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      08-04-2013, 01:24 PM   #1

Drives: 335i Sport Black on Black
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Several tyre pressure questions

I've got several questions regarding my tyres and their inflation, hoping someone knows the answers.

Firstly, my car was ordered with Nitrogen in the tyres, however the dealer swapped out some 20" 361s for me prior to delivery, and they swapped the TPM from the original wheels to the 361s. They have N2 on the dust caps, but is there any way to tell for sure?

Secondly, I have coded my car to display temps and pressures in the status. Today I noticed at startup with the tyres at about 90F the pressures were about 33 front and 37 rear - does that sound right for cold temps? For Nitrogen? Tyres are Pirelli P-Zero 225/35-20 and 255/30-20.

Lastly, I thought Nitrogen didn't change pressure as much with temp - but on a fairly sedate and short trip to the coffee shop (in Texas heat) the temps went up to 105F range, and the pressures increased in the front by 3, rear by 2 - is that expected? Or have I got air not Nitrogen?

Any thoughts much appreciated.
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      08-05-2013, 02:41 AM   #2
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First your pressures probably have to be higher even.
It is a long story , but I have found out that , for savety of low Aspect ratio tyres ( the 35 and 30 in your sises), are given to high maximum load by the tyre-makers. You mayby even have to substact 20% of that what is written on the sidewalls.

If you want I can determine a save lowest pressure for your situation, then give me the needed data and I will calculate.

Then about the nitrogen filling ( further N2) .
Every ideal gas, and N2 and O2 (oxigen) are pretty ideal, water (H2O) is not, expands the same at temperature rising.
N2 needs yust a bit more energy for the same temperature rising then O2, but in the long run the tyres inside gets the same temperature with N2 100% then N2<>80% ( as in normal air).
And so also the pressure-rising is the same.
100% N2 contains no water , by the proces of making it. Water in the tire makes pressure somewhat unpredictable because it can chanche from gas to liquid in the temperature range that happens inside tires. Also the gas in the tire can dissolve in the liquid water and get un-dissolved , wich also helpes at this pressure unpredictability.

Made a spreadsheet for it, to play with the temperature or pressure , and see what it does to the other.
In this map download the pressurecalculationwithtemp spreadsheet by RIGHT clicking and then choose download. Also a simple pressure calculator there.
From this map you can navigate my complete public map of skydrive that belongs to my hotmail adress with same username as here.

You cant check if you have N2 filled tires.
If you fill with 100% N2 the empty tyre contains 1 bar/14.5psi with 80%N2 and 20% O2 ( a bit different but this for easy calculating).
this gives a %O2 belonging to the pressure .
A tire filled with 3 bar/44psi overpressure, gives 4 bar/58psi real pressure inside the tire. This makes at 100% N2 leaving a % O2 of 20%/4=5%O2.
Then you can open the valve and let the pressure drop to zero ( so 1 bar real pressure) and fill again to 3 bar with 100% N2 , and the %O2 drops to 5%/4=1,25%.

But this all is of no use , because in time the % O2 will go to 5% again.
O2 from the outside diffuses trough the rubber to the inside, against the high pressure.
Suprising and unbelievable you shal say.
But this all has to do with partial pressure, that is the same then inside as outside the tire. Then there is a balance, and there is diffusing yust as much O2 into as out of the tires. At that point only N2 is diffusing( and not leaking, there are no little holes in the tire) out of the tire per saldo, but about 5 times as slow as O2.

So enaugh for one post, first work this out, mayby you need some oxigen already.

Last edited by jadatis; 08-05-2013 at 02:52 AM.
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