One for the tyre experts....

[Mikey_s]

As far as I understand it F1 tyres are filled with Nitrogen rather than air.

I'm a chemist by training and I have given quite some thought as to why this might be. As far as I can determine there are relatively few technical arguments for why you would use Nitrogen to inflate the tyres rather than just using compressed air.

Firstly, the atmos is more or less 80% nitrogen in any case, so any move to an alternative gas relates to the other 20% (which is mostly oxygen).

Secondly, in respect of pressure variability, Nitrogen and oxygen behave more or less as Ideal gases, so pV=NRT applies - no difference there regarding temperature/pressure variations

It can't be weight - Nitrogen has an atomic weight of 14, Oxygen 16 and in the big picture the air in the tyres doesn't weigh much (in any case, why not then replace it with a lighter than air gas if that was the issue?).

So what is it? I would like to know as it confounds m why they do it...

Thank for you answers;

[manchild]

Nitrogen is not used only in F1 but for passenger car tyres too (check some local tyre repair shop for Nitrogen compressor). Nitrogen is more stable and better adaptable on temperature changes which occur in F1 during pit stops or safety car. It lowers the risk of tyre self-deflating under sudden change of temperature... so. the reasons why it is used in F1 are safety and performance reliability.

When it matters passenger cars it is used for same reasons as in F1 but there is also fight against corrosion of rim and valve including the better overall care for pneumatics inner wall since there is no oxygen which is not the case with air inflated tyres.

[MrT]

The reason nitrogen is used is that air contains moisture (its not pure air) and nitrogen doesn't. The mositure in air has non-desirable effects in terms of pressure change with heat as it is not a pure gas.

Anyone in any form of serious motorsport will use nitrogen instead of air.

MrT

[scarbs]

Neither F1 tyre supplier uses Nitrogen to fill the tyres. they have used dried air for some years, as this is easier to ship across all the circuits and tests, as it doesn not have the thermal expansion issues of un dried air.

However, the teams may use small nitrogen cylinders to alter tyre pressures in the pit.

Scarbs...

[manchild]

Neither F1 tyre supplier uses Nitrogen to fill the tyres. they have used dried air for some years, as this is easier to ship across all the circuits and tests, as it doesn not have the thermal expansion issues of un dried air.

However, the teams may use small nitrogen cylinders to alter tyre pressures in the pit.

Scarbs...

It's not that I doubt your first-hand sources Scarbs, but why than all the websites dealing with this problematic mention that F1 teams use nitrogen or mixture of nitrogen and some other gases (including some very fresh articles)?

[ginsu]

Secondly, in respect of pressure variability, Nitrogen and oxygen behave more or less as Ideal gases, so pV=NRT applies - no difference there regarding temperature/pressure variations

Using the PV=nRT equations tells us that because of the molar weight difference of nitrogen vs. oxygen the volume due to temperature changes would be significantly less. Using the molar weight ratio, I calculated about 13% less change in volume. Obviously, a race engineer would want to remove as many variables as possible and this is certainly one of them.

[ginsu]

I love 'How Stuff Works', here you go:

Many race car teams use nitrogen instead of air in their tires because nitrogen has a much more consistent rate of expansion and contraction compared to the usual air. Often, a half pound of pressure will radically affect traction and handling. With track and tire temperatures varying over the duration of a race, the consistency of nitrogen is needed.
Nitrogen pressure is more consistent than normal air pressure, because air typically contains varying amounts of moisture due to changes in the relative humidity on race day. Water causes air to be inconsistent in its rate of expansion and contraction. So, a humid race in the southeast United States or a dry race in the desert western United States could make for unpredictable tire pressures if "dry" nitrogen were not used.

Nitrogen is also used in the high-pressure tires on large and small aircraft.

[Mikey_s]

Thanks for the input Ginsu - I am sure that there is a good reason why they do this, but I am still struggling with the technical explanation!! I am perfectly happy to accept that I am wrong, but I am struggling to understand the issue...

To address the two points raised;

If we assume that the volume of the tyre is effectively fixed (at least in the context of this question the volume is the same regardless of whether you use N2, or air) the gas laws tell us that a given volume of gas will contain the same number of molecules - therefore the variables are;
1. The mass of the gas (20% of O2 at the volumes of gas in the tyre would actually make a negligible - probably immeasurable - difference in mass)
2. The mole fraction of each gas in the tyre – I don’t know what the volume of the tyre is, but for the sake of the argument let’s consider it to be 22.4 litres (convenient as this happens to make 1 mole under STP) the mole fractions are going to be the same in relative terms as the percentage of each gas – therefore the pressure change with temperature will be the same.

Both gases will act in exactly the same way in respect of the temperature/pressure equation because both gases are perfectly elastic under the conditions we are concerned with.

The second point you raise... well I think Wikipedia is not correct. Both gases will behave the same, both are ideal gases for the purposes of this discussion and the pressure volume relationship relates to the kinetic energy of the gases - the pressure temperature relationship must be the same for a confined volume. Furthermore, both gases can be dried, so the water content shouldn't come into it either.

Perhaps the reason is as simple as N2 can be purchased in bottles, whereas a compressor is heavier and more bulky…?

[DaveKillens]

When you fill a tyre from a ntrogen bottle, it contains almost no water molecules. Compressed air, although, can contain a much higher percentage of water molecules (and usually does). It's this water/steam expansion they are workng to avoid. If you have just nitrogen then when the tyre heats, the expansion is predictable and repeatable. If the medium is just compressed air, then the odds are that you will have some degree of moisture content. Then when the tyre does heat up, not only do you have the regular air expansion but the unwanted and unpredictable effects of water/steam expansion.

[Guest]

The mole fraction of each gas in the tyre – I don’t know what the volume of the tyre is, but for the sake of the argument let’s consider it to be 22.4 litres (convenient as this happens to make 1 mole under STP) the mole fractions are going to be the same in relative terms as the percentage of each gas – therefore the pressure change with temperature will be the same.

You're right. I was thinking the units for 'n' were g/mol. The molar mass doesn't influence the PV=nRT equation. I've always thought the answer to this question were in the properties of n2, but I think the reason is negative properties of o2:

The balance of the volume of air is mostly oxygen, which is bad for your tires. Oxygen promotes breakdown of the rubber, so your tires should last longer with pure nitrogen. Furthermore, nitrogen molecules migrate through rubber more slowly than oxygen, so your tires should lose pressure more slowly. pop. mechanics

I am definitely not a chem. engr. so I don't know why n2 would migrate more slowly, shouldn't they be smaller molecules after all?

[jezzwa]

Oxygen promotes breakdown of the rubber, so your tires should last longer with pure nitrogen.

this is because Nitrogen has a strong triple bond holding the two molecules 2gether, so N2 is quite unreactive unlike oxygen.

[Guest]

Nitrogen retains less heat than oxygen and therefore allows tires to run cooler.

I read this in another article that prompted me to look up some data:

N2 Gaseous phase

* Gas density (1.013 bar at boiling point) : 4.614 kg/m3
* Gas density (1.013 bar and 15 °C (59 °F)) : 1.185 kg/m3
* Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9997
* Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 0.967
* Specific volume (1.013 bar and 21 °C (70 °F)) : 0.862 m3/kg
* Heat capacity at constant pressure (Cp) (1.013 bar and 25 °C (77 °F)) : 0.029 kJ/(mol.K)
* Heat capacity at constant volume (Cv) (1.013 bar and 25 °C (77 °F)) : 0.02 kJ/(mol.K)
* Ratio of specific heats (Gamma:Cp/Cv) (1.013 bar and 25 °C (77 °F)) : 1.403846
* Viscosity (1.013 bar and 0 °C (32 °F)) : 0.0001657 Poise
* Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 24 mW/(m.K)


o2 Gaseous phase

* Gas density (1.013 bar at boiling point) : 4.475 kg/m3
* Gas density (1.013 bar and 15 °C (59 °F)) : 1.354 kg/m3
* Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9994
* Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 1.105
* Specific volume (1.013 bar and 21 °C (70 °F)) : 0.755 m3/kg
* Heat capacity at constant pressure (Cp) (1 bar and 25 °C (77 °F)) : 0.029 kJ/(mol.K)
* Heat capacity at constant volume (Cv) (1 bar and 25 °C (77 °F)) : 0.021 kJ/(mol.K)
* Ratio of specific heats (Gamma:Cp/Cv) (1 bar and 25 °C (77 °F)) : 1.393365
* Viscosity (1.013 bar and 0 °C (32 °F)) : 0.0001909 Poise
* Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 24.24 mW/(m.K)

*specific gravity/volume - 13% difference
*viscosity - 13% difference

I think the answer may lay in the specific gravity. Crucially, n2 is
less than water, but o2 is greater. Could this account for migration?

Heat capacity does not show a significant difference.

[guest]

I may be wrong but in F1 they do everything for speed and performance. If I was in a race team it would not be long before i suggested hydrogen as a fill gas. It is lighter. Yes it is flamable but by the time the tires are on fire you have other things to worry about. If flamability was important then helium would be the next logical suggestion.

Just becaust the cylinder is nitrogen labeled/coloured does not mean that is what is in it.

Yes it is not a lot of weight saved but it is the most important weight saved - unsprung rotating mass.

I have not read the rule book on this matter but based on how much they spend to reduce this most important mass already I would suggest they are filling with the lightest legal filler.

[jezzwa]

If I was in a race team it would not be long before i suggested hydrogen as a fill gas.

hydrogen atoms are so small that they would leak out of the valve so tyre pressure would drop as the race progressed

[guest]

And if you think F1 can't seal hydrogen in a tire for two hours - more power to you.