Nitrogen does not expand or contract when heated or cooled as much as the other gases in air. So you don't get the pressure change that normally comes with temperature change. Instead of 5-20 psi change you get like 1-3 or so.
it's primarily the moisture making the big change in cold vs hot tire pressure. i fill up my air tank to 125psi with nitrogen prior to going to a race weekend and use it for adding dry "air" instead of airing from my little compressor that always blows wet here in the south. the compressor is used for blowing stuff off with the air gun. ymmv. tim
Gas laws Another racer myth. Here is the formula describing how all gases behave when heated or cooled: Gay-Lussac's Law: The Pressure Temperature Law This law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature. P is proportional to T Same as before, a constant can be put in: P / T = C As the pressure goes up, the temperature also goes up, and vice-versa. Also same as before, initial and final volumes and temperatures under constant pressure can be calculated. P1 / T1 = P2 / T2 = P3 / T3 etc.
Using N2 controls the pressure rise on track versus air. It is especially noticeable when there is humidity present. We use it at the race track quite often, as do many of the bigger teams. We did not use it at MotoGP. Other teams might have, but it wasn't a directive from Bridgestone. They (Dunlop) use 'the gas' at Daytona, which is N2 with a refrigerant.
What others have said about the moisture in the air is the difference between 100 N2 and air you normally fill with, you could do the same thing with any compressor that has had the moisture scrubbed from the air.
Hell I had a car salesman tell me you cant mix regular air with nitrogen in the tires since it could explode because regular air has oxygen. I about fell over laughing after calling him a dumbass and asking him what the hell does he think he's breathing since nitrogen makes up 78% of the air he's breathing. God I think people are just freaking stupid.
Nerdz werdz... Nitrogen(N2) use for consistent tire pressures is all about the lack of water vapor. That's it. Coefficients of Thermal Expansion(TEC) between dry atmospheric air, N2 and CO2 are negligible. To get the best effect, you need to purge your tires twice. Once - still leaves moisture, thrice (M.Python ref.) - has no further benefit. When purging, leave 1psi in the tire to prevent the introduction of atmospheric gases. FWIW, N2 does indeed expand when heat is applied...that's why it's a #1 ingredient in simple explosives. It's what makes things go Boom...rapid expansion of N2 from applied heat. Why N2 and not some other element? It's abundant, easily harnessed and reacts to heat immediately. In another application, it's the whole reason the requisite pressures develop in your combustion chambers to force your pistons down the cylinder...burning air/fuel mixture(rapid oxidation) = lotsa pressure from the just as rapidly expanding N2. You get roughly 4000psi from 4000º when twistin' the throttle in anger...it's over in milliseconds then immediately starts to cool(relative term) and condense. For all N2's hype, I just use CO2 in my tires. A CO2 cylinder lasts longer as you'll get about three times the volume of gas when it's dispensed. It turns to liquid under pressure so you don't have to carry a 3500psi N2 rocket...you will, however, be in the 800-900psi range on a full bottle of CO2 and the bottle needs to stay vertical to use the gas. Local welding shop fills my 15lb-er for $14. It'll last a ten round plus season of "axles off" with an air gun when swapping out wheels for rain or shine and seating the beads of new tires on those wheels for every event. I'm sorry. What was the question?
The guys I pit with. ......we all use $20 bicycle pumps. Cheap, lightweight, easy to transport, quiet, easy to use.....lol
1 psi still leaves 15.7ish psi of absolute pressure in the tire. That's still a pretty large volume of "air" with all that nasty moisture in it. I would think it would take quite a few purge cycles to eliminate all the air/moisture from a tire unless you added a 2nd air valve to use as a bleeder.
It's about percentages. You're not after perfectly pure N2, just less H2O. If you have less water vapor in the tire, you have less pressure rise, which is the goal. For instance, the amount of difference in rise at Barber with N2 versus air was much greater than at Miller because of the relative humidity. It was almost insignificant at Miller. It does make a difference in the way the tire behaves and how long the grip lasts or we wouldn't do it.
Funny story... A few years back I was working with dirttracker Bryan Smith. As we were getting ready for the Bike Week races we had our nitrogen bottle filled which was primarily used for air tools but we always ran in our tires as well. He went on to win the Daytona shorttrack that year. When we got back to Michigan somehow (don't remember) I discovered that the tank had been filled with helium instead of nitrogen. When Barry, Bryan's dad, got home from work that night I inflated a tube from the tank and told him to inhale a little, which he reluctantly did...when he started talking like Alvin the chipmonk we all had a good laugh about it. Oh well...I guess you had to be there.