Is it really better to fill your tyres with Nitrogen rather than compressed air?

In the workshop we are seeing and creasing number of cars turning up with blue or green valve caps, which are designed to show tyre shops and garages that the tyres are filled with Nitrogen, not the usual compressed air, and hopefully tell the business that someone has made the choice to fill their tyres with nitrogen and they should top them up with the same gas.

So I thought it was about time I talked a little about Nitrogen filling of tyres and lets take a look at what advantages there may be.

A few years ago we had a representative from a large workshop equipment manufacturer visit us to talk to us about the potential of selling us a Nitrogen compressor for the workshop for inflating tyres.

As the chap in charge of JMG Porsche, I do believe in buying the very best equipment, spending thousands on air conditioning machines, air conditioning diagnostic tools, on the car brake lathes, state of the art alignment equipment and not to mention the thousands it costs to have the latest Porsche factory and aftermarket tools and diagnostic computers. So we are not shy of buying kit, especially if it has a real benefit to the workshop or our customers, or both.

However, after polling our customers back then, it seemed no one was bothered about having nitrogen filled tyres, it had not become an interesting item on the forums, in car magazines and no one was talking about it on TV.. It did not have the buzz that nitrogen in tyres has today, it was strictly a top level motorsport thing.

So we decided to not bother with buying the Nitrogen machine and stay with compressed air for tyre filling and topping up.

Why is Nitrogen used in top levels of motorsport and what is the theory that it is better in the tyres of a road car or track car?

Nitrogen molecules are larger than Oxygen molecules (even though Oxygen molecules technically are heavier) which means that Nitrogen molecules are less likely to leak from the tyre compared to oxygen molecules, meaning in theory that you need to have less frequent top ups.

Nitrogen also expands and contracts less with temperature change. During a race, or very hard driving, tyres heat up, on a race track far more than you could ever do on the road, but none the less, due to that temperature change, any gas will expand as it heats up, and contract as it cools, as it expands it will increase the pressure in the tyre, and as it cools it will decrease the pressure in the tyre.

With any car, there will be a pressure at which the tyre has the very best handling and traction combination, so in an ideal world, you want the tyre to always be at as close to the pressure you have chosen as possible.

On a race track, this means that when the car is sat on the grid, the tyre pressure will be far less than in lap 5.. This is one reason why F1 teams put tyre heating jackets onto their cars prior to a race and take them off at the very last moment before the start, to try to make the tyre temperatures as close as possible at the starting grid, as they will be at during the race.. In racing, consistency is important.

Also a car tyre at a specific pressure will also help the car return the very best fuel economy, unfortunately this is not always the best pressure which will return the best handling or traction, but it does mean that keeping your tyres at consistent pressures, come snow or sunshine may help maintain a constant fuel economy. But also during a road trip, once the tyres have been running at motorway speeds, they may warm up by a few degrees and that will cause a pressure increase.

More recently, the filling of road and track car tyres with nitrogen has grown in popularity, especially in the performance car world. Many more tyre workshops now have nitrogen machines which have also come down in price since we were first offered one years ago.

The motoring press, both printed, the internet and TV based have talked about its advantages, and we were thinking of joining the crowd and buying a machine, so it was very much on my mind when Tiff Needel of the TV show 5^th Gear decided to specifically test the benefits of nitrogen in tyres, and the results were interesting, but the shows presenters did not elaborate on the results or analyse why their findings were not in line with common expectations.. Yes, that's right, they found that nitrogen tyres performed worse than air filled ones in many tests, including lap times, stopping distances and more.. But why??

I am not one to just follow the crowd and just buy something like a nitrogen machine because everyone else has one. But also as a full time engineer and part time scientist, I always refuse to accept results unless I am sure the testing was performed in a scientific way and unless the results have some kind of scientific explanation.. So 5^th Gears results bounced around in the back of my mind for a couple of months.

In May of this year (2013) I suffered a knee injury at the Nurburgring 24 hour race weekend, which involved an ambulance ride, a German hospital, transportation back to the UK and eventually an operation, with some complications which found me not only going stir crazy with limited mobility, but also with some time on my hands.

During this time I continued with a couple of automotive research projects, but also began to think and later dissect the findings of 5^th Gear, which were still bothering me.

All technicians should understand science, I believe one of the most powerful tools at my disposal at the workshop is a good understanding of thermodynamics, Physics, chemistry as well as engineering. One very useful part of this understanding, are the gas laws of science and engineering, which calculations helped in my understanding of nitrogen versus free air for the inflation and top up of tyres.

The first important detail to mention is that free air, the air that surrounds us, and is compressed and used to inflate tyres, is already 78.09% Nitrogen, 20.94% Oxygen, 0.93% Argon, 0.03% Carbon Dioxide and 0.006% other gasses... As long that is, that the air is dry.. We will assume all good workshops have driers installed on their air compressors, but in the free air around us, sometimes it can be up to 3% water vapour and only 97% made up of the gasses just mentioned... But for our calculations, we will assume that the air is dry.

So, we might assume that when your tyre is inflated with a traditional compressor, it has 78.09% nitrogen ,and when it is inflated with nitrogen, it is filled with 100% nitrogen... However, the light bulb moment is that this is very much wrong.

Once I started to think about this, it became very obvious that there is a big difference between how a Formula-1 car is inflated, and a local tyre shop will inflate your tyre with nitrogen.. An F1 team fill the tyres with the wheel and tyre in a vacuum cabinet, not sat on a workshop floor.

The air that surrounds us is pressurised, its actually pressurised to 14.5 psi, what is also called a “Bar” of pressure. When you inflate a tyre to 30 psi, you are actually inflating it to 30 psi above atmospheric pressure, which is an absolute pressure of 44.5 psi or 3.06 Bar.

So, when a car tyre is installed and inflated in a workshop, it already contains 14.5 psi of air, with only 78.09% of that gas being nitrogen.

Lets just say that a typical car tyre, when not inflated, has a volume of 15 litres... before that tyre is inflated in a workshop, that tyre will contain 11.7 Litres of nitrogen, 3.14 Litres of oxygen and the rest will be other gasses such as Carbon dioxide, Argon and other gases.

When that tyre is inflated to 38 psi, that tyre will receive another 54 Litres of gas, which if it is filled with nitrogen, will be 100% nitrogen. But overall, that tyre will only be filled with 95% nitrogen, because of the other gases, such as oxygen, which were already in the tyre before it was filled. (yes, 95% does take into account the nitrogen in free air)

In contrast, this is an improvement over a tyre which is filled with just free air, which will have a final percentage of 78.09% Nitrogen and the remaining being other gasses, but only a difference of 17%, but still an improvement.. So why the poor results that were found by 5^th Gear??

Lets go back to a scientific fact, no tire is completely air tight, gas molecules can even pass through the structure of the tyre itself (a problem with having a tyre flexible is that it can not use a material which is completely air tight), not to mention through its sealing bead, which will never be perfect... The other scientific fact is that oxygen and some other molecules are smaller, and so will leak out of a tyre before the larger nitrogen molecules.

What this means is that if you inflate a tyre with free air from a traditional compressor, and then leave it on the car for a few months, you will find it needs to be topped up.. when that tyre is at only 78.09% of its original pressure, it will be filled with pure 100% nitrogen , as the oxygen and other gasses will have leaked out!

Hang on.. does that mean that a tyre filled up months ago with normal air, has a higher percentage of nitrogen in it than a tyre inflated yesterday with pure nitrogen?? You bet-ya!

But also, a tyre inflated yesterday will loose that 5% of inpurities over the coming weeks or months as it looses its pressure, and if topped up will also be full of 100% pure nitrogen.

The big thing is, do you worry about if your tyre has 79% nitrogen (fresh air), compared to 95% nitrogen (filled with pure nitrogen) or would you rather have the 95% to 100% of pure nitrogen you would get from just having your tyres filled with fresh air and topping them up on a regular basis with normal fresh air. (once a few weeks or months had passed and you have had to top them up)

What it really boils down to is 95% nitrogren, versus 100% nitrogen.. the 95% being what you have when you fill up your tyres from scractch with nitrogen and also what you would have after a few months of having your tyres filled and topped up with a traditional air supply, or 100% what you would have a few months after a from scratch nitrogen fill up and allowed time for the impurities to filter out and you have continued to top up with pure nitrogen.

What I can tell you, is the difference between 95% and 100% nitrogen is probably something that a F1 driver who can achieve lap times consistently 0.1 of a second within each other, would not be able to notice on the track, so why would you do it for the road?

So the one question is, why did 5^th Gear find that nitrogen filled tyres performed significantly worse than air filled ones?

From what I can make out from the video footage, it seems that the car they used first ran tests with normal air in the tyres, probably air that had been in there for months, or even since the car was new. So of course, if you have been reading everything before this point in the story, you will know that this may mean that rather than running on air filled tyres, it would have been running on approaching 100% nitrogen filled tyres (depending on how recently the tyres had been filled and topped up).

For their second test, using nitrogen filled tyres, they then let the air out of the tyre, fresh air will have flooded in, and then filled it with nitrogen, which would have meant that the tyre had at best 95% nitrogen.

The conclusion from this is that 5^th Gears experiment may have been flawed, but even they said there was very little, although noticeable difference between the two sets of results, but perhaps through the assumption that an old tyre will be pure normal air and one filled with nitrogen will contain only nitrogen their mistake actually reversed the results.

My own conclusion is that the garage will not be getting a nitrogen machine. We have a deal with a local tyre company that if we swing past on a test drive we can top up tyres with nitrogen whenever we want, during a service for those customers with nitrogen valve caps, but in the real world, it really is a waste of time and money...

A nitrogen machine would be an expense which someone would have to pay for, and as always in business, its the customer who carries the costs.

Likewise, next time you're having your tyres changed, if the company offer you nitrogen filling instead of normal compressed air for only £5 per tyre, save your money and put it towards something that will really improve your car.