And as a general safety rule, especially if one does not control regularly the tire pressures, better having a little bit more pressure than less.

 

Spot on. This is why the nitrogen in the tires scam is exactly that- a scam. If nitrogen didn't expand at exactly the same rate as air, they'd all be driving around with under inflated tires.

 

Nitrogen is a scam if you have to pay for it. Nitrogen does not hold moisture like air does. Thus less corrosion to the inner wheel rim. Another favorable characteristic is that the molecules are larger than O2 and less air leakage over time.

 

Also false.

 

Real reason of tires holding better (only a bit) pressure over time with nitrogen is that it diffuses much more slowly (a factor 4) through rubber than oxygen.
But don't forget that nitrogen is already 4 vols of 5 in air.

Also nitrogen provided in gas stations is generally dryer than air, some benefit wrt to inner corrosion of alloy rim sealing surfaces in the long term.

But all in all quite a small benefit only.

 

Source please?

If this was true, and assuming there's some magic property of rubber that acts as an oxygen-only filter, tires would gradually become oxygen free all on their own. No need to pay for nitrogen.

Gas station compressors are no different than domestic garage units when it comes to moisture separators, not that tire/rim corrosion is much of a factor in the first place.

 

Mikey,


All tires eventually lose air pressure with time as the gas diffuses through the tire. Nitrogen has a lower rate of diffusion from tires than oxygen, so the tire retains it pressure longer. The permeability of a gas through a material is a function of the gas and the material. For example, the permeabilities of nitrogen and oxygen through poly-isoprene are (http://mathscinotes.com/wp-content/u...rePressure.pdf)

• Oxygen:
• Nitrogen:

Where STP standpoints for standard temperature and pressure. As you can see, the permeability of oxygen through synthetic rubber is substantially greater than that of nitrogen.


In the following link, see a graph showing the pressure decrease over time, significantly stronger with air compared to 100% nitrogen:
http://www.composite-agency.com/arch...n-in-tyres.pdf

 

Liquid or compressed Nitrogen, while expensive to initially get setup, has advantages over atmospheric air, which includes Oxygen, Argon, Carbon dioxide, Neon, Helium, Methane (CH4), Krypton, Hydrogen, and Xenon. Oils and acids also get introduced into shop air.

 

Thanks for the links and the accompanying theory, but in practice tire rubber is nowhere near a consistent enough substance to act as a filter that favours nitrogen over oxygen. Nitrogen molecules are 300 picometers in dia. while oxygen is 292 picometers. There's very few materials that would act as a trap for one but not for the other.

There's also the reality that a large part of typical leakage is at the tire/rim interface and not through the tire carcass itself.

A year long study done here 10 years go at the onset of the nitrogen scam indicated so little difference in pressure loss between nitrogen and air (1.3psi over a 1 year period) that the results were declared inconclusive.

Nothing wrong with nitrogen if it's free but most places charge for it.

 

There are sometimes thresholds wrt "molecule dias" as you call that (VdW radius is just a calculation).
And reactivity of oxygen compared to nitrogen vis a vis organics is another world. Chemical paths and channels, membranes permeativity, transitory states, so on so forth...
Bring your own evidence before dismissing others', come with your 10y study. Peer reviewed?
I don't say the cause is ascertained but I provided elements and a graph coming from experiment, you just come with arguments.
Fact is that pressure loss is (slightly) reduced with dry nitrogen, no more, not saying it's worth.
Btw, I just fill my tyres with air.

 

WOW I totally did not expect starting an innocent tire pressure thread would send me running to my college chemistry books.

I think regular 'air' and 30/32psi work just fine for me.

 

At the track day I attended 10/01-10/02 I arrived with 32 in the tires. The car sort of wallowed around when I adjusted direction in corners. After a cool down period I moved the pressures up to 35. That helped some. After another cool down period I moved them up to 38. After that the car felt planted and responsive with no wallowing.
Remember though, this was at speeds of 70-110 mph on a track.

Tom

 

With my 255/40 R20 tyres (not the standard size for 20" rims, higher flange for some more comfort), I adapted the pressures recommended by Jaguar, using 2.2 bars front and 2.6 bars rear (which translates to 32 and 38 psi respectively).
That is for a use mostly on highways at 130 km/h (ca. 80 mph), loaded with 4 persons and luggage.
In town, it's a bit too firm if I keep these pressures and I should go down to 2/2.2 bars.

 

FWIW,

Here's a summary of the nitrogen vs. air test done by Consumer Reports.

Tires - Nitrogen air loss study

The pressure loss represents that lost by imperfect tire/rim sealing, valve loss as well as from the tire itself. The test can be considered flawed for many reasons but the upshot is the difference between the two gasses (1.3 psi over 1 years) is not significant enough to prove very much. The real world number also differ significantly from the study linked to above.

 

Thank you for the link that helps understanding your beliefs.

In the study you quote, I observe that the "pure" nitrogen filling is 95% pure, so that you have 5% air remaining in the "nitrogen" tyre.
When both tyres (the one with "pure" nitrogen and the other with air) are inflated at 30 psi, that means more or less 28 psi nitrogen and 2 psi oxygen partial pressures in the "pure" nitrogen tyre, and 24 psi nitrogen an 6 psi oxygen in the air tyre respectively (air is composed of ca. 80% nitrogen and 20% oxygen).
After one year, 3.5 psi are lost in the air tyre and 2.2 psi lost in the "nitrogen" tyre.

Actually if the escape is four times quicker for oxygen than for nitrogen (order of magnitude as in the formulas I provided above), and as the standard partial pressure of oxygen in air is four times less than nitrogen, 3.5 psi loss in the air tyre corresponds more or less to about 1.75 oxygen loss and 1.75 nitrogen loss for the air tyre, assuming that we have first order kinetics with flows proportional to concentrations.
Similarly, with the partial pressures of oxygen (2 psi instead of 8 psi) and nitrogen (28 psi instead of 24 psi) in the "nitrogen" tyre", and with the same higher diffusion rate of oxygen, the 2.2 psi loss in that tyre may well be about 0.4 oxygen loss and 1.8 nitrogen loss.

Of course these figures show orders of magnitude, and there is certainly at least 0.1-0.2 psi of uncertainty in the measurements, but all in all, the data you present are consistent with a four times quicker loss of oxygen compared to nitrogen through rubber, as I indicated.

Your basic mistake in dismissing a substantially quicker loss rate of oxygen was to focus on what you perceived as a relatively low difference in pressure loss in the experiment you quote between "nitrogen" and air, without noting the substantial bias of the 95% purity factor.
Lastly, I see nowhere in your link a causality of imperfect tyre/rim sealing on the pressure loss. Maybe your own "feeling"?