Bruce SEA 993

So I was talking with a buddy of mine about Nitrogen in the tires and came up with this take on it. First the assumptions. Air is 80% nitrogen and 20% oxygen. This is for ease of calculation. The refill assumes that all the oxygen leaks out and you are left with all nitrogen. This may be extreme but shows how this works.

First time fill of your tires: 80% nitrogen and 20% oxygen.

Oxygen leaks out and you refill. 80% N + new "air" = 96% N and 4% O (20% refill of air * 80% N = 16% N additional)

Second time you fill your tires back up: 96% N + new air = 99.2% N and .8 O (4% refill of air * 80% N = 3.2% N aditional

Third time you fill your tires back up: 99.2% N + new air = 99.84% N and .16% O (.8% refill of air * 80% N = .64 N additional)

And so on..after you top up your tires 3 times you have mostly Nitrogen. I can see why race teams use Nitrogen as you don't keep filling them up within a few hours...not a few months. On a street car you fill them up and check them three times and you have mostly Nitrogen.

Garth S

Nope .... O2 does not preferentially leak out - it is actually a slightly larger molecule that N2, and osmotic separation via a rubber membrane is a non issue.
However misguided, my take on N2 usage is in post 5 of this recent N2 thread .