Nitrogen?

[Brett Dikeman]

On Mar 22, 2006, at 12:27 AM, SuffolkD at aol.com wrote:

> At the track some of the guys use it.
> Claims go from less expansion (pressure) at speed & temp

If you fill a tire (we guessed 10L in volume; this isn't too  
critical) with 36psi of Nitrogen, and a second with 36psi air (which  
is 80% Nitrogen) at room temperature and then raise both to 200  
degrees F (guesstimated off Hoosier's site on ideal tire temps for  
various track tires), you'll have a pressure difference of 8.8E-8  
psi, according to a friend who plugged in all the variables into  
MAPLE.  Keep in mind this would be dramatically lower for a passenger  
vehicle where tire temps stay in a much narrower range.  Any moisture  
throws all this out the window, especially if the tire temperature  
exceeds the boiling point of water.  Nitrogen, CO2, or scuba tank air  
are all very dry sources of air.

All the claims for Nitrogen are pretty silly.  The ones I've read  
thusfar:

-Gas escaping faster.  Well, air doesn't escape out of the tire very  
quickly in the first place...and again, air is 80% nitrogen.  How  
much air really "escapes" from the tire over the course of, say, a  
month?  Outside air temperature fluctuations strike me as much more  
important.
-Oxidation of the rim and the tire.  The rim is usually clearcoated  
and painted throughout, so the metal won't oxidize; if they're  
unpainted steel wheels, the steel requires moisture to rust.  As for  
the tire- I think it has far more to contend with outside, like UV  
light from the sun...
-Nitrogen retains less heat.  The heat capacity of the air inside the  
tire is rather small, and the heat capacity of Nitrogen and air  
(measured in J/g-K..ie Joules required to raise one gram one degree  
Kelvin) are within a couple hundredths of each other (around 1-1.06 J/ 
g-K, I think.)

Someone please feel free to double-check the calculations and such...

Brett