why does air pressure continually drop in a tire?

it appears to be a hermetic system. Where and how is the loss? I’m sure this has been answered over and over again, but please pardon me on this lovely spring morning.

If it has been answered, does anyone want to have a go at why a tyre would explode?

From Is it better to fill your tires with nitrogen instead of air?

holy batman!
so the air filters through porous tire walls? I didn’t know that tire tubes were porous

That’s almost unbearably sad.

tires seem to go flat faster if they are static. if the tire is used they seem to hold the pressure longer.

I seriously doubt much of that. From what I remember O2 and N2 are fairly close in size. So given that it’d kind of be a surprise if replacing one of those gasses with another would have much of an effect. (O2 is a little heavier than N2, 32 AMU vs 28 AMU) Plus now that I think of it basically what you’ve done is increased the partial pressure of N2 vs an air filled tire of the overall same pressure. So given that there is a great difference in the concentration gradient in the N2 tire I’d expect it’d would actually leak faster.

Seriously though at least from my experience a decent tire doesn’t leak that fast with air anyway. (I only need to add air once every few months. When I needed to add it faster than that it was because of problems with the tire like old valves or slow leaks.)

Anyway like someone else has written yes rubber is porous to gasses. (Which is why a balloon filled with helium will deflate. The helium moves out and air moves in. The gasses move from regions of higher partial pressure to lower partial pressure. Helium is fairly small plus it helps that a balloon is actually quite thin.)

what i strange is that i have fixed many flat tires on bicycles for example and after you apply the patch, you submerge it, inflated, in water… and you see no air escaping. sounds like some sort of osmosis.

I think that’s pretty much what it is. Rubber is in effect semi-permiable to gasses and there is a higher concentration of a gas inside the tire than outside. The gasses move from a region of higher concentration to one of lower through the rubber. Of course the thicker the walls of the tire the slower this goes.

At a molecular level, pretty much everything is porous. At least a little bit.

Tire rubber, and the joint where tire rubber touches the metal rim, are close to impervious. But not *perfectly *hermetic. So over time, a few molecules leak out.

As long as there is a pressure difference between inside & outside, there is a force driving air molecules out. So it is a one-way process until the pressure difference is lower than the porosity. Which is why a tire will leak down from 50 to 40 more quickly than from 40 to 30 or 30 to 20 or …

Finally, as the tire gets very low, the shape changes enough that the fit of the tire to the rim begins to degrade. And as even microscopic gaps open up where the tire meets the rim, the rest of the air escapes quickly, in hours rather than days. And then that tire is truly 100% out of air pressure.

I do this as well (under water ‘test’) to see if the leak was repaired. However, if enough time was given, there would most likely be tiny amounts of accumulation(s) of air ‘bubbles’ where teeny ‘holes’ allow molecules to escape. It might take days or weeks, but air will escape eventually. Not the best way to put it, of course, but a quick immersion test does not allow enough time to visualize the slooooow leak of air (and pressure)

As far as nitrogen use, I do remember quite well how nitrogen, when used in white-water rafts/inflatable gear, will not respond to temperature changes like atmospheric air does. On cold mornings, I would have to ‘pump-up’ my raft to a usable pressure while a fellow that had inflated his raft with nitrogen had no such duty. I would bleed off pressure throughout the (sunny/warming) day so as not to exceed pressure limits of the seams. I did hear about folks that had their nitrogen-filled rafts ‘exploding’ from dropped cigarettes or sparks/etc, so there was a risk to using such a lower-maintenance (and quite spendy, I hear!) gas within their rafts :wink:

this reminds of what physicists have been telling us forever. that there is much more space between molecules than we think. that even a table is really not solid.

Actually it’s my understanding that this is a 2 way process. Molecules of gas move both into and out of the tire at the same time all the time. However since the pressure is higher in the tire there’s more of them so there’s a better chance one will migrate from inside the tire to outside.(The rate from inside to out is greater than outside to in.) Once the pressure is equalized then they would move in and out at the same rate.

So how could a nitrogen-inflated raft explode?
It sounds to me that maybe some other gas (like butane) was used instead.
How about the wheel material? I understand that steel wheels are pretty good, but that cast aluminum/mag wheels can be porous, and pose pressure.

If we are talking about passenger cars and trucks they don’t have tubes, just the tire sealed to the rim.

My bad on nitrogen…it was a mix used in the raft. Not thinking clearly at that time of morning. Oops. I did not double-check myself.

Per Cecil in 2007

Movement of gas through a solid is described by permeation, which includes both diffusivity and solubility. The diffusivity is a function of molecular size, but the solubility isn’t (or is less so, at least). I posted some actual numbers a long time ago on permeability rates through rubber in this post, which is probably not worth C&Ping. Bottom line is permeability doesn’t closely scale with size at all.

You are correct that permeability scales with partial pressure, but continue along your line or thought above: If the permeability rates of oxygen and nitrogen were identical (which they’re not), replacing O[sub]2[/sub] with N[sub]2[/sub] would have exactly zero effect: more N[sub]2[/sub] would leak out of the tire, but a commensurate amount of O[sub]2[/sub] would leak in, resuluting in the same net loss of pressure over time.

I still doubt this would make a difference regardless. Lets think of the situation where O2 leaks significantly faster than N2.(IE we consider the case where O2 leaks in almost no time while N2 doesn’t leak at all.) About 1/5 of the pressure in air is from O2 and 4/5 are from N2.(At least that’s what I remember the partial pressures are from chem class.) So suppose I had a tire inflated to 30 lbs. So 24lbs is from N2 and 6 is from O2. So the O2 leaves and we would expect to see the tire drop to a pressure of 24lbs.

Next we add air again. However we still have the old 24lbs of pressure from the N2. The new air (6lbs) is made up of N2(4.8lbs) and O2(1.2lbs). So after this second pumping of plain old air our pressure consists of 28.8lbs from the N2 and 1.2 from the O2. In effect everytime you add air you would be only replacing the escaped O2 and you would quickly end up with almost nothing but N2 if there were a significant different in diffusion rates precisely because we’re assuming a selectively permeable barrier.(The N2 would stay behind and the O2 would leak out every time you added air.)

I was a real skeptic regarding nitrogen filling, but there have been enough studies done now to show that there is a real-world advantage to nitrogen filling.

A long-term trial conducted through a large trucking company showed that nitrogen filling resulted in a 6.1% increase in fuel economy when no pressure monitoring was done, or 3.3% even when all tires were kept inflated to the right pressure at all times. They also found a decrease in tread wear and longer tire life.

One factor that hasn’t been mentioned is that nitrogen is inert, and oxygen is extremely reactive. So you get some oxidative breakdown of the rubber tire from the inside as well as the outside with air fills, but you avoid the oxidation on the inside with a nitrogen fill.

Now, this was with heavy duty truck tires. Whether you’d see the same benefit in a car is another question. Yet another question is whether the savings would be greater than the cost of a nitrogen fill.

But it definitely has an effect.

I got new truck tires last July. Dealer offered nitrogen for $5 a tire. $20 seemed worth trying. I checked my air pressure last week. That’s 8 months and it still doesn’t need any. Dealer said he’d add more (when needed) for free.

So far, I think nitrogen helps.