[Invincible]

Bicyclists running skinny tires with high pressure know that tires have to be pressurized quite frequently.  The pressure is high, volume is small, so it doesn't take much loss to cause significant pressure drop.

Spare donut tires for cars have the same problem.  Surface area-to-volume ratio is higher AND pressure is higher.  They usually run around twice the pressure of normal tires.  I don't check my spare tire as often, but when I do check it every few month, its at about half the pressure its supposed to be.

From reading bicycle related topics, CO2 deflates even faster than ordinary air despite higher molecular weight due to higher solubility in tire rubber.  

What's an ideal fill gas that isn't too toxic and compatible with rubber? Nike used SF6 for their air shoes for its resistance to diffusion loss.  

The gas would have to resist condensing at 100psi or so at temperatures expected to be seen by vehicle tires.  

Would pure nitrogen fill fare somewhat better?  By pure, I mean evacuating tire/wheel in a chamber, then filling with nitrogen to purge oxygen.  What about argon?  Sulfur hexafluoride? 

I assume Kr and Xe would fare better than Ar, but I excluded them since they're too expensive to be practical.  I suppose SF6 is out too as it's too environmentally unfriendly. 

[rajan]

for some reason the big thing to do now is to fill your tires with N2. i'm not sure about the science behind it, but I think i remember hearing some stats supporting its use.

pros for N2: some (possibly fraudulent) data seems to support it, it's cheap, easily purified, inert, nontoxic
cons for N2: probably doesn't do that great of a job in reality

the CO2 thing you brought up is intriguing, and confusing... why, if the solubility of the CO2 is higher in rubber, doesn't the CO2 stay in the rubber, as opposed to diffusing out on the other side of the tire? behind this question lies some doubts in my mind that the solubility of CO2 is measurably different from any of the other gases in the atmosphere in rubber.

in any case, it would be interesting to know which gas is indeed best. my vote is N2. you wouldn't even need a pump, just a dewar of liq N2.

[Borek]

http://www.chemicalforums.com/index.php?topic=9490.0

[stewie griffin]

Just like my car tires, I like to rotate the air in my bike tires 4 times a year.... I use winter air, spring air, summer air, and fall air during the respective seasons. 

[tactical328]

to my understanding, automobile tires are filled with nitrogen gas to keep tire pressure stable with respect to temperature,  this however is not a property of gas ie the ideal gas law, but due to nitrogen gas being 'dry' low h2o content compared to  air from a compressor.

any of the more knowledgeable folks feel free to let me know if this is correct or if i'm just talking outta my ass.

[Invincible]

To whoever linked me to nitrogen vs air thread, this thread isn't about nitrogen vs air, this is about what the ideal gas is.

Nike used sulfur hexafluoride to pressurize "air" shoes and it had to make changes in polymer to keep the diffusion loss to an acceptable level when they switched to nitrogen due to environmental reasons.  

I think SF6 would be great for car tires, although for bicycle and commercial vehicle tires running ~150psi, it may condense into liquid in arctic climate.  

As for CO2, 20oz bottle of soda will go flat in a few months, unopened, but a 12oz can of soda usually stays good well over a year.  CO2 sneaks through polymers and elastomers quite well for some reason.

Also, to make pure N2 fill, the tire would have to have two valves at least to flush the remaining oxygen containing air out of it.   

[Borek]

To whoever linked me to nitrogen vs air thread, this thread isn't about nitrogen vs air, this is about what the ideal gas is.

Yep, but parts of the discussion still apply.