Mark Browne wrote:

I am not sure what you have taken from this thread. Conventional wisdom is
that the measured pressure increase is due to liquid water flashing to steam
above the boiling point of water. It has nothing to do with the fraction of
oxygen or nitrogen in the fill gas.

I don't place much weight in "conventional wisdom" when it comes to
physical phenomenon that follow well defined laws of physics.

Water will not" flash to steam" at the pressures and temperatures you
describe.

In the turns NASCAR and F1 cars run peak
tire temperatures between 225 and 250 degrees F. I leave it to you to offer
an alternate explanation of the measured 4 to 16 PSI jump (nominal 30 PSI)

If there was liquid water in the tire at the start of the race, at say
80 degrees F, all but the tiny amount required to saturate the filling
gas would still be liquid. The filling gas will follow the gas laws.

At 34 psig the gas temperature would have to reach approximately 280
degrees F to evaporate any liquid water in the tire.

At 46 psig the gas temperature would have to reach approximately 290
degrees F to evaporate any liquid water in the tire.

I have no idea what the tire volume is but if you do you can calculate
the weight of water present in the filling gas as a saturated vapor at
atmospheric pressure and temperature and if you know there is liquid
water flying around in the tire you can calculate what temperature and
pressure it takes for that liquid to change state.

under racing conditions. This increase is enough to completely scuttle
chassis tuning. While you are at it, explain how switching from running
"air" to dry nitrogen combined with a few forced purge-fill cycles
eliminates the effect - the tires pressure changes pretty much as predicted
by PV/T = PV/T.

IT looks like you are ignoring the vapor pressure of water and you
probably do not calculate the partial pressure of the water vapor in the
air filled tire. You are using the wrong gas law to begin with and when
you get a dry tire with a dry gas the tire acts as predicted.

This stuff is not conjecture - it is measured data. If it does not
match your expectations - perhaps it is time to reexamine your
expectations.

I am only a simple mechanic, it is my place to follow the laws, not to
change them.

The gas laws are not predicated on anyone's "expectations" they are
physical phenomena that scientists and engineers have used for a couple
of hundred years with great reliability and repeatability. It appears
that the only place they are held in abeyance is the race track.

If I am missing something here I would really like to know what it is.
It is an interesting subject.


Rick