N2 better than O2 (errr actually 78% N2/air)…and is it less reactive?......yup but how much better? Measurably better?

Bottom line guys, no matter what you do you are going to get the pressure fluctuation in your tire with changes in temperature irrespective of whether it’s completely desiccated N2, O2, or air and it’s not dependent upon your selection of either of these gases, unless you’re using wet shop air. Keep in mind air is 78% N2 anyway so all of this is why I said buy a $10 desiccant cartridge, run your air through it slowly to give it a chance to work, and call it a day.

Gotta go. FL State is mounting a comeback against Auburn.

Best,
K