Originally Posted by
Slaninar
Exactly. After the oxygen leaks out and you repump the tyres, there should be around 95% nitrogen. After several times you add air to your tyres, they will be more and more nitrogen. Theoreticaly at least.
Yep.
I did some calculations using the figures on the Prestacycle site (which sells a Nitrogen inflation system) for the rates at which oxygen and nitrogen escape. Starting from a mixture of ordinary air, you'll have nearly pure nitrogen (plus residual moisture) when the tire slips to about 70% of its initial psi. If you wait until then to refill, the newly refilled tire will be about 93% nitrogen and will hit pure nitrogen at about 90% of psi. Refill again at that point and you'll be at 98% nitrogen. From there, vigilant re-inflation will keep you very near pure oxygen using just a simple floor pump. The key is to let it drop to 70% before the first re-inflation.
Of course, most people probably let their tires drop to around 80-90% of ideal psi before topping it off, so they're condemned to always hover at around 90% nitrogen.
Yes, I'm a geek.
To offer something a bit more practical, finding tubes that minimize air leakage is probably a much better investment than inflating with nitrogen.