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Nitrogen in tires

Discussion in 'Wheels & Tires' started by CRKrieger, Oct 12, 2009.

    • Member

    CRKrieger

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    After looking around a bit, I discovered that I had not posted all the very good reasons not to bother with nitrogen in your tires, but I did tell several people to look for it. I apologize for that oversight. If you would like to read a thread where I did some of the research and posted it, it is here at MyE28.com. For those who don't wish to read all of our E28-style witty repartee (It does explain where some of my 'attitude' here comes from.), here is a summary:

    1) Prevention of oxidation: Some suppliers and users claim that the absence of oxygen in the tire inhibits oxidation and hardening of the rubber. While that is true, it only addresses the inside of the tire carcass, not the outside. Tires oxidize and age primarily from the outside, not the inside. If they oxidize inside, they use up the 21% oxygen in there and it is not replenished until you pump in more air (which, unless the tire was totally empty, will leave much less than 21% oxygen in the new atmosphere inside). The outside is constantly bathed in a 21% oxygen atmosphere, not to mention lots of other ugly stuff (sunlight, ozone, etc.) that will make them harden and crack. Tires degrade from the outside, not the inside. The existence of pumped-up 30-year-old TRX (or 40-year-old XZX!) spare tires is proof of that. So as an oxidation preventative, the effect is real, but inconsequential.

    2) Stability of pressure: A gas is a gas is a gas. Boyle's and Charles's Gas Laws are universally accepted as a physical/chemical fact. No gas, regardless of its atomic or molecular weight, behaves any differently inside a tire. While the mass might vary by a barely measurable amount (some gases are heavier than others), the pressure does not. For those who seriously care about weight savings, nitrogen's molecular weight is 28 amu and oxygen's is 32. However, given that air is 78% nitrogen in the first place, the average molecular weight of air is only about 28.5 amu - effectively the same as pure nitrogen. If somebody with a better engineering background than I have would like to calculate the interior volume of a typical mounted tire, I'll be happy to calculate the mass differences for a few selected gases.

    3) Dry gas: Another claim is that nitrogen, as a dry gas, makes the pressure more stable because there is no water vapor in it. This only matters when the water changes states. Frozen or liquid water (doesn't matter which it is) in the tire at normal pressures must vaporize to increase the pressure. That is only going to happen above 100ºC - but wait. That's only at normal atmospheric pressure at sea level! Tires are at least twice that pressure! So the boiling point of that water increases dramatically. Even if you could manage to get your tires hot enough to boil any water in there, it is likely to be an inconsequential amount affecting the pressure very little. Besides, there's a very simple solution to this: dry air. Anybody can put a dry air filter on a compressor line for about $10. That's a lot better than 600 times that for a dry nitrogen generator.

    4) Less migration: Nitrogen is a smaller atom than oxygen. That, too, is true. So nitrogen suppliers would have you believe that oxygen leaks out of small holes that the nitrogen can't get through. Oxygen's covalent radius (size, when bonded to another atom of oxygen) is 73 picometers. Nitrogen's is 75 pm. BFD. When you recall that a molecule of either nitrogen or oxygen is somewhat dumbell-shaped (consisting of two of those covalent radii) and that gas molecules simply bounce around anyway, any 74 pm-sized holes in the tire would block most oxygen molecules as well. If an air-filled tire lost all its oxygen, that would be only 21% of its fill. Refilling it with air would leave it at about 4.4% oxygen (21% of the 21% of gas replaced), so if you have a bunch of pesky 74 pm holes letting all your oxygen out, two air refills will get you a fairly pure nitrogen filled tire (less than 1% oxygen). Tires lose air because they leak gases, not a gas.

    5) Nitrogen is inert: Pure unadulterated BS. Nitrogen is one of the most plentiful active elements we know. Life would be impossible without it. Ever heard of 'amino acids'? The 'amine' is hydrogen and nitrogen. Ever heard of 'nitrous oxide'? Nitrogen & oxygen. Either a 'shot' of horsepower or an emissions headache (NOX). Plants cannot survive without nitrogen. High explosives are largely nitrogen compounds - like the ones that blew up the Murrah Building in Oklahoma City. The only thing nitrogen isn't is flammable ... or a lachrymator.

    6) I'm just now seeing claims that nitrogen somehow keeps the tires cooler. I'm at a complete loss to understand how anyone could believe this. I'm definitely waiting for someone to come up with even the slightest piece of evidence that it's true. Nitrogen doesn't conduct or reflect heat any differently than any other gas.

    The bottom line is that a nitrogen generator costs about $6000. If you've been dumb enough to buy one, you need to convince other people that you were smart in doing so; otherwise, you'll never amortize the thing. So those who try to sell the 'benefits' of nitrogen are very earnest (if not desperately so) in their pitches and they probably really do believe all this made-up stuff. If it's free, go right ahead. Put nitrogen in your tires. It won't hurt a thing. Just don't believe that it makes the slightest measurable difference.
    • Member

    Brian A

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    The cool thing about the BMW CCA website is a safe place to ask "dumb questions". (I certainly ask my share.)
    It is important for us to be charitable here rather than abusive. The former is the best way to encourage follow-up questions to gain clarity and ultimately a better general understanding.

    Your summary of the (non)merits of nitrogen is excellent but you have not invited debate. I too am astonished at the incredible rise in popularity of nitrogen fills. It is really weird how quickly the uptake has been for, what seems to me to be, snake oil. What's up? I am interested in a dialog with both scientists and people who may have just drunk the Nitrogen Kool-Aid. How could everyone but "you and me" have been fooled?
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    DHENRY

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    CR and Brian,
    Several interesting points to ponder, from both sides of the fence:
    The use of nitrogen in military aircraft and NASCAR racing predate the push by dealers to use nitrogen on your Wheaties.
    Strangely, many MBZ and Lexus dealers near us were early proponents..
    At Lexus, I can understand it.
    At MBZ it's curious.
    Their service folks have related tales of 'fill stations' costing several times that $ 6,000 figure.
    They charge about $ 35 for a four-tire 'purge-and-fill'.
    There's got to be some SERIOUS marketing to amortize that! :)>).
    My BMW dealer and the Lexus dealer where we got my wife's car are NOT nitrogen-fill proponents.
    What's the attraction?
    Maybe it's the chrome-and-green cloisonne valve caps?
    Thanks for a good technical analysis.
    Don
    • Member

    CRKrieger

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    I stumbled across some information on the aircraft uses not long ago. Seems that sometimes, an aircraft brake can get really really hot and it will get the wheel equally hot. Since automotive uses don't get that extreme, I didn't pay much attention to it, but apparently, the alloy wheel can catch fire. If that's the case, it makes sense not to have a gas (oxygen) that supports burning inside the wheel. Otherwise, you're looking at a certain tire explosion.

    With NASCAR, it's partially 'drinking the Kool-Aid' but probably more a matter of convenience. The teams carry nitrogen bottles around with them to power the air wrenches and it's easier to use the same bottles to fill the tires than it is to carry around a second set of bottles. That goes throughout racing. I know we did it in the late '80s with an endurance team I worked with - at least occasionally (most tires were overfilled from the mounting trailer, adjusted downward for the track, and used up in a couple of hours, so we never really filled them).
    None of which is surprising.
    I wouldn't settle for less than metal valve stems and roundel caps. :cool:

    tthomidis guest

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    My thoughts on this Nitrogen thing and more...

    I also need to be educated a bit here. I am considering tracking my 2009 M3 E93 at an event and have been seeing conflicting reports about cold tire pressure versus hot. I have also seen things regarding Nitrogen versus regular air. There was also a concern that some users posted about runflat tires versus regular tires. Lots of information to digest and comprehend:

    1. Nitrogen vs. Regular Compressed Air- It seems that there is not too much difference for daily use rather than the key point of Nitrogen being a dry gas and therefore reducing the amount of water vapor inside the tire for corrosion's sake and also pressure variability with regard to temperature under heavy load and heat conditions. For this, I wonder if you could use a simple dry air compressor similar to those used for Air Brush/Art type environments. You could easily attach a fitting and fill your tires with it. I wonder if that would produce a dryer air and give you similar benefits to Nitrogen? This is a simple sub $200 system in most cases. This is just slightly more than the normal small canister based systems usually found at Home Depot or an Auto Parts store with the vapor removal pieces already installed.

    2. My understanding is that the more you deflate to compensate for heavy friction on a race track, sometimes this might work against you and actually spike your tire pressure higher. My thoughts are to leave the car the way it is since it might not matter in the long run because I am not a professional driver and will not notice the difference anyways. Anyone have Nitrogen in their tires and know a decent cold setting to use for Front and Rear with regards to PSI for the racetrack? Would that vary if it was just regular compressed air? If so, what would those recommendations be? I have read about 28-32 PSI F/R range in most cases. Any suggestions would be great.

    3. Runflat vs. Regular. Found out the M series does not come with runflat tires according to most people that post out there. The dealer said it did when I was buying the car and I questioned them and they insisted even though I found no ZP(zero pressure) rating anywhere on the sidewall. I was curious but never really looked into it. After a little digging, I have come to the conclusion that the tires on my 2009 M3 are not run flats. They are just normal Michelin PS2's. This is great since the RFlats are alot more money to replace.

    biomimetic guest

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    Air is about 80% nitrogen. Not oxygen. Pure oxygen tires would be incredibly dangerous. As in kaboom.

    Second, no offense, that's not what inert means. It has to do with pressure changes - which is probably why it's used in airplanes.

    I think N2 is overkill unless you're going for a serious track day. I also think it's possible with the higher pressures, and differences in make of the runflats, that there's more of a difference in handling over te course of a track event or a long spirited drive. Friction coeeficents and tire wall flex being my main point. Is it the head of the driver? Maybe. If it makes the driver better even if it's in their head, does it matter?

    Would I personally bother? No.
    • Member

    CRKrieger

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    There are no "benefits to nitrogen"! That was the major point I had hoped to make. Do not spend money for nitrogen and do not bother spending money for dry air. The water vapor in your tires is negligible.
    We don't deflate to compensate for friction. In fact, we recommend you run your pressures on the high side to avoid excessive sidewall flex and to keep the tires from rolling over onto the sidewalls. Where the tread ends is where the driving part ends. If your tires are rolling past that (any veteran student or instructor can show you the scuffing between sessions), you need more pressure. Period.
    This varies with the particular vehicle and its suspension setup. It does not vary depending on the gas used.
    At a complete loss there ...

    tthomidis guest

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    Great info...

    Sorry for all the newbie comments and questions. I appreciate everyone's candid/honest responses. I guess I am just going to leave the car alone and head out to the HPDE and see what happens. I am sure they will advise me regarding the proper setup since I will register as a novice and have an instructor with me.
    • Member

    CRKrieger

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    You will probably get some preliminary setup information in your registration packet. Just starting out, your learning curve will be steep. Don't obsess over tire pressures. Set them at the highest recommended pressure on the door (glovebox?) sticker and go with it.
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    az3579

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    I cannot believe you guys are still on about this...
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    lcjhnsn

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    Amen Brother!
    • Member

    CRKrieger

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    Do you really want me to dredge up some of the threads you've taken "to infinity ... and beyond"? :cool:
    • Member

    Satch SoSoCalifortified

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    The last word on nitrogen

    I always tell people I use a special compound in my tires. . . a mixture of about 80% nitrogen, along with oxygen, CO2, and trace elements of several other gases. . .

    Somebody apologized for a newbie question. Let me second the voice of reason---was that you, Aftanas?---there ain't nobody nohow who hasn't felt stoopik on one topic or another. All of us here are happy to share our opinions, no matter how wrong they might be! And we understand the motivation behind the questions: If you're heading for your first driving school or autocross, you never want to look like the dweeb you think you are.

    As for tire pressures, you'll hear lots of opinions, most of them passionate. I ran 35 pounds (hot---well, pretty warm) at the last autocross and examined my Dunlop Drizzlers pretty closely to see the wear and roll patterns---but mostly, it was a seat-of-the pants analysis. With rally tires on asphalt, we used to take pyrometer readings across the surface of the tires: If the temperatures are the same in the middle as the outer edges, the whole tread is working. (And I swear if you haul out a pyrometer in the parking lot of your next driving school, your friends will be IM-PRESSED.)

    It's probably best to leave a couple of pounds out of your tires, so when Russ Wiles whips your butt, you can check the pressures and say, "Dang! No wonder! I was runnin' two pounds low!"
    • Member

    CRKrieger

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    Y'know, the latest cheapass test meter I bought actually has a pyrometer probe. At the time, I thought, "Hell, that's about as useless as teats on a boar hog." But now, knowing I can impress Satch with such a simple and pointless exercise, maybe not ... :D
    Been there; done that. :rolleyes:
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    jeffo

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    While it is clear that you do not use nitrogen in your tires, I believe there is some evidence that there may be a small benefit. I fill my tires from a nitrogen cylinder, not a nitrogen generator, as do most of the professional race teams that I have witnessed.
    I do this because many years ago I raced motocross with Fox "air" shocks, and had to recharge them every time I rode. Trying them with air pumps and with commercial nitrogen, the nitrogen "felt" better, and also all the sponsored guys used nitrogen. I soon purchased a small cylinder and regulator, and have had one ever since.
    In my career I have extensive experience with hydro pneumatic heave compensation systems which use either air or nitrogen at high pressures. We always recommend a nitrogen system and believe our testing has substantiated the benefit.
    For me in filling tires, I believe the nitrogen provides a small benefit in more stable pressures due to temperature. With the cylinder in my garage it makes it very easy to fill my tires before autocross events, mountain bikes, and also for normal maintenance on my cars, and motorcycles. A fill of a 50-80 cu ft cylinder costs about $12-18 and lasts me about 6 - 8 months. The convenience alone is pretty much worth it to me.

    Jeff
    95 525i
    00 Z3
    91 525i
    87 325is
    85 K100RS
    99 Suburban (normally use air in this one)

    mose121 guest

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    The real benefit of nitrogen is that tire pressures will not change nearly as much due to temperature variance. If you live in a cold climate, you'll know that your tire pressure monitor warning will come on seemingly at random after the car sits overnight. You come out to work the next morning and it's 10 degress outside. You rush to the car late for work, hit the button and "bluummmmm" ...the light comes on. Now you have to stop to check your pressures in the cold wearing your nice clean work clothes. Not to mention your late for work already.

    This scenario plays out all the time here in Pittsburgh. The temp difference overnight can sometimes be 15-20 degrees or more in the winter. That is usually enough of a change to cause air filled tires to drop enough in pressure to turn the TPM warning on. Unfortunately since 90% of current BMW owners don't even have a clue how to check their own tire pressures, they are forced to go to the dealer for assistance. This is highly inconveinent and really makes people angry. People have even tried filing lemon law for this. Nitrogen does not expand and contract like oxygen does, therefore the tire pressure is more stable during a temperature change.

    Racecars use nitrogen because a cold tire just going onto a car will rise in pressure as the tire is warmed up with use. The variance in each corners tire temp is usually pretty dramatic as all four tires do not work the same depending on track layout. A track with lots of right turns means the left front tire will be hotter than the right front, and also higher in pressure. These pressure differences will often effect the handling of the car and can cause pulling, vibrations, and such. Thus nitrogen gives you a more stable and predictible race car. In racing, it's all about reducing the amount of variables that can take you out of the race in order to be one step ahead of your competition. This is one of those unpredictible variables that they eliminate with nitrogen.

    Airplanes use nitrogen because it's cold up in the sky. The cold temp at altitude causes the oxygen in the tires to contract and thus the pressures fall. The change is gradual and takes some time to happen. The rubber from the tire helps to insulate the air inside, which means it takes time for the air to expand and contract. The change does not happen immediately. As a plane comes in to land, the time it takes to descend is not long enough for the air in the tires to heat up and expand back to normal pressure. And just like driving, landing an airplane with underinflated tires is not usually recommended. :D

    So to recap...

    1. Using nitrogen in a street car saves you the hassle of unexpected trips to the dealer when the warning goes off.

    -side note: I would expect everyone here reading this to be able to check their own pressures.

    2. more stable pressures helps to save tire life

    3. stable tire pressure maximizes fuel economy potential

    So, it could be argued that $40 for a nitrogen fill would more than pay for itself during the life of a set of tires based on fuel and tire savings. It would be interesting to know what the savings would be if a company like UPS used nitrogen in all their trucks. Honestly, I wouldn't be surprised if they do that already.
    • Member

    CRKrieger

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    With all due respect, gentlemen, you're simply displaying ignorance of the basic physical properties of gases. This is the formula for the behavior of gases when temperature (or pressure or volume) changes.

    P1V1 / T1 = P2V2 / T2

    Where P = pressure, V = volume, and T = temperature.

    In a tire, the volume essentially remains the same, over the temperature ranges we're talking about. As temperature drops, you can see that the pressure must drop, as well, to make the equation balance.

    IT DOESN'T MATTER WHAT THE GASES ARE.

    Nitrogen and oxygen and every combination of them as well as all the other gases that comprise plain old everyday air behave in exactly the same way! Nitrogen makes pressure behave no differently than if the gas was pure oxygen (which air is not). The only place at which all gases don't behave the same is when they're near 'absolute zero', which is -273ºC. At all known temperatures on - or above - the earth where tires would be used, all gases behave the same.

    Come on, guys, I'm not only a Car Guy (who has worked in racing), I've been a chemical researcher and I've taught the gas laws. If I could find any way to support any of these claims, even theoretically, I would. But there is absolutely no scientific basis for it. What you 'felt' was the same thing that 'CAI' fans 'feel'. It's the idea that you spent some scratch on this idea and it damn well better work or you look kinda' foolish for spending on it. I'll reconsider when and if I ever see some hard objective data on it. Saying a race team (or the military, or a dealership) does it because this is what they believe means nothing. Show me the numbers directly comparing a dry air fill to a nitrogen fill. All I have seen is rehashing of the same old stuff that the manufacturers of the nitrogen generators have been passing off as 'facts'. To their credit, they're doing a helluva lot better marketing job than those of us who know it's all smoke & mirrors. So now I gotta go buy some Zicam ... :D
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    Brian A

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    My favorite local tire installer recently really starched my undies by charging me $12 to fill 2 tires with nitrogen. Me bad: I didn't check the details of the estimate before agreeing to the work. I didn't notice until I got home. I was not happy.

    I too have studied more than my share of physics and chemistry and agree that basic science says there is no benefit to nitrogen. FWIW, lots of bicycle racers fill their fragile tires with massively caustic CO2 without consequence.

    ... ladies too.

    mose121 guest

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    I live in a climate that hovers b/t 10-35 degrees for about 4 months out of the year. All I can tell you is that by using nitrogen in tires we have cut down on the infamous early morning tire pressure warnings by at least 50% if not more. There has to be some reason for that. Previously we would have to top off air pressures in half of our loaner cars every morning because the warning would come on as soon as you started the car. We haven't had to do this once since putting nitrogen in all of our loaner cars. Can you explain that to me? Also, can you please explain to me how my techs can set pressures at spec in the shop which is 66 degrees, then move the car outside where it's 30 degrees and a few hours later the tire pressures are low and the warning is on in the dash? If you can answer that for me I'll burn our $6,000 nitrogen machine at Octoberfest this year. :D
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    CRKrieger

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    Can you show me the records of all tires filled with air and those filled with nitrogen and the outside temperatures for each date in question? Until you compile that kind of database, this is little more than your vague impression. You don't install parts because 'it worked once'. You use a logical diagnostic process to find enough information to identify the problem first. Here, we're lacking that defining information.

    Tell you what, though. You fill half your cars' tires with air and half with nitrogen, and record the pressures all at the same temperature (This is important! See below.). If the nitrogen generator or the air generator (compressor) heats or cools the gas, then you have a variable you have to address. To do that, I'll suggest you fill them in the middle of the day and then check to see that the pressures are equal at the end of the day. Don't add air or nitrogen to any of them because that will reintroduce the variable. Bleed the higher ones to match the lower ones and record the pressures. Then, park them all outside and wait overnight (assuming the temperature is significantly lower). The following morning, record all the pressures again. While not strictly scientific. this is a close approximation to the experiment you'd need to do several more times to get enough data to eliminate anomalous variables (like one or two tires that might leak slightly, or variability in the sensitivity of the pressure sensors or the slight reduction in temperature that bleeding off gas can produce).
    Yeah; pretty easily, in fact. Using the equation above, here's the math:

    P1=35 psi
    V1=[volume of the interior of the tire, which I assume varies less than 1% and appears on both sides of the equation in the same place, canceling out as a constant]
    T1=66ºF (292ºK - you gotta do this with absolute temperature and this is the Kelvin scale)

    P2=X psi
    V2=V1
    T2=30ºF (272ºK)

    35 psi/292º = X psi/272º

    0.1198 psi/º = X psi/272º

    272º*0.1198 psi/º = 272º*(X psi/272º)

    32.6 psi = X psi = Low tire pressure warning

    So how big is that thing? We can probably burn it out by the Motorplex where I'll be running the autocross. We don't want to be too near the Bridgestone trailer, but we will be near enough the Turn 9 bridge that the safety crew from there can come over and extinguish the fire.

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