Cold tire pressure VS. hot tire pressure

[JeffreyNMemphis]

Hello Pelicans,

My 85 Carrera tire pressure should be 29/36 cold tire pressure. Problem is, when will my tires get cold again? The temperature here in Houston has been almost 100 every day. The mechanic's hand held thermal temp meter was reading 107 degrees when pointed at the concrete.

What pressure should I measure when the ambient air temp is 98 degrees? Should I add or subtract from the 29/36 psi?

Thanks in advance

[Paulporsche]

"Cold" in this instance does not refer to ambient temp. It means the tire has not been driven on for more than 1 mile. A good time to check would be after several hours of not driving or, probably best of all, first thing in the morning.

[dshepp806]

Mine are checked in the AM,..after she's slept all night...

Best,

[911 heathen]

Use a nitrogen fill in your tires as this will prevent (well, minimize at least) any pressure creep.

Since not using standard 'air', I have noticed a difference in handling as being more predictable no matter how long the journey or warm the tires are.

Over here it costs 5 bucks a corner; a good investment at twice the price.

[EarlyPorsche]

You could probably just drain and fill your tires once or twice at the service station and get a close enough cold fill. Try at your own risk but I do it.

[911s55]

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=147

[javadog]

Originally Posted by 911 heathen

Use a nitrogen fill in your tires as this will prevent (well, minimize at least) any pressure creep.

This is a fairly misunderstood thing. The difference in using nitrogen vs. normal air has to do with the fact that the nitorgen doesn't have water vapor in it and compressed air usually does. When you drive your car, the tires get warm from deforming as they roll. This heats the air inside your tires, which raises the pressure. This pressure increase is proportional to the temperature increase. For a street driven car, this is usually of no consequence, as the tires do not run that hot.

When you race, to get the most grip out of a set of tires, they must run at a certain temperature, which varies from tire to tire. Different brands, different constructions, different sizes and different rubber compounds all have an influence on what that ideal temperature might be. Racers are interested in the hot, running pressure. Once they know what pressure they want, they adjust the cold pressure so that the pressure rise from cold to hot gets them the hot pressure they want. Keep in mind that race tires run much, much hotter than street tires. If the temperature inside the tire gets hot enough, any water vapor inside the tire will cause the pressure to rise (between cold and hot pressures) more than what a dry gas would on it's own, thus resulting in higher hot tire pressures than they wanted. So, they run nitrogen, or any other gas that has had the water vapor removed, so that they get the same pressure change every time.

There are a couple of other minor differences but, for the average guy, it's not worth it.

JR

[911 heathen]

Good info Javadog

With nitrogen, I just like knowing when I fill to a certain pressure, it stays at that pressure.

Also, from high school chemistry memory, nitrogen is denser than atmospheric air so overtime you don't lose pressure through leakage. So no constant pressure checking...

[javadog]

Actually, air is around 78% nitrogen anyway, with the rest being mostly oxygen and CO2. And to refresh your memory, your'e not thinking of density but molecule size...

At least I agree that nothing good can come from having water vapor inside your tires.

JR