[Vwdiesel] TDI tires

[Roger Brown]

Only problem I have with the pressure vs. temperature change is that the product of the 
gas constant and molecular weight for O2 and N2 are pretty darn close, within 0.7% of each 
other, so by Boyles Law, that would keep the pressure change very close:
	http://www.engineeringtoolbox.com/individual-universal-gas-constant-d_588.html

I think the main issue is water or water vapor as mentioned by James.  And in some 
applications, the O2 can cause trouble, especially in very high pressure tires like heavy 
equipment and airplanes.  Put an organic material (rubber) and an oxidizer under pressure 
and possibly high operating temperatures and you can have combustion.  But in normal auto 
tires, you are maybe talking 30-40 psi and not very high temperatures.

According to this article, there is a about a 3-4 times difference in the diffusion rate 
of O2 vs. N2 through something like rubber:
	http://www.getnitrogen.org/pdf/graham.pdf
But like Loren mentioned, over time you effectively enrich the N2 in the tires as the O2 
leaks out.  But since there is like 4 times the partial pressure of N2 as O2 in the air, I 
guess that sort of equals out, so you are losing about equal amounts of N2 and O2 in a 
given time.

I have a pair of quite old tires on my 4x4 that I am trying to wear out.  They were spares 
for many years and once they hit about 10 years old, I put them on the road to wear them 
out before they died of old age.  Actually, they are pushing 15 years old now and just 
show the faintest amount of cracking on the sidewalls.  They have been filled with good 
old air the whole time and have done a fair amount of time off road which includes airing 
down to ~15psi and back up to 30-40psi each trip, so they have seen a lot of oxygen and 
water vapor in those years.  I don't have a dryer on the compressor on my truck.  They 
still hold air very well, I only top off the air in them maybe twice a year unless I take 
them off road.  If there were a tire that should have succumbed to oxygen in the air, 
those should qualify.  In fact, here is picture of those tires back in 1998:
	http://www.4crawler.com/4x4/Santiago/Images/Santiago_07.jpg

One of the big things for the off-roaders is to use CO2 for airing up tires.  Has the 
advantage of packing a lot of gas into a small tank and fills a tire pretty fast.  Down 
side is the cost, runs $20-30 to fill a tank and that is enough for maybe 5-10 fill ups 
depending on the size of the tires and how low and high you run them.  But I guess I am 
just too frugal, I can't see paying for air.  Sure, I spent money on the compressor for 
the truck, but as long as there is a charge in the battery, it'll pump air for very low cost.

And I do notice that the smaller tires on my VW diesel do lose air pressure at a faster 
rate than the 33" tires on my 4x4.  Likely related to the tire's volume to surface area.


On 2/29/2012 5:53 PM, dieseltdi at verizon.net wrote:
> According to my son, the mechanic, N2 use has little to do with leakage or anything else.  Nitrogen doesn't expand and contract near as much as oxygen during heating and cooling cycles so you don't get high pressures during driving and lower pressures when it is sitting.  Also eliminates seasonal pressure changes due to winter and summer temp swings.  Hayden




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   Roger