| Re: [GMCnet] Tire size and blow outs [message #96699] |
Sat, 21 August 2010 15:41  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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All this talk of covalent bonding had better stop :-)
Over the years I've heard the claims and wondered, and wondered. There seem to
be two claims - one is that tires with nitrogen will leak less and the other is
that nitrogen-filled tires will have less pressure change wtih temperature.
Like Emery said below, using molecular size as the reason just doesn't seem to
hold water. However, there are a couple of explanations that make sense to me.
One is that the oxygen in air can react (oxidize) with the rubber over time.
Some some of the oxygen is then removed from the air in the tire, reducing the
pressure. Does that effect have enough clout to noticeably reduce the pressure?
I'm not sure, but I doubt it - the Chemists will have to chime in.
The other explanation that I think makes more sense is most compressed air
contains water vapor. In fact, unless descacent (sp) dryers are used the
compressed air will be close to saturated with water vapor. Take air from an 80
psi tank and expand it into a 60 psi tire and the amount of water vapor is still
substantial. Water vapor has a molecular weight of about half that of air (does
it have a smaller radius, Emery?) and so could migrate through the rubber. The
second thing caused by water vapor is that at low temperatures it can condense,
and this would significantly lower the pressure. The biggest advantage of
nitrogen might be that it is relatively dry.
No, I don't use Nitrogen, but do have some of those blue valve stem caps:-)
Gary Casey
> Nitrogen is a smaller molecule and will not leak down as fast with butyl
> tubeless tires. Usually used in truck tires. I wouldn't pay anything extra
> for the use of it. I don't use it and can get it free.
>
> Ken O'Rourke
> Greenville, SC
Hi Ken
I think you mean "is a larger molecule" although barely.
Molecular size a bit tricky. As a quick comparison, one can use the covalent
radius defined as 1/2 the distance between two identical covalently bonding
nuclei. This is measured in picometers (1 pm= 1x 10-12 m). Nitrogen's covalent
radius is 75pm so the length of a nitrogen (N2) molecule ought to be 4 X 75pm or
300 pm. A molecule of oxygen (O2) ought to be just a shade smaller 4 X 73pm or
292pm. So an oxygen molecule is a little less than 3% smaller than a nitrogen
molecule.
Emery Stora
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