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Why not Helium in tires?


JayW

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I have noticed a lot of tire shops selling nitrogen as a filler, but am not convinced the benefits are significant. There was a recent episode of "Mythbusters" (a great show by the way) where they tested to see if a raft filled with Helium would fly. The answer is no, but all of the rafts were a lot lighter than when filled with room air.

 

So why not use this property of helium to reduce the unsprung weight on motorcycle and car wheels? I know BMW has put a lot of R&D into developing light wheels, and this seems a simple and easy way to shave off a few more ounces.

 

Jay

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St0nkingByte

Did you see the Mythbusters where they filled footballs with helium? Helium just doesn't generate much lift at all. In a motorcycle tire you would probably shave just an ounce or two off the wheel weight with helium.

 

Pure (or nearly pure) nitrogen is used in tires because nitrogen molecules are larger than oxygen (and others) and thus leach out of the tires much slower than the regular air mix. Historically this is used in fleets where a small percentage increase in tire life and/or mpg can mean alot of money. For the regular consumer its a nicety because it means maybe you don't need to check your tire pressure quite as often.

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From my kart racing days, I remember using nitrogen mainly because it doesn't react to temperature changes as quickly as air does. Nitrogen was more stable.

 

And as in most forms of motorsport, tire pressure was critical for performance.

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Helium would be a poor choice, for the same reason that nitrogen is a good choice - molecular size. Helium molecules are much smaller than nitrogen molecules, and would leak out of your tire relatively quickly. You no doubt have seen what happens to helium-filled rubber balloons the day after a birthday party.

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I find that carrying a helium tank around tends to offset the difference. But if you don't, you never know how much "regular" air is in the hose at the single-hose filling stations. I'm not going to pay 10% more for diluted helium thank you very much.

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Pat_Da_Geeeze_Donahue

The nitrogen fill for tires is more temperature stable only because it's DRY. Moisture in the air is what causes the expansion and contraction as temps go up and down respectively.

 

The helium choice is already mentioned due to molecular size.

 

Pat

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So why not use this property of helium to reduce the unsprung weight on motorcycle and car wheels? I know BMW has put a lot of R&D into developing light wheels, and this seems a simple and easy way to shave off a few more ounces.

 

Two reasons:

 

1. It would leak out in no time flat. Helium is a single atom (He), unlike Nitrogen or any other normal gases that are always paired (N2, O2, etc). This means that helium ATOMS are much smaller than nitrogen or other gas MOLECULES. The result is that helium leaks through nearly all attempts to contain it (other than metal containers).

 

2. The difference in mass would be utterly insignificant compared to the wheels and tires. Don't forget that what matters is not "unspring weight" (there is no such thing), but "unspring mass". Helium may be lighter than air, but it does not have negative mass!

 

Bob.

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The nitrogen fill for tires is more temperature stable only because it's DRY. Moisture in the air is what causes the expansion and contraction as temps go up and down respectively.

 

Sorry! You just failed your physics 101 exam! grin.gif

 

All gases expand and contract in direct porportion to absolute temperature. Water vapour is no different than nitrogen or oxygen or helium or whatever.

 

For example, if you put 36psi of ANY gas in your tires at 0 degrees C (32 F, or 273 degrees Kelvin), then warm the tires up to 10 degrees C (50 F, or 283 degrees Kelvin), the tire pressure will increase to.....

 

283/273 x 36psi = 37.32psi

 

Increase the temperature to 20 degrees C, and that original 36psi (at 0 degrees C) is now 38.64psi!

 

It makes no difference whatsoever WHAT gas you inflate your tires with (as long as the pressure/temperature does not cause the gas to condense to a liquid), the above pressure change will occur. Mind you, as I mentioned in an earlier post, helium would be a dumb choice since it leaks through materials like rubber that other gases cannot get through.

 

Bob.

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ShovelStrokeEd

Thanks, Bob. You saved me a bunch of typing.

 

Just for some further information, the cross sectional area of a Nitrogen molecule is 16.2 angstrom^2, Oxygen is 14.1 angstrom^2. It is doubtful that there will be a significant difference between the two in terms of permeation through rubber when you consider that O2 is only 21% of air, 78% being N2 anyway. 1 Angstrom is 1.0x10^-7 mm. Pretty darn small.

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I'm confused. Maybe someone can enlighten me...

 

The atomic number of nitrogen is 7, so a single atom of nitrogen is composed of 7 protons, 7 neutrons, and (in a non-isotope form) 7 electrons. Its atomic mass - the total number of protons and neutrons - is 14. Oxygen is the eighth element, so it is composed of 8 of each particle per atom (on average.) Its atomic mass is 16 (approximately.)

 

Why do people keep saying that nitrogen molecules are bigger than oxygen molecules? If nitrogen atoms are less massive than oxygen atoms, wouldn't nitrogen molecules be less massive? What am I missing? Is it a bond-angle thing? Or is it all just bovine digestive effluent?

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Thanks, Bob. You saved me a bunch of typing.

 

Just for some further information, the cross sectional area of a Nitrogen molecule is 16.2 angstrom^2, Oxygen is 14.1 angstrom^2. It is doubtful that there will be a significant difference between the two in terms of permeation through rubber when you consider that O2 is only 21% of air, 78% being N2 anyway. 1 Angstrom is 1.0x10^-7 mm. Pretty darn small.

Indeed this is small. What is interesting is that the size of a water vapour molecule is somewhat smaller than that of N2 or O2. This is because it consists of one large oxygen atom and 2 much smaller hydrogen atoms. N2 or O2, by comparison, are 2 "large" atoms bonded together. The end result is that water vapour can diffuse through things like rubber tires (both into and out of the tire depending on the partial pressure of water vapour on either side) significantly easier and faster than the "air" (i.e. nitrogen or oxygen).

 

Just a useless fact-of-the-day grin.gif

 

Bob.

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I'm confused. Maybe someone can enlighten me...

 

The atomic number of nitrogen is 7, so a single atom of nitrogen is composed of 7 protons, 7 neutrons, and (in a non-isotope form) 7 electrons. Its atomic mass - the total number of protons and neutrons - is 14. Oxygen is the eighth element, so it is composed of 8 of each particle per atom (on average.) Its atomic mass is 16 (approximately.)

 

Why do people keep saying that nitrogen molecules are bigger than oxygen molecules? If nitrogen atoms are less massive than oxygen atoms, wouldn't nitrogen molecules be less massive? What am I missing? Is it a bond-angle thing? Or is it all just bovine digestive effluent?

 

The size of the neucleus has nothing to do with the size of the atom itself. This is the same thing as saying that the size of the solar system has nothing to do with the diameter of the sun. The size of an atom is determined by the size of the electron "cloud" swirling around it, and this is thousands of times larger than the diameter of the neucleus.

 

Nitrogen ATOMS are in fact slightly smaller than oxygen ATOMS, but not by much. But in the real world, this is irrelivant. What is relevant, is how big are their respective MOLECULES. The size of these molecules has a lot to do with how tightly the covalent bonds hold them together, and that has a lot to do with the chemical properties of each type of atom.

 

Curiously, EVERYTHING we experience about any substance, whether its appearance, its chemical properties, its "solidness", its mechanical properties, its susceptibiity to magnetic fields, its interaction with light, or ANYTHING AT ALL (except matters relating to radiation, and mass) are all the result of "Quantum Electrodynamics", which is a fancy term for the interaction between electrons and photons. The neucleus has little to do with what we experience in the everyday world.

 

Bob.

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ShovelStrokeEd

Simple, really, if anything about this discussion can be deemed simple. You are confusing mass and size. They don't relate directly as things like the electron cloud and the fact the molecules are comprised of more than one atom enter into it.

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So why not use this property of helium to reduce the unsprung weight on motorcycle and car wheels? I know BMW has put a lot of R&D into developing light wheels, and this seems a simple and easy way to shave off a few more ounces.

 

.

 

Jay

 

Two reasons:

 

1. It would leak out in no time flat. Helium is a single atom (He), unlike Nitrogen or any other normal gases that are always paired (N2, O2, etc). This means that helium ATOMS are much smaller than nitrogen or other gas MOLECULES. The result is that helium leaks through nearly all attempts to contain it (other than metal containers).

 

2. The difference in mass would be utterly insignificant compared to the wheels and tires. Don't forget that what matters is not "unspring weight" (there is no such thing), but "unspring mass". Helium may be lighter than air, but it does not have negative mass!

 

Bob.

 

That makes a lot of sense, Bob. I knew there had to be a good explanation. Do you use N2 in your tires? I haven't bothered since room air is mostly N2 already, but I might if it were as cheap and easy to access as the stuff we breathe.

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ShovelStrokeEd

I can't speak for Bob but I do use it. The humidity here in Florida makes using a shop air compressor problematic. I bought a large bottle of Nitrogen at my local welding supply for not a lot of money, use a singe stage regulator to go from the bottle 2000 PSI or so down to 150 PSI and I use it for my tires as well as air tools, with an oiler. Of course, as much as I am on the road, I wind up using my little 12v compressor about half the time anyway. blush.gif

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Sorry for the name mix-up. I guess I don't use N2 because I don't have ready access to it, and regular air has worked fine since the first pneumatic tire was invented. I check my pressures regularly too. My Helium question was one of interest and curiosity, rather than something I was actually considering using.

 

Jay

 

Jay

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I can't speak for Bob but I do use it. The humidity here in Florida makes using a shop air compressor problematic. I bought a large bottle of Nitrogen at my local welding supply for not a lot of money, use a singe stage regulator to go from the bottle 2000 PSI or so down to 150 PSI and I use it for my tires as well as air tools, with an oiler. Of course, as much as I am on the road, I wind up using my little 12v compressor about half the time anyway. blush.gif

Actually, I need to get a source of N2 as well, but for a different reason, namely to repressurize my Works rear shock from time to time. At $30 a crack at the local bike shop, for a couple minutes of work and 2 cents worth of N2, I want to do it myself. It is annoying that I disassemble the shock, clean it out and reassemble it, and refill it with oil myself, but have to pay $30 for some guy to blow it up with N2 again!

 

Bob.

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ShovelStrokeEd

Your local welding supply would be a good source. No need to go with Ultra High Purity stuff, just specify dry Nitrogen.

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Joe Frickin' Friday
Nitrogen ATOMS are in fact slightly smaller than oxygen ATOMS,

 

Actually, since we're splitting hairs, Nitrogen atoms are slightly larger than oxygen atoms (1.4 angstroms for N versus ~1.3 for O), despite having a lower A#. It's the whole thang with electron shells and periodic-table behavior: as you go from left to right across the table, the atomic radius decreases (as you stuff more and more electrons into the outer electron shell), then the atomic radius takes a jump as you start over at the left end of the next row down in the table.

 

What is relevant, is how big are their respective MOLECULES. The size of these molecules has a lot to do with how tightly the covalent bonds hold them together, and that has a lot to do with the chemical properties of each type of atom.

 

Funny, in my chemistry textbook there is a table for diatomic bond length: I see that N2 has a bond length of 1.10 angstroms, and O2 has a bond length of 1.21 angstroms.

 

So if nitrogen atoms are slightly smaller than oxygen atoms; and nitrogen molecules are slightly smaller than oxygen molecules; then why does nitrogen diffuse through a tire carcass more slowly than oxygen? Or is this yet another myth being perpetuated?

 

From what I read many moons ago, dry nitrogen is beneficial for trucks and airplanes for a couple of reasons:

 

1. no moisture. For trucks, this means your steel wheels will be able to last for a zillion miles. For airplanes, this means that after an extended cruise at 30K feet and sub-zero temps, there will be no condensation inside the tire to cause imbalance/vibration when they spin up with 130+ MPH upon landing. Not an issue for bikes, which have aluminum alloy rims and don't experience such wide temperature swings.

 

2. no oxygen. With air at 100+ psi, the partial pressure of oxygen would be pretty high. This would accelerate corrosion of steel wheels and breakdown of the tire carcass. Using pure nitrogen takes the O2 out of the picture, again prolonging wheel and tire life. Not an issue for bikes, which (again) have aluminum rims, tires that get changed every 10K miles, and air at only ~40 psi.

 

Regarding helium: as an extremely rough estimate, the total two-tire volume is about 29 liters. Nitrogen is about 3.75 grams per liter (at 42 psi), helium is about 05358 grams per liter (at 42 psi). Filling tires with He instead of N2 would give a net reduction in vehicle mass of 95 grams - about 3.3 ounces.

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ShovelStrokeEd

Or is this yet another myth being perpetuated?

 

Yup. More likely, any free O2 running around is combining with the carbon in the tires. Ozone, another component found in air, is probably the bigger culprit there and will, over time, deteriorate the rubber. Again, not a big factor on motorcycle tires as for me, useful life of a rear tire is about 4 months, fronts about 6. Not much opportunity for breakdown.

 

Ain't splittin' hairs fun though? clap.gif

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Joe Frickin' Friday

Lotsa junk out there on N2 tire fills, including some serious BS from what most laypeople might regard as respected journalistic sources:

 

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

 

http://www.sptimes.com/2005/09/28/Tampabay/Nitrogen_in_your_tire.shtml

 

http://www.treehugger.com/files/2005/10/inflate_your_ti.php

 

http://www.moderntiredealer.com/t_inside.cfm?action=art_det&storyID=1207

 

http://www.truckstoptravelplaza.com/2000/n1/tirewheel.html

 

http://www.branick.com/nitrogen_faq.php

 

Here's an extremely long discussion (even by BMWST's standards) about nitrogen tire fills:

 

http://www.eng-tips.com/viewthread.cfm?qid=120996&page=1

 

Pay attention to the posts by Pisgahchemist; he's got a rock-solid grasp of the physics/chemistry/thermodynamics behind it all, and he's unerringly precise in his explanations. thumbsup.gif

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Actually, since we're splitting hairs, Nitrogen atoms are slightly larger than oxygen atoms (1.4 angstroms for N versus ~1.3 for O), despite having a lower A#.

Interesting. After I made the original off-the-cuff statement that N2 atoms were smaller than O2 atoms, based on having one more electron, I started to think about it and realized that I may have been wrong (and obviously I am).

 

What may explain Mitch's point that as one goes left to right on the periodic table, the mean diameter of the atom (i.e. the electron cloud diameter) decreases, may be that the in any given row, the added electrons end up in the same outer shell. This would imply the diameter remains the same. But the increasing number of protons in the neucleus implies that the greater total positive charge of the neucleus may tend to pull ALL the electrons to tighter orbits.

 

I'd ask Richard Feymann to chime in on this, but alas, he's dead! grin.gif

 

Bob.

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Lotsa junk out there on N2 tire fills, including some serious BS from what most laypeople might regard as respected journalistic sources:

You're right; there is some good science here that is mixed up with some excellent Cow Exhaust.

 

One point of interest is that if oxygen diffuses through a tire as dramatically fast as some are claiming (an exaggerated claim in my opinion since there isn't all that much difference in size between N2 and O2 molecules), there would be an interesing result.

 

Each time your tires leak down a pound or two, most of the leakage would be O2 leakage (if the claims are to be believed). This means that there would be a greater percentage of N2 left in the tire. After several cycles of refilling and leakdown, the tire will have to end up with almost pure N2 in it!!

 

This then means that if true, after one first fills a tire with air, the initial leakage rate will slowly decrease (after several cycles of topping up the tire's air) to a much lower value as the percentage of N2 in the tire gradually increases to nearly 100% N2.

 

The point is that I do not see this happening in reality, which tends to strongly imply that there is NOT a significant difference in diffusion rates between N2 and O2 as some of the literature claims.

 

Bob.

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ShovelStrokeEd

You're right; there is some good science here that is mixed up with some excellent Cow Exhaust.

 

OMG, are you bringing Methane into the discussion as well? grin.gif

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You're right; there is some good science here that is mixed up with some excellent Cow Exhaust.

 

OMG, are you bringing Methane into the discussion as well? grin.gif

Hey! That's a great idea! Methane as a tire inflation gas! Apparently is is used to inflate cows, so why not tires?

 

Bob.

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I've got to go check my cold fusion reactor.

Seriously, and not a word of a lie: I am taking fourth year quantum physics right now, and my prof is a nuclear physicist. Last week he said that he is not convinced that cold fusion is entirely cow waste.

 

Back on subject - Nitrogen in a bottle is dry, cheap, portable and may be helpful on the track.

 

Density of helium is 1/7 that of air. Density of air at 30 psi and room temperature is 6 oz per cubic foot. There is maybe a cubic foot of air in a pair of tires, so there are several ounces to be saved. In bulk, helium costs a couple of cents per cubic foot. Helium is non-reactive, and will not oxidise the interior of the tire.

 

Proof that not all information is useful. dopeslap.gif

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Seriously, and not a word of a lie: I am taking fourth year quantum physics right now, and my prof is a nuclear physicist. Last week he said that he is not convinced that cold fusion is entirely cow waste.

He's not the only one. It is apparently clear that there is SOMETHING going on that is not well understood. It is unfortunate that in the hysteria that followed the original "proof" that this was not fusion, everyone has dropped this like a hot potato.

 

Helium is non-reactive, and will not oxidise the interior of the tire.

That is rather academic, for two reasons:

 

1. The tire will not hold helium for long, given helium's tendency to leak where other gases do not. This is why helium is used for leak tests.

 

2. If you fill a tire with any pure gas, you will get diffusion INWARD of those components of air that are not represented inside. As physics guy, you are no doubt familiar with the law of partial pressure.

 

Item 2 is likely to be a relatively minor effect, to be sure, but item 1 will require constant refilling of the tire as a result of helium's fast leakage rate.

 

Bob.

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Interesting that no one so far has noted that Costco fills all newly mounted tires with nitrogen......... I even got green valve stem covers with my last set of tires!

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Interesting that no one so far has noted that Costco fills all newly mounted tires with nitrogen......... I even got green valve stem covers with my last set of tires!

What's the significance of the green covers? Surely it can't be claiming that nitrogen is more environmentally friendly than pure air!

 

Actually, since the first thing Costco's customers will do when their tires a a bit low, is to head to the nearest gas staion and refil the tires with air, one has to wonder why Costco even bothers to use nitrogen!

 

Bob.

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Hey, anyone know of a place where I can fill my tires with nitrous??? dopeslap.gif

THAT'S more like it! And if you get bored, you can just take a few whiffs out of the tire and laugh yourself silly! grin.gif

 

Bob.

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Firefight911

Easy now big fellas. The last thing we want here is a horrible sequel to 'The Fast And The Furious'!!! Oh, wait, they already did that!

 

Who's going to make the bypass line from the nitrous equipped tires to the intake for that "added" boost? grin.gif

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OK...Now with all of that said....can it be explain in Laymens terms? eek.gifdopeslap.gif

 

Geez, sometimes I wish I didn't grow up in the sixty's and seventies.... thumbsup.gif

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You're right; there is some good science here that is mixed up with some excellent Cow Exhaust.

 

OMG, are you bringing Methane into the discussion as well? grin.gif

 

Hey, aren't Methane molecules even larger than N2? Why just allow all that exhaust to contribute to global warming? If we could somehow harvest that gas, get some brown valve stem caps.... tongue.gifdopeslap.gif

 

Jay

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ShovelStrokeEd

OK, Duane, here it is in a nutshell.

 

Helium - no good as it will leak out very fast.

Nitrogen - good stuff but no magic. It expands at the same rate as air and leaks at the same rate as air. Biggest advantage is it starts out dry and comes in portable bottles.

Methane, NOX, all that - we were kidding.

Plain old air - adequate to the task. Only caveat is to consider the source of the stuff. A crusty old compressor that hasn't had its tank drained in the last year is likely to have some water mixed in with the air and that is bad.

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OK, Duane, here it is in a nutshell.

 

Helium - no good as it will leak out very fast.

Nitrogen - good stuff but no magic. It expands at the same rate as air and leaks at the same rate as air. Biggest advantage is it starts out dry and comes in portable bottles.

Methane, NOX, all that - we were kidding.

Plain old air - adequate to the task. Only caveat is to consider the source of the stuff. A crusty old compressor that hasn't had its tank drained in the last year is likely to have some water mixed in with the air and that is bad.

 

thumbsup.gif Thanks!

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