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I was wondering: If you check the pressure on your bicycle tyres every week and top up the pressure with air then how long would it take without getting a puncture before the nitrogen content of the tyres is enriched to 99%? Also is pressure a variable? I'm not quite sure - because the molecular sieving action of rubber tyres is mostly due to solubility, but perhaps diffusion also has an effect.
It won't?
Assuming your leaky rubber leaks different elements at different rates (and I've no idea whether that's true or pseudo-science), if you're continually topping it back up again then it'll tend towards what you're topping it up with, which is ~80% nitrogen IIRC.
If you have a bottle of Coke and every time you pour out a drink you top it back up with water, eventually you're going to end up with mostly water not mostly Coke.
If you have a bottle of Coke and every time you pour out a drink you top it back up with water, eventually you’re going to end up with mostly water not mostly Coke.
On first reading that sounded like a reasonable analogy, but on second reading it doesn’t sound like the same thing is happening. Isn’t it more like letting orange juice with bits settle and pouring some of the more ‘watery’ juice away, then topping up with bitty juice. How long until it becomes very bitty? (Now that is a terrible analogy).
Ooh!
No, it's a great analogy, and I hate you for it.
Is this bike on a treadmill?
No, it’s a great analogy, and I hate you for it.
Aww, thanks - I shall wear that hate with pride 😀. It is a little clunky though, although relatable which always helps.
So I suppose the question first becomes, what elements actually leach and at what rates?
If nitrogen never leaches then it's exactly your analogy, you're never disposing of your orangey sediment just continually adding to it.
If it leaches after everything else has gone then it's wholly dependent on you topping it back up at the right point in time, once all the water has gone but before you've reached the bits. So your first fill would be when you've lost 20% and then increasingly shorter intervals that I can't be arsed to do the maths for.
If it simply leaches slower than everything else then it gets complicated...
Tyre deflation is proportionately more due to oxygen vacating the tyre because it is more soluble in butyl rubber than Nitrogen.
So before you top up that partially deflated tyre it has a higher proportion of Nitrogen in it than air.
When you top up the tyre with air you bring the mixture closer towards that of air but not completely because you are only partially refilling the tyre.
Then the next time you refill the tyre you put in even less air because there was proportionately more Nitrogen in the tyre so less gas escaped.
People will notice this empirically, the first week after they've had a puncture they'd have more pressure drop than for a tyre that has not had a puncture for a while.
Perhaps you are right that the 99% nitrogen would not be reached as some nitrogen does escape the tyre also - perhaps a better question would be what %age level does the Nitrogen level tend towards?
I go tubeless with sealant. How does that fit in?
Oxygen is ca. 2.5 times more permeable than nitrogen through butyl rubber IIRC. Given some every day assumptions about pressure loss (e.g. a 23c road tyre at 100psi loses 5psi per day just on standing?) and twisty's schedule of re-inflation (e.g. we re-inflate with air to 100 psi after 1 week), it should be straightforward to get a rough picture of the nitrogen enrichment rate.
A complication to this simple picture is that the rate of deflation will change according to the composition of air in the tyre. Whilst one could simply approximate this as constant (e.g. 5psi per day as above), this is unsatisfactory as the basic premise of the question is that different gasses permeate through membranes at different rates.
I'd sketch this out myself but have some fig rolls to attend to.
the rate of deflation will change according to the composition of air in the tyre
Very good point. If it gets to 98% N it will only deflate at a tenth of the rate it did at 80% N - but that's assuming all the deflation is down to loss through the butyl rubber, if there's a physical leak path the rate will only depend on pressure. Loss of pressure through the rubber and replacement by air will (until convergence) increase the percentage of N, whereas loss through physical leaks and replacement by air will push the N content back towards 80%.
If you have a bottle of Coke and every time you pour out a drink you top it back up with water, eventually you’re going to end up with mostly water not mostly Coke.
Nah, with an appropriate tap of the bottle whilst uttering woo, you end up with super strength Coke.
A guy at work anecdotally stuck his head in a nitrogen tank , to end it all.
I'd like to think in his last moments , he worked it out.