What kind of air in tires?

In a recent post someone is busting about the same kind of air in their tires, well I'm a crew guy in Div.1 and we wondered once if anyone used an inert gas(Argon,Nitrogen?) for filling tires instead of compressed air? Heat in the tires should not change the pressure after a burnout or is an increase in pressure necessary? Any thoughts?

 

from: http://kekule.chem.csus.edu/gaslaws/charles.html

Charles' Law:

For a fixed amount of gas (fixed number of moles) at a fixed pressure, the volume is proportional to the temperature.

In other words, as the temperature increases, the volume increases. (When you heat a balloon the volume of the balloon goes up.)

 

You're both correct as Boyle's and Charles' laws describe the behavior of an ideal gas. For all practical purposes, common gases -- nitrogen, oxygen, argon, helium, etc -- behave as ideal gases.

The kicker, however, is water which is ever present in air. If water vapor gets into the inflated tire, it can condense on the interior surfaces. If the tire pressure adjusted with moisture present, when the tire gets hot the pressure will increase more than expected. The moisture (liquid) turns into water vapor (gas) increasing its volume far in excess of what ideal gas behavior predicts. Varying moisture contents of air -- as will happen with varying ambient temperature, barometric pressure and humidity -- will lead to varying pressure increases.

NASCAR Nextel Cup teams use nitrogen in their tires primarly because it is very, very dry. Thus they get consistent -- and predictable -- behavior in terms of pressure increase as the tires heat up. Well dried compressed air will behave like nitrogen and is certainly more affordable.

And, yes, I do have a degree in chemistry.

 

No problem Will...just finished my grade 12 high school chemistry class at the end of January...seems to be one of the few things that has stuck...unlike other subjects...

 

A top fuel buddy used, I think, Nitrogen in his tires as a weight saving idea. Said nitrogen weight less than oxygen. Fact or fiction?

Paul

 

Nitrogen has a molecular weight of 28 and oxygen 32. Thus nitrogen is 12.5% lighter than oxygen. However, oxygen comprises only 20% air. Air then has an effective molecular weight of 28.8. So nitrogen is only 2.8% lighter than air.

Given this small difference and the weight of tire and wheel assembly compared to the gas inside, I don't think the weight difference between air and nitrogen would have a noticable effect.

Also consider that water has a molecular weight of 18. It's lighter than either nitrogen or oxygen. So increased humidity means decreased air density -- lighter air. (Odd, isn't it, that very humid, foggy air is considered 'heavy air' when it is actually lighter than dry air?!) In fact, 7.4% water will give air the same effective molecular weight as nitrogen. (Don't confuse this percent water with the percent relative humidity you hear in weather reports -- they're different.)