On Wed, 30 Jan 2008 17:45:51 GMT, "Leo Lichtman"
<
[email protected]> may have said:
>
>"John Everett" The web site makes the claim that oxygen migrates through
>tire rubber
>> four times faster than nitrogen. Starting at first inflation nitrogen
>> comprises 78% of the gas filling a tire. Each time you top up a tire
>> thereafter you're adding mostly nitrogen (78%) to a mixture becoming
>> more and more nitrogen rich. One can easily see that with age the gas
>> in one's tires asymptotically approaches 100% nitrogen. ;-)
>^^^^^^^^^^^^^^^^^^^^^^
>By the same token, a person who buys tires at Costco, for example, starts
>out with almost pure nitrogen in the tire. Typically, that person will top
>up the tires with air for the rest of the tire life. So the nitrogen
>concentration decreases, and asymptotically approaches the asyptote in your
>paragraph.
>
>Has anyone ever kept and reported tire inflation data comparing the top-up
>frequency requirements for tires filled with air vs. nitrogen? I doubt it.
Actually, several truck fleet operators I used to visit maintained
such logs when evaluating which of a selection of tires to purchase
based on samples obtained. It was quite important to them to minimize
the air loss rate, and if a given tire's compound proved more porous
in actual use than another, the information would make a big
difference in their decision. In point of fact, they found that air
loss rates for various brands varied by a considerable range, and
*all* of them lost air at a rate which indicated that mere
permeability of gases was secondary to other mechanisms in the
results. These tracking efforts were undertaken about 20 years ago; I
haven't been in touch with the folks involved much since, but I ran
into one of them a couple of months back. I asked about his
experience with the new tires, and whether the nitrogen fills were
making a difference. He said that they had, as is now recommended,
swapped over to N2 fills for their trucks when replacing the tire or
performing a service that required deflation in the shop, but they
hadn't seen any difference *at all* in the pressure loss rate after
the change.
The only change he had seen was that a few less tires were
exploding...but it hadn't stopped. Given the usual scenario that
obtains which leads to such an explosion, that's hardly surprising.
It is only a risk when a tire is run underinflated, and the drivers
are largely pretty good about checking inflation frequently (as in, at
least once per day). If a noticeable inflation loss short of a flat
is present, it is very likely due to a still-embedded puncture. The
driver will almost always reinflate with onboard compressed air in
that case, and proceed on his run; repair will happen after the run.
(If the tire is nearly flat, and he feels it's already unsafe to
continue, he'll call for road service to come to his location and
patch or replace the tire...but as he's not making any money when he's
standing still, the drivers tend to avoid doing that if at all
possible.) If the puncture leaks enough, the tire will get a quick
refll of compressed air several times, bringing the O2 content up to
external levels...and if the leakage rate is enough to case pressure
sag to the pouint that the tire overheats badly before the next stop,
an explosion can still happen.
Car and bike tires simply can't get hot enough for this to be a
problem.
If N2 loss rates through a tire were low enough that using 100% N2 was
going to make a big difference, a tire that was originally inflated to
100 psi would sag to about 80 psi and then lose pressure much more
gradually. My experience, on my van and cars, is that pressure loss
is far more linear well past the 20% loss point, so I have no
confidence that permeability of the tire compounds is the majority of
the cause of pressure loss. In particular, the rear tires on my van,
which require 80psi as the running pressure, regularly sag about 5 to
6psi per week until they reach 45 to 50 psi, after which it slows to
about 4 to 5 psi per week. (The van spends several months of each
year unused during the period when I'm not on the road to
conventions.)
The whole "Use Pure Nitrogen To Inflate Your Tires" thing for purposes
other than explosion reduction is based entirely on theoretical
considerations, not real world results. In theory, theory and
practice should mirror one another; in practice, this is seldom the
case.
Oh, lovely. My case fan just started smoking. Later!
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