Cyberpunky

I went to my tyre place today, where I bought my tyres from. I got sold nitrogen for them with life long top ups. I have not had a top up in about 5 months as car was off the road for a couple of months getting a new turbo.

Anyway guy there tells me it didn't need any nitrogen as tyres had about 37 PSI when they normally have 35 PSI, so he just let some nitrogen out. The tyres were cold as it is very close to my house and car hadn't been driven.

I can't see how that could be right and am wondering if he just told me that because it was close to closing time and they just couldn't be bothered doing it. Is it even possible for tyres to go up in pressure after several months ?
Seems to defy the laws of physics to me
TIA
peace
Cyberpunky

Jim Michaels

Tire pressures increasing over time seems strange to me too. I assume that your own tire pressure gauge read lower than 35 psi, or you wouldn't have bothered to take them in. How many of the following hypotheses can be rejected?

Hyp. 1: The tires were actually overinflated before they sat idle for several months.

Hyp. 2: The ambient temperature is significantly warmer now than it was several months ago.

Hyp. 3: Your tire pressure gauge reads too low.

Hyp. 4: The tire guy's gauge reads too high.

Hyp. 5: The tire guy didn't want to mess with it because it was near closing time.

Hyp. 6: Pure nitrogen maintains a more constant pressure over time and temperature than does just mostly nitrogen; i. e., air.

Hyp. 7: Pure nitrogen (as opposed to just mostly nitrogen) expands over time, so one has to keep bleeding off nitrogen instead of adding air. [If this is true, I'm switching to nitrogen too]

Cyberpunky

Yeah my money is on Hyp. 5. Think I will have a word to the manager next visit. Why they offer these services when they can't be bothered dealing with it gets me *shrugs*

Forgedwheeler

According to Avogadro's law, (physics) all gasses expand and contract at the same rate when heated or cooled. This includes air, oxygen, nitrogen, helium, cyanide or any other gas for that matter.
The only variable would be the partial pressure caused by minute amounts of water vapor. Really humid air will expand by about .5 PSI when heated from 70 degrees F to 180 degrees F.
If the pressure ACTUALLY increased, it is because the temperature in the tire increased. This can occur from running the tire or, most likely, from changes in ambient temperature.
Save your money. Dry compressed air is scientifically the same, and 78% Nitrogen anyway.

Cyberpunky

I paid an extra $8 per tyre, when I bought the tyres, for a life time of refills, so it isn't going to cost me anything (more) to stick with nitrogen. According to the spiel when I was sold nitrogen, it is a lot less likely to need topping up, compared to air, as the molecules can't permeate the rubber to escape, like air allegedly does. At the time I figured it was worth the money just to have some one fill my tyres for me for life lol

Forgedwheeler

There is some science to support the reduced pressure loss. The Nitrogen molecules are slightly larger than the oxygen molecules. So they will permeate the rubber more slowly.
But when you do the math, the nitrogen advantage is only about 2-3 lbs less lost per year in a normal size tire.
Nothing wrong with Nitrogen, as long as you don't have to go too deep into your wallet.
It just won't increase gas mileage, reduce tire wear, increase lap times, save the whales or reduce ozone.
You probably guessed, correctly, that this is a pet peeve of mine.

Cyberpunky

D'oh! that was the other reason I went with it

Kevinmacd

The only thing I see nitrogen a benefit for street car is if you have a set of expensive aftermarket wheels they will not corrode internally since here is a lot less moisture.

Forgedwheeler

Theoretically, you are correct. However, it is almost impossible to evacuate a tire of compressed air without a vacuum chamber. Any residual moisture or air would contaminate the "pure nitrogen fill" (usually 95% to 98% nitrogen), eliminating this "benefit". Modern alloy wheels are relatively unaffected by moisture INSIDE the tire anyway.
However, wheels live in a hostile environment and are subjected to damage from abrasion, rocks and gravel, road salt and chemicals and acid cleaners in car washes. These are far more hazardous.
If you could drive in a 100% pure Nitrogen environment, you would be free from most of the above hazards. But you would find that most challenging.