Miata Mailing List: February 1995, Message #7
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   From: Tom Manson 
Subject: Re: Good air for tires
   Date: Wed, 1 Feb 1995 06:59:18 -0500

Dale,
On Wed, 1 Feb 1995, you wrote:

> There is a reason nitrogen is better than air. I'm told the problem with
> air in tires is that the water vapor in it expands drastically when
> heated, causing a big change in tire pressures as the tires heat up.(I've
> seen it change by 4 lbs during a single lap in an autocross.) Bottled

So it's not the nitrogen, but the water vapor?  Air of course is 79% 
Nitrogen, 20% Oxygen and 1% trace gasses.  The pressure in a tire 
increases 1 psi for every 10 degrees outside temp (tire carcass or ambient 
air temp) from what I've heard (I've never done the calcs for that one 
Mac) regardless of the minor difference represented by the 20% oxygen.  Thus 
Matt R correctly referred to the ideal gas law.

> nitrogen is dry, and expands very little when heated, therefore the tire
> pressures are much more stable. It's also a very inert gas, so it doesn't
> rot the tires. 

I'll go along with that.  The 20% oxygen is probably slightly more active 
than the if it were another 20% nitrogen.

Tom Manson		Frederick, MD		Product Assurance Manager
President,		301-694-6721 (H)	Orbital Sciences Corporation
Chesapeake Area Miata	301-220-5670 (O)	Greenbelt, MD 20770
****************************************************************************
		Manson@fred.net  	(Frederick, MD)
		Manson@fsd.com	 	(Germantown, MD)
		TMANS@oscva.orbital.com	(Dulles, VA)
		Manson@VAX1.fs-gb1.com	(Greenbelt, MD)