Nitrogen in Car tyres

[Andy Champ]

On 08/10/2010 09:08, Halmyre wrote: .

Twice actually. H2 molecules vs He atoms. :P

Andy

[Halmyre]

Damn you and your extensive knowledge of chemistry.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Halmyre saying something like:

Well, just be careful when docking it.

[Andy Dingley]

This is Usenet. We have plentiful reserves of monomoronic hydrogen over in alt.energy.free.browns-gas

[Reentrant]

But the buoyancy of hydrogen is only 8% greater than helium

[Andy Dingley]

Buoyancy here has more to do with the weight of what you're displacing (considerably more) than the H /He difference. Even vacuum isn't hugely better.

[Tim Streater]

Amazing how many folks think that H/He/hot air gives you lift, innit, when its actually gravity.

[John]

Since the atomic weight of Nitrogen is 7 and Oxygen is 8 and they both have 2 atoms per molecule, the oxygen molecule is actually slightly larger than the nitrogen one. Consequently, it will take longer to diffuse through the tyre wall.

John

[dennis]

Atomic and/or molecular mass alone do not determine size.

O2 has a double bond. N2 has a triple bond. Hence the size will be different as (apparently, its been decades since I did any real chemistry) the triple bond is about 3% shorter than the double bond.

[John]

Sorry, I forgot that one. It means that N2 will diffuse even faster than air then so we are actually better off using air than N2 in our tyres. Now if you want REAL fuel savings, go to solid tyres, preferably solid metal. Just look at how ecconomical trains are.

John

[Richard Russell]

Do you dismiss what it says here:

are links to some of the original research documents, including this from University of Bologna:

[Tim Streater]

Is that why they need such a huge annual subsidy, then?

Some years ago I needed to go to Glasgow from Cambridge for a meeting. Train was around £300, compared to £50 or so by whoever. To say nothing of the extra time it would have taken.

I'm inclined to think that these plane/train "cost" comparisons don't take the cost of the infrastructure and its maintenance into account. Cost of the HS1 line again? Six billyun, was it? That's £100M per mile.

[Alan]

In message , Dave Liquorice wrote

Judging by the poor state of the air machines at my local petrol station(s) I'm not sure that anyone using them would have tyres pressurised to within 5psi.

[Andy Champ]

who make the equipment to inflate tyres (or tires!) with nitrogen.

get his doctorate? Who funded it?

Andy

[Fred]

But which handbook pressure did they use? I think mine has one pressure for light loading, another pressure for car full of family and luggage, and then another pressure still for fuel economy.

[Dave Liquorice]

One would hope the one for the loading of the vehicle when they checked it. The +/- 5 psi may well cover the different pressures anyway, though I haven't checked what the range is for my car. It rarely runs "loaded" so don't know what the loaded pressures are off the top of my head.

[John]

Apparently a cylinder of Nitrogen costs about £17 for a refill. Enough to do about 50 tyres. Better than running a compressor to give free air away.

[Richard Russell]

Well, given a choice between a learned thesis and the prognostications of the denizens of UseNet, I know which I believe! Can you challenge the calculations in the thesis which conclude that a tyre filled with N2 loses pressure at only half the rate of a tyre filled with air?

Richard.

[Dave Osborne]

Yes, usenet every time ;-P

Can you challenge

The thesis asserts that the internal pressure in the tyre falls by 3% a month. If you start at 32 p.s.i., you would get the following pressure drop:

Months Pressure

1 31.0 2 30.1 3 29.2 4 29.3 5 27.5 6 26.7

This analysis neglects pressure loss through an imperfectly-sealed rim and through the valve, which could only make the figures worse.

In the real world, *my* tyres never lose pressure at this rate, unless there is a slow puncture. Secondly, the student should have done the experiment and presented his results to back-up his theory and his schoolboy mathematics.

Further, the author goes on to assert (section 3.5) that

"[...] tyres have a service life dependent on a number of factors, including:

- Deterioration of the lining due to oxidation;

- Fabrication defects;

- The quality of the road surface;

- Formation of cracks on the outer surface of the casing due to ozone and normal oxidation."

No mention of the fact that the service life of a tyre is over when the tread has worn down then? No mention of cracks in the external rubber due to UV degredation? No mention of the variation in wear characteristics between different tyre compounds; no mention of driving style, no mention of heat ageing?

He goes on to talk a bout a US study in the 1960's that "evaluate[d] the effect of oxidation in the inner tube. All six tests clearly demonstrated that using de-oxygenated air as the inflation medium had a positive effect on the life and wear of the tyre. The results demonstrated that if the oxygen concentration inside the tyre is reduced by 6%, the life of the private car tyres was increased by 22 %, and that there was an evident reduction in wear. From this we can calculate that using de-oxygenated (completely oxygen-free) air for inflation would increase average tyre [something - life, presumably] by 25 % in the case of private cars and 40-50% for trucks."

He makes no citation of the study, and takes no pains to explain the causation and correlation of the supposed miracle improvement in the life of the tyre. If the assertion were true, we would all have been filling our tyres with nitrogen for the past 40 years. The government would probably have made it illegal by now to fill tyres with compressed air.

How many tyres have you ever had to change because they were "worn from the inside"? Of course, he might be allowing for tyres to be re-treaded, but he doesn't say so.

Any reduction in wear on the outside (all other things being equal) can only be as a result of keeping the tyres inflated at the right pressure. As far as I can tell, the only well established scientific fact is that under- or over-inflated tyres wear more rapidly that correctly inflated tyres.

Finally, the inclusion of irrelevant bullshit marketing bollocks from tyre manufacturers at the end only serves to reinforce the fact that this thesis is severely flawed and makes a poor effort at scientific rigour and no effort at all to provide experimental evidence of even the most rudimentary kind to support his theorems.

[Richard Russell]

Near the beginning of the published extract (section 3.2) it states "To treat the problem analytically, we must make a few simplifying assumptions". Then, before the section you are referring to, it states "This calculation requires us to make further simplifying assumptions in addition to those introduced in paragraph 3.2". Clearly the absolute rates of pressure loss are dependent on all those assumptions, and may not be at all accurate, but that is no reason to doubt the conclusion that the rate of loss using N2 is about half what it is with air.

Richard.