[bowman088]

I was reading the tire pressure thread and instead of hijacking it I decided to make this one.

I have heard from a lot of people that filling your tires with nitrogen lowers the pressure change when the tire heats up, but I also heard that it doesn't do anything. As far as I know th gas in the tire has no effect on the pressure change. So if pressure change isn't the reason that nitogen is better than why do people use it?

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[sasilverbullet]

Before you get the plethora of responses you get to hear from someone that spent 4 years in the Air Force working on airplanes...



The Air Force (can't speak for the airlines) uses pure nitrogen in all their aircraft tires. Reason is the tires go from bone cold to super hot in a matter of seconds. One of the big properties the experts told me it has over air is it's DRY!!! And the nitrogen doesn't expand as much when it gets hot.



There, done...now get ready for a million different answers... <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />

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[RPM]

Not sure a million answers are necessary. Silverbullet hit the nail on the head. Skip the physics lessons (I always did) - simply put, nitrogen does not react to heat/cold cycles the way O2 does. As a result your tire pressures stay fairly constant instead of fluctuating. This is particularly key during fall and spring when outside temps can vary tremendously day to day. I have on occasion found my O2 filled tires as much as 5-7 pounds low after just a few days when temps are up and down.



Of course the solution to this with O2 is a good pressure gauge and the memory to remember to check. Nitrogen just gets you off the hook for this task. My tire guy fills with nitrogen so now I'm set for the Porsche, still checking the other cars when I remember. I probably would not spend the $40-60 that tire shops/dealerships want for a 'nitrogen plan' where they do an initial replacment fill and then refill as necessary for a period of 1 or two years.



Always check tire pressure when tires are cold.

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[flash]

the primary reason that aircraft use nitrogen is not for the above mentioned reasons (though the point is valid)



they use nitrogen because it does not support combustion - the fire rate is greatly reduced by the use of nitrogen - it is a safety thing, pure and simple



keep in mind that they check tire pressures every day, and fill them up every day



air is already 78% nitrogen - the nitrogen you get at the stations is only 95% - not a huge difference - tires still lose their pressure and require filling - are you going to go back and refill with nitrogen every time? if not, you will soon end up back at your original 78%



on a race car, where you fill your tires every event, this might make sense - on a street car it's just plain silly - unless you have a nitrogen tank at home or your local gas station, and are wiling to pay the cost, i don't get it



yes, it works - yes it reduces tire wear, pressure loss, and pressure change from heat - changing your oil every week helps increase engine life too, but who is going to do that?



here are some links further explaining the nitrogen thing:



http://blogs.consumerreports.org/cars/2007...-nitrogen-.html



http://www.getnitrogen.org/



http://www.nitrogendirect.com/N2Info.htm



http://www.straightdope.com/columns/070216.html



best solution - find the compromise pressure that makes you happy, and check and reset it often

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[gryphon]

I tried the Costco nitrogen thing on two Lexus RX400h's and my 968. All lost about 4 psi over two months. Given the inconvenience of going back to Costco to have them topped off, requiring about a one hour wait, I dumped the nitrogen and refilled them all with good old air. I can once again check and adjust my tired pressures with nothing more than pulling down my air hose from the garage ceiling.



Tom

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[Georges]

So I see that blowing up the tires by mouth has the advantage of reduce the O2 content. Haven't yet figured out the effect of the extra moisture.

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[Anchorman]

How about: wet = bad; dry = good; temperature effect on the gas volume/pressure = same (physics doesn't lie, whether you want to skip it or not).

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[raydog]

Yes Sasilverbullet, you are correct. Being an aircraft mechanic for many years, many heat and cool cycles can cause condensation. That is why dry nitrogen is used in aircraft tires... along with stable tire pressure. Flash, fixed gear aircraft also use nitrogen. Don't think combustion would be an issue when the wheels are hanging out in the air at 100+ mph

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[bowman088]

so it is basically just to get dry air in the tires and the actual gas doesn't make much of a difference since air is mostly nitrogen anyways?

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[Cloud9...68]

[quote name='bowman088' post='50348' date='Apr 3 2008, 11:55 PM']so it is basically just to get dry air in the tires and the actual gas doesn't make much of a difference since air is mostly nitrogen anyways?[/quote]

I would say that's correct. Air and nitrogen behave exactly the same as far as response to temperature and leakage, so the only thing I can think of that would be different is if nitrogen tends, for whatever reason, to be generally available in a drier form than air. But compressed air that's been through a drier should behave exactly the same way.

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[bowman088]

Okay, I just didn't think that the moisture would make a noticable difference.

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[Georges]

Some say Physics?



First I thought PV = nRT and the type of gas doesn't matter.



The type of gas must matter because the atomic weight will affect how fast T changes and the heat transfer rates from rubber to gas. That implies we should look at specific heat capacity.



Specific heat = 29.1 for Nitrogen, 29.4 for oxygen, 28.8 for hydrogen and 4.2 for water. Hey water is bad. Best result is pure oxygen at 29.4. OK if you don't mind your tires igniting at some point. Well, no they won't ignite but their life span will suffer due to oxidation.



All right, I see why water is not good to put in tires. I'll use care to fill the tires on days with low humidity. No more doing it by mouth!!



This derivation is based on high school chemistry class I attend in previous century.





<img src="/forum/images/smilies/968/closedeyes.gif" class="smilie" alt="" />

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[craigawoodman]

Interesting thread and most answers are correct. Having worked with earthmover tyres, the biggest issue is them catching fire and/or exploding (if they catch fire internally - combustible gases are given off which can ignite and explode). Nitrogen was proposed to reduce the chance of a tyre fire and explosion. THe problem is you mount the tyre onto a rim and it has air in it. So the initial volume of the tyre is pressurised at atmospheric pressure with air which contains oxygen. If you inflate with pure nitrogen - you have to get the oxygen content to be below 5% by volume. This takes five (5x) inflating and purging prior to getting close to 5% oxygen by volume. For tyres that are 55 inches wide, on a 63 inch rim and stand about 3 metres tall and weigh 4.5 tonnes - it takes a lot of nitrogen to achieve this.



Real value/benefit for car tyres - absolutely nil.



For aircraft - issue is water vapour - which will condense and freeze at -35 degrees, and therefore make the tyre out of balance (and go from zero to 350km/h very quickly). Not a good scenario.

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[Cloud9...68]

Craig,



That's why some racers use wheels with two fill valves - one to evacuate the air in the tire, and one to then fill it up with nitrogen.



I agree the benefits of nitrogen on a car, particularly a street car, are marginal at best, but I remember hearing from a fairly reputable source that since nitrogen is a significantly less oxidizing medium than air, the rate of corrosion of the wheel at the mating surface of the tire is reduced, keeping that interface smoother, resulting in a wheel/tire that will hold its pressure better. But who knows how significant of a difference this really makes?

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[BeBe]

Use Helium so your car would be lighter! <img src="/forum/images/smilies/968/biggrin.png" class="smilie" alt="" />

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[craigawoodman]

Cloud - how many really corroded rims have you seen (internally)?



For earthmover tyres - we add sealant into the tyre, so we add around 50 litres of liquid (although - technically it is a corrosion inhibitor) - but the rims and components still rust.



Most car rims are some form of alloy rather than dead mild steel - so tend to rust less than mild steel.



For the cost and hassle - hard to justify. If the tyre goes down - how do you add pressure unless you add air - which negates the benefit. If you run the tyre under-inflated, it will impact the tyre life more than putting air into it.

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[Ryan]

I'm with Greg...use Helium for less unsprung weight.

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[flash]

i seriously doubt the sealing surface is an issue. that seems to hold up for a very long time without issue. i would tend to think it holds longer than the selectively permeable membrane of the tire itself. the schrader valve leaks more than the tire seal.



i'm glad to see others also realizing what a bunch on nonsense nitrogen in tires really is. this is one where a little science is twisted to sell a product. with air already being 78% nitrogen, it's almost impossible to see any benefit from pure nitrogen filled tires in a street car, and still very hard to see it on all but the highest performance race cars. on a long race nascar car or indy car, maybe. controlling the expansion would be a benefit, but it is so minimal that unless you were really out there at the edge of the envelope, and had all the onboard data logging tools to record it, there are so many other variables that would outweigh any effect that you couldn't tell if it did anything or not

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[craigawoodman]

Lets not forget that if you release all of the Nitrogen from an earthmover tyre (at 110psi cold) - you can create a low oxygen environment in the workshop/area where you are discharging it. It is expensive to get and in most applications add zero benefit over clean, dry air.



As per previous post - unless you mount the tyre in a nitrogen filled container - it is bloody difficult to get the oxygen content below 5% anyway. Even if you constantly purge the tyre through two valves - it would be interesting to see how much purging is required to obtain a relatively pure nitrogen environment.



Nitrogen, Oxygen, Carbon Dioxide, etc all are assumed to behave like nobel gasses anyway thermodynamically - so there should be no difference - as per above - add water vapour and that is another equation all together. Add oil through an on-line oiler and you add another bad issue.

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[tamathumper]

If you're going to fill your tires with Helium, you'd better hurry up,... <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />



http://www.njherald.com/story/18435987/h...ning-issue

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