Here are some other comments I located on another website
Engineering Tips Forum.
The follwing are some of the key points:
The question about nitrogen in tires has come up in other lists on the web as well. Let me clarrify a few points.
1. Air is 78% nitrogen, N2, and 21% oxygen, O2. So even if you put air in the tire, it's already 78% nitrogen. Many of the so called nitrogen generators don't produce much more than 90% nitrogen.
2. At relatively low pressures (ie tire pressures) N2, O2 and water vapor will all behave as ideal gases, and follow PV=nRT. Pressure will increase or decrease to the same extent as the temperature increases or decreases regardless of which gas is in the tire. (Even at 300 psi, which is about 20 atm, there is little deviation from ideality.) Therefore the comments about N2 not changing in pressure as the temperature changes are without merit.
3. The rate of effusion (or diffusion) of a gas through a porous membrane depends on the molar mass and to some degree on the molecular diameter. N2 and O2 are almost the same size and N2 is lighter than O2 (28 g/mol vs 32 g/mol) so if either gas were to effuse out of the tire, nitrogen would do it more quickly. Luckily, tires are designed not to be porous membranes.
4. N2 and O2 both have essentially the same specific heat capacity, about 1.0 J/gK, and thermal conductivity, about 0.00026 W/cmK. Water vapor has a specific heat capacity of about 2 J/gK. But remember, water vapor will constitute less than 1% of the air in the tire. So the idea that N2 has different heat handling properties is also without merit.
5. The ozone, O3, in the atmosphere, which is a ground level pollutant, will do a great deal more damage to your tires than the O2 inside the tire. For instance, don't leave a condom out in the air in Los Angeles for a few days. It will develop lots of tiny holes and weaken.
It has been said that dry air is preferably to air with a lot of water vapor. As a tire heats up, the very small amount of H2O present will be in the vapor state which may contribute to the overall pressure very slightly.
Several have suggested that N2 in a high pressure tank is more portable and requires no electricity. That would make sense, particularly for aircraft tires.
I find no reason to believe that N2 is going to produce a "better ride" or "better handling".
The bottom line is that for general passenger car tires or truck tires there is nothing to be gained (other than portability) by using nitrogen rather than air. The biggest gain will be $$$ by the companies that sell nitrogen handling equipment and the tire merchants that appeal to ignorant customers. And who is the biggest loser? Yep, the consumer.
Regardless of how much you want a gas not to increase in pressure when heated, it's just not going to happen. The laws of chemistry and physics apply all the time. PV=nRT is true all the time, for every gas, be it air, nitrogen or water vapor. Saying that a gas does not increase in pressure as temperature increases is nonsense.
Also, don't worry about the "water" in the air. It exists as water vapor, a gas, and is simply part of the total pressure in the tire. As the tire heats up, the water vapor will continue to be a gas and be governed by the equation PV=nRT, just like air, which is 78% nitrogen.
Every gas is going to escape from a tire, regardless, as long as the pressure inside the tire is greater than the pressure outside the tire. The gas molecules are going to diffuse through the walls of the tire at a very slow rate. It doesn't make any difference what the gas is, although there are small differences between the diffusion rates of oxygen and nitrogen. Since nitrogen is slightly less massive than oxygen (28 g/mol vs 32 g/mol) Graham's law predicts that nitrogen will diffuse slightly more rapidly than oxygen.
The bottom line is that you can't set aside science in favor of hype and wishful thinking.
This was copied from a post by Mike Jones on
www.eng-tips.com.
