Nitrogem

[jberks]

So do they also mount the tyres to the rims by remote-control in a nitrogen-filled workshop - having first baked all components to drive off any residual moisture?

Or are the tyres fitted, then evacuated with a vacuum pump before filling with nitrogen?

Fair point, though the volume of air in an uninflated tyre is minimal compared to one at 30psi so relatively the moisture content will be much less. That said, a dryer on the air compressor (which I suspect most have anyway) will have the same effect.

 

[Splatt]

Reading one tyre study report they conclude that AS AN AVERAGE a car tyre warms up and the pressure builds up by 1psi per 5 mins for the first 20 mins and by no more than 1psi from that point onwards.
That is, that from cold you can expect up to a 5psi tyre pressure increase from cold start.

Also that when charging the tyre with nitrogen the air is first purged from the tyre. If that is any more than taking the valve out and sucking it flat I dont know !

 

[S80]

Thinking a bit more about it, I guess the tyre could be 'flushed through' with nitrogen - kept below the pressure required to 'pop' the bead onto the rim.

Jberks - i'm not sure I agree with your comment regarding the amount of gas in an unflated tyre being 'negligible'. My physics/chemistry education ended more than 30 years ago, but:

Basic gas law is pV = nRT (1)

where p = absolute pressure
V = volume
n = molar amount of gas
R = gas constant
T = absolute temperature

To a first approximation, the volume (in cubic whatevers) of gas in a deflated tyre is no different to that in a fully-inflated one - assuming it wasn't sitting on the flat :neutral:

So - for the simplest case of a single gas, and at constant temperature, we can re-arrange (1) to give:

p = k.n where k is a constant (2)

Now, p, the absolute pressure of gas in the 'flat' tyre is 1 bar (atmospheric pressure). By inflating the tyre to a gauge pressure of, say, 2 bar (~30 psi on your tyre pressure gauge), the absolute gas pressure is raised to 3 bar.

So from (2), we can deduce that we have only added just twice the molar amount of gas that was present initially.

Must get a jacket with elbow patches....