Why was nitrogen used in race car tires?

Dear Car Talk

Dear Car Talk | Mar 01, 1995

Dear Tom and Ray:

The recent inquiry about using nitrogen in tires caught my eye. I was in the automobile trade in the 1920s and 1930s. The only time I remember using nitrogen in tires was at the Indianapolis speedway. Race cars there used to go 500 miles without pit stops or changing tires. Some drivers used nitrogen in their tires to keep the pressure constant. Air-filled tires would expand as the air got hot. But nitrogen filled tires would not expand because nitrogen doesn't expand like atmospheric air does.

TOM: You sure sound like you know what you're talking about, Andy. But so does my brother, and I've learned not to trust him either.

RAY: The last time I took high school chemistry...

TOM: Notice he says "last time," because there was a first, second, third, and fourth time he took it before he finally passed it!

RAY: As I was saying, the last time I took chemistry, atmospheric air was made up of 80 percent nitrogen. And nitrogen did expand when heated, just like every other elemental gas on the planet. And since I haven't read of any breakthrough discoveries in the composition of atmospheric air or seen any applications to amend the old PV=nRT equation, I assume that nitrogen still expands just like air does. So I'd have to disagree with you, Andy.

TOM: My guess is that nitrogen was used in race cars because it is inert, or non-combustible. So that if there was a crash--especially a fiery crash--and the tires popped, the nitrogen would not fan the fire the way atmospheric air with oxygen would. Of course, I'm just guessing, too. But at least my guess doesn't alter the basic principles of modern chemistry.

RAY: No. His guesses often alter the basic principles of logic, but not modern chemistry.

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