Nitrogen in tyres

[gordski]

I know I'm probably being a bit dim, but my good lady wife had a flat tyre caused by a nail. I took the wheel off and popped down to my local tyre shop. Nice guys help you out not rip you off type of place. On the bill I noticed they had marked - nitorgen refill £1.15. It's not the money that my question relates to, but why nitrogen is good old fashioned air not good enough? Wifes car is not high performance (well its a 307 1.6l version). Any ideas?

[pincher]

I think (but am happy to be corrected) that nitrogen 'leaks' out of the tyre slower than normal air, therefore you don't have to pump them up quite as much.

Can't really see the point myself though.

[gordski]

I didn't understand either, since when the 4 tyres were changed recently (yes sods law getting puncture in brand new tyre) they were air filled.

[pincher]

Probably one of those 'standard charges' - a bit like the litre of washer fluid you get charged every service even if you filled it right up 5 minutes before you dropped it off

[ferrisbueller]

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

[annodomini2]

Given air is ~80% nitrogen I wouldn't have thought so, but did they ask her to put it in before they charged her?

To me it sounds like bulls**t! But they still should've asked!

[denisb]

On a hard driven car the main benefit of nitrogen is it is 'dry'. As moisture expands more than a gas this means your pressure doesn't increase so much with tyre temperature.

On your average shopping trolley it might naturally leak a little slower (tyres are usually a tiny bit porous) but the difference would be minimal.

[Mave]

ferrisbueller said:

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

Why is that? Doesn't Nitrogen follow the same gas laws as air, ie PV=mRT?

[aberdeencraig]

Nitrogen is more stable under pressure, as was previously mentioned, it is less affected by changes in temp. i believe a lot of F1 teams use nitrogen in tyres as do a lot of aircraft.
Nitrogen pressure testing is becoming more and more common in industry, pressures up to and sometimes exceeding 20k psi and is used for the same reason as above and, as also mentioned earlier the lack of hydrogen and oxygen in its make up.
why you would need it on an average car, who knows.

[ferrisbueller]

Mave said:

ferrisbueller said:

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

Why is that? Doesn't Nitrogen follow the same gas laws as air, ie PV=mRT?

Is m not different then? Why don't you dig out your steam tables and let us know - don't know where mine are...

[geoff82]

I thought the reason nitrogen was used is becuase chemically it is very inert and does not react with much unless specific conditions are met.

[Ahonen]

Nitrogen on a road car is pretty pointless, really.

Dunlop Motorsport uses dry air, which is perfectly fine for almost all motorsport applications.

[Mave]

ferrisbueller said:

Mave said:

ferrisbueller said:

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

Why is that? Doesn't Nitrogen follow the same gas laws as air, ie PV=mRT?

Is m not different then? Why don't you dig out your steam tables and let us know - don't know where mine are...

Don't know what you mean by "is m not different then". Thinking about it, I guess gamma is different between nitrogen and air, but I would have thought you wouldn't be able to measure the difference in pressure it makes with a tyre gauge

[ferrisbueller]

Mave said:

ferrisbueller said:

Mave said:

ferrisbueller said:

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

Why is that? Doesn't Nitrogen follow the same gas laws as air, ie PV=mRT?

Is m not different then? Why don't you dig out your steam tables and let us know - don't know where mine are...

Don't know what you mean by "is m not different then". Thinking about it, I guess gamma is different between nitrogen and air, but I would have thought you wouldn't be able to measure the difference in pressure it makes with a tyre gauge

Sorry. I could have worded that better. I meant that in the tables there are tables for various gases and their properties at various temps. I think from those that you'd be able to calculate the relative change you'd see in pressure for the different gases for a given change in temp. I suppose you'd assume V as a constant and no leakage.

The "is m not different" thing is based on the fact that if R and V are constant then a given change in T would cause a resultant change in P that would be related to the differing m's.

Feel free to tell me that's all bollox

[Pigeon]

Mave said:

Thinking about it, I guess gamma is different between nitrogen and air, but I would have thought you wouldn't be able to measure the difference in pressure it makes with a tyre gauge

Air is 99% diatomic molecules, so I wouldn't expect it to be significantly different.

[smhmotorsport]

Ahonen said:

Nitrogen on a road car is pretty pointless, really.

Dunlop Motorsport uses dry air, which is perfectly fine for almost all motorsport applications.

but most race teams then bleed and replace with nitrogen. when your running a 200mph prototype 2 psi can make a hell of a difference affecting ride height,damping etc. but on a road car i would say its pointless. too many variables to make it worthwhile.

[nel]

Obviously the answer must be to use helium to reduce the unsprung weight...

[mave]

smhmotorsport said:

Ahonen said:

Nitrogen on a road car is pretty pointless, really.

Dunlop Motorsport uses dry air, which is perfectly fine for almost all motorsport applications.

but most race teams then bleed and replace with nitrogen. when your running a 200mph prototype 2 psi can make a hell of a difference affecting ride height,damping etc. but on a road car i would say its pointless. too many variables to make it worthwhile.

Well, just did a quick hand calc. For tyres with 32psi at 0C, at 100C air would be 34.96psi, nitrogen would be 34.88psi. Even at 200C, the difference is less than 0.2psi, therefore I can't see why this would be the key reason

[smhmotorsport]

this sort of explains the issue,www.toyo.com.au/tech_info11.html but i have to say in all my time racing we never ran dry air in the tyres.

[ATG]

ferrisbueller said:

Mave said:

ferrisbueller said:

is it down to the fact that Nitrogen keeps a more constant pressure with varying temps when compared to humble air?

Why is that? Doesn't Nitrogen follow the same gas laws as air, ie PV=mRT?

Is m not different then? Why don't you dig out your steam tables and let us know - don't know where mine are...

No the "m" isn't different. It is just the amount of gas expressed in terms of the number of molecules. It's usually written as an "n" not an "m".

This subject was done (to death) a few weeks back, and I think it's fair to say that the conclusion was that the primary benefit of nitrogen was that it was dry. If your tyres are going to get sufficiently hot that condensation inside the tyres will boil, you might benefit from dry gas. Is that going to be relevant on a road car? Nope. It also appears to be true that nitrogen difuses more slowly through the tyre than oxygen ... but given that my air-filled tyres don't need reinflating at all frequently, I can't see how that is supposed to be important.

to be perfectly clear, it isn't that there is anything special about nitrogen ... it's just that nitrogen happens to be an easy way of getting hold of a very dry gas.

p.s. back of the envelope suggests filling with Helium would shave about 20 grammes off the mass of the car.

Edited by ATG on Tuesday 25th July 13:04