blazinlow89 said:
Moisture content seems to have a larger effect on pressure changes.
It's truly sad that a web site like that has to resort to distortions and incomplete science to try to persuade people of the minuscule benefits of nitrogen.
Will my tire pressure fluctuate while driving if I use nitrogen?
You will still see pressure changes with nitrogen while driving, but overall your tires will run cooler and at a more consistent pressure than if they were filled with air. Nitrogen does not contain the moisture and other contaminants found in compressed air, so nitrogen filled tires will fluctuate less in temperature and pressure than air filled tires under driving conditions, even at high speed and at high temps.
Not one word about the MAGNITUDE of the difference caused by the moisture in air and (presumably) not in nitrogen. This next paragraph contains the meat of the argument, but again, totally ignores the opportunity to post up some math as a proof.
Ambient air contains moisture, nitrogen does not. If moisture is present it contributes to a greater change in pressure simply because at lower temperatures water condenses to become a liquid. The liquid form of water occupies very little volume and contributes only a negligible pressure to the tire. But at higher temperatures, such as those in a running tire, water evaporates inside the tire and becomes a gas which increases pressure in the tire.
So their claim is that at a cool tire-filling temperature, the relative humidity inside the tire is such that the water vapor condenses out as liquid water, and when you drive off, the self-heating in the tire vaporizes the liquid, and the new equilibrium point of pressure/temperature/and humidity gives you some amount of extra unanticipated pressure in the tire.
1) How much EXACTLY is this extra pressure? The industry shill writing that web site is silent on the issue. I'm not doing the math for them, but I'd be shocked if it's more than 1-2 PSI extra.
2) MUCH MORE IMPORTANT QUESTION:
Why is this extra pressure a BAD THING? Why isn't it possible to see the increased pressure as reducing your rolling resistance, a GOOD THING? And who cares if the pressure goes down when it's cold out? Perhaps the reduction in pressure is compensated by more stiffness in the sidewalls, and the tire ends up having similar cornering performance at cold as when it's hot? The nitrogen-concentrating industry isn't going to perform those experiments because they might disturb the great rip-off.
3) On the other hand, their claims are that O2 permeates through the rubber 3-4 times faster than N2 molecules. I followed the math in the PDF paper, but the conclusion was never presented. Yes, I agree O2 molecules will permeate faster than N2. But how fast is fast? In the average tire, if it was filled with PURE O2, how many PSI per month/year/decade/eon would it lose?!?! Never quantified. The science is incomplete and can't be used to form any conclusions unless that critical detail is presented at the end of a formula. useless exercise until the equation is completed.
4) The site runs its numbers assuming tires are made of rubber. Of course, tire manufacturers are not stupid, and are motivated by their tread wear guarantees to have as low a permeability as economically possible, so they ALL INCLUDE INNERLINERS!
NHTSA Tire Aging Test - analysis of the composition and permeability of innerliners
http://www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle Research & Test Center (VRTC)/ca/Tires/811296rev.pdf
Ooops, what's this? A report I hadn't seen before from NHTSA with the real poop:
http://www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs/Crash Avoidance/2009/811094.pdf
As an example, one set of light-truck tires were inflated to 411.3 kPa (59.6 psi) with air that contained 21 percent O2 and 78 percent N2. At the end of 90 days the remaining pressure in the tire was 388.6 kPa (56.3 psi), an average inflation pressure loss rate of 1.72 percent/month (1.03 psi/month). When the light-truck tires were inflated with 99 percent N2 gas, the pressure at the end of 90 days was 398.6 kPa (57.8 psi), an average inflation pressure loss rate of 1.24 per cent/month (0.74 psi/month). Over the 90 day period the percent N2 in the air-inflated tire increased to 80 percent, and the percent N2 in the nitrogen-inflated tire decreased to 98 percent, due to the faster permeation of oxygen gas through the tire components in both directions, in and out, depending on the balance of partial pressures. If we continue to replenish the inflation gas with the original gas mixture (air or 99% pure N2) over a period of three years; the approximate predicted inflation pressure loss rates for the air-inflated tires, now containing 85 percent N2, would be 1.57 percent/month (0.94 psi/month), relative to a predicted inflation pressure loss rate for the nitrogen-inflated tires of approximately 1.31 percent/month (0.78 psi/month), and now containing 95 percent N2.
5) So this data shows the measured permeability of the tested tires is around 1 PSI/month lossage with air, and around 3/4 PSI/month lossage with 99% pure N2. Of course, because you cannot possibly purge all air out of a mounted tire unless you run MANY, MANY inflation/deflation cycles with pure N2, which the tire store concentrators are not even giving you, the real-world benefits are even worse than this data.
6) AND FINALLY - since O2 *does* permeate faster than N2, even a regular air-filled tire, over time, and if not purged, will end up with less percentage of O2 in it as a natural process you don't even have to pay for!
If this doesn't put it in terms we can all understand I'll eat my hat. It's a rip-off, incapable of being defended by the N2-concentrator industry, and ratified by published and very scientific data by the NHTSA.
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