How does tire pressure effect tire temperature?

[Chad Stapleton]

If this is a real issue, why has nobody yet used a pressure relief/regulating tire valve to vent the pressure as the tire & gas heats up ?? But i guess you could have a problem if your tires cooled much under yellow flag running !

[Michael Polizzi]

will using tire snot to mount a tire induce water to the interior of the tire? I have been told not to use WD40 as this can break down the rubber of the tire.

michael polizzi 69n
full bore karting

[Mark MacDonald]

Chris, I didn't read through all of these posts, but I thought I'd share. I was at the Cleveland Grand Prix with the MTM Karting booth when these two chemists came up. They explained that the expansion is based on Molarity and the like. Claiming the best thing to use would be Hydrogen and Helium at a 30% to 70% mix, as Hydrogen is not explosive at less than 30%. Unfortunately I have not had the chance to test it Do take into account all the variables. I would actually suggest to just call up Bridgestone or Goodyear, they should have good ideas about it.

Mark MacDonald
MTM Karting
330.497.3585

[Tim Doll]

Chris

I "am" a rocket scientist (well, sort of, masters in Aero Engineering). Bottom line - it's a trick questions. Go find a text book on gas dynamics and look up the "Ideal Gas Law". The ideal gas law basically says that any gas will react the same to changes in pressure/temperature at a constant volume. In other words, "air", nitrogen, pure oxygen, argon, helium, even water vapor will all react pretty much the same in a tire with respect to changes in pressure due to changes in temperature of the tire (the catch being it be in vapor form - not an issue with most gasses but is an issue with water vapor).

Now, there are some differences in heat transfer and such that can make a small difference in the way that different gasses react in a race tire. BUT, the question your teacher asked - what's the difference between nitrogen and dry air (which is ~79% nitrogen and ~20% oxygen)? There isn't any, as stated by the ideal gas law.

Tim

[Chuck Bunnell]

Tim is right, and so was Lynn many posts back, that any gas will behave the same way following the ideal gas law. PV=nRT This is just basic chemistry and is true. Following from that is Boyles' and Charles' laws with respect to using the ideal gas law. For a constant mass of gas, like that trapped in a tire, we can say the following: P1*V1/T1=P2*V2/T2 This can be simplified if we hold any one of the variables constant. If we hold volume constant like in tire we have P1/T1=P2/T2 or P1*T2=P2*T1 which tells us that increasing the temperature will increase the pressure in direct proportion. The units for pressure and temperature must be in absolute units, for example psia and R. psia is pounds per square inch absolute. What you see on your tire gauge is actualy psig, or pounds per square inch gauge. Add ambient pressure to the gauge and you will get psia. Ambient pressure is typically referred to as the "barometric pressure" on the weather report and is just the ambient pressure in absolute measure. Typically it's going to be about 29 inches of mercury which is around 14.3 psia or so. It will vary dependent on the weather and your altitude. Degrees Rankine, R, is degrees Farenheit plus 460. Absolute zero is 0 R or -460 F. Are we having fun yet?

The real trick to remember here is that liquid water *can* be dissolved in a gas. It's a very small quantity and can be confused with the amount of water vapor also carried by air. But that vapor really wants to be a liquid at room temperature and makes things messy. The difference between dry nitrogen out of a gas bottle and ambient air is the water content. Dry air and dry nitrogen will both increase pressure at the same rate with respect to temperature, but the addition of liquid and gaseous water to air causes differences.

Regarding the hydrogen and helium. I think they were yanking your chain. First hydrogen and helium and the smallest molecules and are the most difficult to seal. We can probably handle that at least. Hydrogen is a fuel and requires and oxidizer to burn. Helium is one of the noble gases and is not an oxidizer and is actually fairly inert. Their "30%" number is just talk. Hydrogen in air has the widest explosive range of anything I've ever looked up. From 4% to 96% concentration in air you can ignite this stuff. I doubt you'll ever see anybody put this in their tires or use it at the track. Common bottled nitrogen will be the ticket for most everybody.