Text 5286, 91 rader
Skriven 2005-01-21 07:38:52 av Robert Bashe (2:2448/44)
   Kommentar till text 5282 av Roy Witt (1:10/22)
Ärende: Tire inflation
======================
Roy Witt wrote to Robert Bashe on Thursday January 20 2005 at 15:28:

RW> Tires rot from being exposed to the elements.

To be honest, I haven't seen a "rotted" tire since I was a kid. The
antioxidants they put in the rubber are pretty effective nowadays. And the
problem with tires is not water anyway, since any tire is exposed to a lot of
water in its lifetime and must withstand that, but with things like traces of
ozone in the atmosphere and any free radicals running around... they can lead
to accelerated ageing of a tire surface and the small cracks you sometimes see
on old tires, but unless the tire is very old indeed the cracks aren't deep.

Incidently, I used to work for Grace (Dewey and Almy), where the plant made
things including rubber masterbatches for tire manufacturers, so I have some
idea of what goes into such materials.

RW> Compressed air has water in it, that's the nature of the beast. And
RW> why they put drain cocks on compressor tanks. The air is usually mixed
RW> with the oil from the crankcase and crap that makes it through the
RW> filters, not to mention what's carried in the water that's naturally
RW> in air...

That's why air compressors have traps and filters in the lines. That kind of
stuff doesn't get into tires, at least not in appreciable amounts. Incidently,
the main problem is compressor oil, but that's removed in a trap.

I've translated stuff for a company that makes compressed air treaters,
incidently, and you can get very pure compressed air indeed, but such a degree
of purity is not really necessary for things like tires. It's mainly used for
stuff like pneumatic controls in industry or compressed air supplies for
hospitals.

Oh, and remember another thing: even if you want to put nitrogen into a tire,
you have to compress it...

RW> Have the scientist visit a tire shop and have the workers show them
RW> the insides of tires when they remove them from the wheel.

Curiously enough, I've done just that, in fact quite a number of times over the
years. I'm a scientist with papers to prove it, and also have worked on cars
enough to know what tires look like, inside and out. The rubber dust you often
see on the inside when a tire is removed from the rim is just due to the
abrasion caused by the tire constantly flexing under load.

RW> The bead will be the worst off, as it gets moisture from both sides.

Yeah, 99.99% from outside and 0.01% from inside <g>. No, if there's any problem
with the bead it's that it isn't completely tight and capillary force brings
water from the outside between the tire and the bead.

Compressed air is drier than any atmospheric air and certainly isn't causing
any moisture-related problems. The air behind the compressor is hot and when it
cools, moisture condenses and can be removed. Then when the tire is inflated,
the air in the line leading to the tire expands since the tire is never
inflated at the pressure the compressor can produce, and the expansion cools
the air, again condensing any residual moisture which can then be removed with
a trap in the line. Remember that cold air contains less moisture than warm
air.

RW>>> A really crazy theory...

RB>> Not at all. Compressed air is great for making a small fire a large
RB>> one. More oxygen in a given volume.

RW> Flat tires are usually the cause of race car crashes. No air in those
RW> tires to contribute to a fire.  They worry more over the possibility
RW> of spilled fuel contributing to a fire.

So then it's six one way, half a dozen the other whether you use compressed air
or compressed nitrogen in that particular case...

RW>>> ... as the fire is fed by oxygen in the air surrounding it. If the
RW>>> tire were filled with sand, the tire would still burn.

RB>> Right. But not at the same rate.

RW> Bob, once compressed air is released to the atmosphere, it no longer
RW> is a threat. That happens as soon as there's a hole in the tire to let
RW> it out. It's not like compressed air is going to stay compressed while
RW> a fire is burning around it.

No, that's true... so in that case the argument, if valid at all, would have to
relate to fires inside a tire, perhaps caused by internal friction when the
tire has to flex for lengthy periods at high speed. Frankly, I don't know...
this is merely one of the reasons I've seen for people using nitrogen instead
of compressed air in racing car tires.

Cheers, Bob

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