"Mark Browne" wrote in message
news:Q_Zpb.109411$Fm2.94923@attbi_s04...



Conventional wisdom is
that the measured pressure increase is due to liquid water flashing to
steam
above the boiling point of water.

There are a few simple things you could do to eliminate the liquid water in
the tires:

1) Install a water separator on the air line between the compressor and your
inflation nozzel. Standard equipment on most air systems.

2) Don't mount the tires outside in a driving rain!

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)
under racing conditions.

Are you saying that as the tire temperature changes from 225 and 250
degrees F (685 to 710 degrees R) the tire pressure changes from 30psig to
46psig ( 44.7 psi to 60.7psi at sea level) ??

pv/t = PV/T . IF the volume stays constant (not a good assumption) so
44.7V/685 should equal 60.7V/710
0.065 does not equal 0.085. Nope, something else going on here.

Hmmm, sure doesn't follow the steam tables. According to the steam tables
publised by the American Society of Mechanical Engineers (the accepted
standard) the internal temperature would have to be up to around 294 degrees
F to have boiling water create that kind of pressure. Nope, isn't steam.

And you say this doesn't happen when you use Nitrogen?

You said that this happens in the turns, so I can explain why you would get
a sharp increase, but I can't explain why you didn't see the same increase
using nitrogen. In the turns, the tire is going to be subjected to a
significant amount of lateral force. This force is going to distort the
shape of the tire, hence its volume will decrease. As you decrease the
volume, the pressure will increase. You are changing a second variable in
the PV/T equation.


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

What other gases did you try this with? Did you try dry air? Did you try
carbon dioxide?
The water theory would have been trival to eliminate by simply eliminating
the water, did you do that?
Did you try adding a little water to nitrogen and seeing if it behaved just
like air?

Numbers are wimps, if you torture them enough they will confess to anything.
To throw out the ideal gas laws because your measurements didn't agree, and
then say "Conventional wisdom is that the measured pressure increase is due
to liquid water flashing to steam" is absolutly conjecture.

Maybe there is something else going on. Too bad you didn't follow the
scientific method properly and try to figure out what it was. Change one
variable at a time and you have a much better chance of establishing what
the cause and effect relationships are.

Nitrogen is a funny gas. At sea level pressures and room temperatures it is
generally inert and safe to use for many applications. If you increase the
pressure, that is no longer true. SCUBA divers all know that under a few
additional atmosphers of pressure Nitrogen does BAD things to the human
body. Oxyen is even worse, double the pressure of oxygen and it suddenly
becomes toxic in its pure state. Perhaps what is really going on is that at
those pressures and temperatures the oxygen in the air is reacting with the
rubber compound of the tires, makng them more pliable.

Rod