Tyre pressures

[Dangerous Fish]

It shouldn’t make any difference to pressure changes whether your tyres are filled with damp air or dry nitrogen. Boyles Law tells us that for any gas (or gas mixture) Pressure/(absolute) Temperature is constant if volume is constant. There may be arguments for using nitrogen but I don’t think they have anything to do with pressure. If there is liquid water in the tyre the argument would be different, but under any normal conditions the air in tyres should only contain water vapour - a gas.

I'm certainly not sold on the idea of having N2 in my tyres. Even when offered for free I don't bother. It's nigh on impossible for a tyre fitting shop to purge all the existing air from the tyre before filling unless they had very specialist equipment and in any case I'd have to invest in a bottle of N2 to keep topping up my tyres which I'm not going to be bothered with.
The benefits are minimal unless you're an F1 team maybe.

Water vapour doesn't act as a gas in all situations. Vapour pressure increases with temperature, so water vapour in your tyres will produce an overall increase in pressure as they get hot. The only real benefit of N2 in tyres outside of top level motorsport is that it leaks less than O2 so should maintain the set pressure longer.

 

[markharro]

I had to walk all the way out to the barn again <g>.. you're right. 19" stated.

Oh dear I couldn't ask a final favour? Would you be able to do the barn walk again either today or tomorrow and check that the full tyre code matches what is shown on my own tyres which is 245/45 R 19?

This would be extremely helpful as that would then pin down my correct pressure to the 45psi your sticker shows. Many thanks in anticipation.

 

[pgkevet]

I didn't have to walk back to confirm 245/45 R19 - not quite that senile yet <s> - just lazy about typing last time...

Now if summon worked properly I wouldn't have to walk anywhere. As it is the quadbike is the best way to get to the bottom of the garden to go pick blueberrues...

 

[markharro]

That's fantastic thanks v much

 

[AndrewGR]

Water vapour doesn't act as a gas in all situations. Vapour pressure increases with temperature, so water vapour in your tyres will produce an overall increase in pressure as they get hot. The only real benefit of N2 in tyres outside of top level motorsport is that it leaks less than O2 so should maintain the set pressure longer.

Water vapour (as distinct from water in liquid or solid (ice) form) is a gas, just as nitrogen and oxygen are. Water has a molecular weight of 18, nitrogen 28 and oxygen 32. Molecule size is related to molecular weight. So if anything is going to diffuse through the tyre it will be water vapour. The molecular sizes of O2 and N2 are so close I don’t think you would notice the difference. Air is mostly nitrogen anyway - 78%. All these gases expand similarly with temperature.

I believe the argument for using nitrogen in tyres is that it doesn’t cause the tyre to deteriorate from the inside by oxidation. In high pressure aircraft tyres it is safer in a fire if a tyre explodes - might also be a consideration in F1. For the average motorist I believe it is a con to extract money from people - a bit like super pure over sized copper speaker cables.

 

[pgkevet]

An oxygen molecule is a smaller (volume) molecule than a nitrogen molecule. - Inertion

 

[Dangerous Fish]

Water vapour (as distinct from water in liquid or solid (ice) form) is a gas, just as nitrogen and oxygen are. Water has a molecular weight of 18, nitrogen 28 and oxygen 32. Molecule size is related to molecular weight. So if anything is going to diffuse through the tyre it will be water vapour. The molecular sizes of O2 and N2 are so close I don’t think you would notice the difference. Air is mostly nitrogen anyway - 78%. All these gases expand similarly with temperature.

I believe the argument for using nitrogen in tyres is that it doesn’t cause the tyre to deteriorate from the inside by oxidation. In high pressure aircraft tyres it is safer in a fire if a tyre explodes - might also be a consideration in F1. For the average motorist I believe it is a con to extract money from people - a bit like super pure over sized copper speaker cables.

You keep saying water is a gas. It isn't. Gaseous water i.e. water vapour behaves almost like a gas, but the temperature would need to be over 100°C in the tyre for all the moisture to turn to gaseous water. A tyre on a Model 3 driving around UK roads is unlikely to get anywhere near that temperature, so the moisture inside will be vapour. Some of it may even have condensed and be liquid. That's why when the temperature rises the tyre pressure goes up more than if it contained no moisture.

 

[AndrewGR]

Pgkevet - I stand corrected on molecular size and accept the argument why this is true.

Electric Dream - I assure you that water vapour in air is a gas. Air can contain a substantial proportion of water vapour at temperatures below 100. Indeed it can contain gaseous water below 0. That’s why snow evaporates and why you can dry clothes on the line even if the temperature is below 0. When air containing water vapour is cooled to the point it is saturated (100% humidity) further cooling causes the gas to condense into liquid water droplets. In the air this forms clouds at height, fog near the ground - and ‘steam’ over a kettle.

 

[Dangerous Fish]

Pgkevet - I stand corrected on molecular size and accept the argument why this is true.

Electric Dream - I assure you that water vapour in air is a gas. Air can contain a substantial proportion of water vapour at temperatures below 100. Indeed it can contain gaseous water below 0. That’s why snow evaporates and why you can dry clothes on the line even if the temperature is below 0. When air containing water vapour is cooled to the point it is saturated (100% humidity) further cooling causes the gas to condense into liquid water droplets. In the air this forms clouds at height, fog near the ground - and ‘steam’ over a kettle.

Whatever. You're not going to accept the science, so I'll leave it at that.