Nitrogen Tyre Inflation

[bernmc]

Went down to my local Britannia today, and saw a sign advertising their new Nitrogen Tyre Inflation service. They're now inflating all new tyres they fit with nitrogen, and will do any others for £2 a tyre. Not too bad eh. Might pop in with my VR4 sometime.

And for those of you who've never heard of it:


Why fill my tyres with Nitrogen?
Why would you want to do this? The benefits of Nitrogen filling are as follows:

Improved comfort of ride
Improved safety
Increased fuel savings
Improved life of tyre Nitrogen has long been the accepted gas medium for filling aircraft tyres, racing tyres and heavy mining and construction vehicle tyres. Nitrogen is used for safety reasons and to ensure that tyres are always at a constant pressure. Compressed air, the traditional medium for inflating car tyres, contains both oxygen (21%) and nitrogen (78%).
The rubber tyre is like a membrane, through which oxygen permeates three times faster than the nitrogen. The result is that the oxygen slowly leaks out through the rubber walls, and the under-inflation leads to higher tyre wear with a consequent decrease in safety and comfort, and higher fuel costs.

[richy rich]

sounds intresting

[amsoil]

Big benifit is that the nitro is dry so no moisture goes in and this means that the pressure stays constant (so I'm told) I wish the person offering it well as all the peole down my way have give up with it. Joe public just seems to think that air shoud be free and doesn't care that this free air will cause £25 quids worth (or whatever) of extra wear on the tyres.

[Nick VR4]

Its £20 here for 4 tyres and free re-inflation after
I have to do my tyres every week they loose about 4-6 psi and I bet there not open on a Sunday here

[WOODY72]

Nick, you must have a problem with your tyres if you are losing 4-6lbs a week, i.e. around the sealing area. Granted all tyres are porous through their butyl liner inside, nitrogen or not, some faster than others, but not 4-6lbs if all is ok. All alloy wheels oxidise, especially around the sealing area and have to cleaned up almost every time you have new tyres fitted. If your local fitter doesn't check this, he isn't taking care over his job, and your safety which is most important.

[lancerevo3]

id like to give it a go just for curiosity sake

[ianb]

Costco routinely fill (and will do top ups) with Nitrogen, no exta charge, but they only sell Michelin tyres,but at excellent prices. I have Exalto XL which are good.

[mattnz]

If it is true that oxygen escapes three times faster than nitrogen, then you should have predominantly nitrogen in your tyres anyway after a few top ups.

[Scanny]

i just had the front tyres swapped and asked for nitrogen (£1.25 per tyre) but i really dont understand why its better than oxygen or why i shouldnt be checking the pressures regularly. apparently i dont need to which goes against everything i was ever trained on about driving

[Davezj]

nitrogen does not expand as much as air when heated.
if you try increase the volume of a gas (by heating) in a confind space the presure will rise, and conversly lower temp the gas will contract lowering the volue and consiquently the pressure.

nitrogen is no better than air when you have a constant temp but vary the temp e.g. driving heats the tyres up the pressure will increase more with air than it does with nitrogen. would you notice the difference on the road normal driving, i would have thought not.

[Scanny]

so air is constantly expanding and contracting the tyre wall which will cause obvious stress even if it is quite minimal. nitrogen causes less stress and therefore damage to the tyres so there is less risk of the molecular structure being stretched which would allow air to escape. again, we are talking about miniscule levels but that is still above 0%

[Davezj]

No that is not what I meant, all I meant was you tyre pressures will not be as constant with air in the tyres, it is the moisture content more than anything that expands and contracts with heat.
Eg air filled cold tyre set at 36 psi, will be more like 40 psi when you check them when they are hot, after high speed driving.
Where as dry nitrogen filled tyres will have less of a change from cold to hot.

[elnevio]

There will be no difference, as it is essentially a fixed volume. You increase the temperature of the gas inside (whatever the gas is, be it air, pure nitrogen, or methane! etc), then the pressure goes up by an amount proportional to the increase in temperature. I'd have to check, but it's Boyle's Law, or something like that, maybe the Ideal Gas Law. Can't remember all my physics now, although I did post up about this on a different thread a few years ago.

What is a benefit of nitrogen is that it apparently doesn't permeate through the tyre walls, whereas oxygen does, albeit quite slowly. However, this is quite negligible, when you consider the mixture of gases in normal air. Air is already 78% nitrogen, with oxygen being 21%, leaving the last 1% being comprised of CO2, water vapour and a few other trace gases.

So what this means is that you fill your tyres with air to a pressure of say 36 PSI. We will ignore the effects of temperature for the moment.

Assuming everything but the nitrogen escapes, then a month later, you get almost pure nitrogen left in the tyre, but the pressure will have dropped to 28 PSI. So you top up the tyre with more air. Now, you are only adding enough to get it back up to 36 PSI. When only the nitrogen is left once more, the pressure will only have dropped to 34 PSI, as you only added 8 PSI's worth of air (yes, I know that pressure is not a measure of volume, but in an enclosed space of fixed volume, it is directly proportional, so works for this calculation).

Top it up again to 36 PSI, and when everything else but the nitrogen escapes once more, the pressure will have barely dropped, i.e. still be more than 35.5 PSI.

So you end up with nitrogen anyway!! The water vapour content is minimal, so I can only see its effect as negligible at best - other than perhaps the reduction in likelihood of corrosion on the wheel.


Now, I brushed aside the effect of temperature earlier to demonstrate the above. But bringing it back into the equation, and it turns out that it has a much bigger effect than most other influences! Richie, you are right to think that the advice to 'not need to check your tyres again' goes against everything you already know, because it does, and it's wrong. You set your tyres to 36 PSI with nitrogen when the tyres & wheels are cold, and the ambient temperature is say 0C, then check them two weeks later with an ambient temperature of 15C, they could be a few PSI higher. Factor in differences from getting the tyres warm (accelerating, braking, cornering), and the pressure can rise further.


Nitrogen certainly could be a goer, especially as it gives you a 'pure' starting point. But I would still check them regularly, and add plain old air when necessary - the pressure will change with temperature, even if the volume inside doesn't. It is true that nitrogen acts in a more 'stable' way than the air mix, but the differences can be measured in tenths of PSI units. This is the sort of thing that is relevant to F1 teams, not road cars, where no driver can notice a whole PSI or two difference in their road tyres, IMO.

As for the gas/air/whatever expanding/contracting and stressing the side walls, etc., I would have to estimate the effect of this to be hugely outweighed by the normal stresses on the tyres (weight of the vehicle, acceleration, braking, cornering, etc.), probably by a ratio of thousands.

[Davezj]

Well said nev, I sit corrected.

[Nick Mann]

I understand it to be exactly as Don mentioned near the start of this thread. The major advantage is the fact that you are not compressing air with a moisture content (and then forcing the moisture out of the gas with the pressure) so you get a dry gas in the tyre rather than a tyre full of moisture.

Oxygen isn't a lot different to Nitrogen in physical size, so I can't imagine the oxygen gets out of the rubber significantly more quickly TBH?

[Davezj]

If you put helium in the tyres it would leak out like a sieve.

Good job we don't use that!

I only mention it because we use it as a leak testing gas at work.