On Fri, 30 Nov 2007 11:21:27 -0800, SMS ???• ?
<
[email protected]> may have said:
>The permeability is only one advantage in automobile tires, but it's
>very real.
You say that, and then go on to this:
>Remember that a typical driver will likely _never_ check
>their tire pressure during the life of their tires, which could be
>several years.
You fail to state who *will* check the pressure; clearly, the tires
will fail due to underinflation early if the pressure isn't checked in
that period, or else the issue of pressure loss isn't a real concern.
Since overwhelming common experience it that it *is* a concern,
somebody must be checking and inflating the tires. If your assertion
is correct (which I don't buy, but will allow for the sake of
discussion) then it's being done by someone transparently to the
customer. Since a lot of oil change service places also check tire
pressure, I suspect that this would be the primary source of inflation
maintenance for the owners that you describe. 100% N2 won't be used
for those pressure corrections. (Costco does not provide oil change
service at their local stores. According to the counter man at the
one nearest me, very few of their customers come back just for tire
pressure checks...and the tires they check often require some pressure
adjustment.)
>The other advantage for automobile tires is that without the presence of
>water vapor or oxygen the expansion due to heat will be less,
Okay, one last time: Oxygen and nitrogen have virtually identical
thermal expansion rates, and those rates are so small as to make no
difference at all. In any event, tire *UNDERINFLATION* is what
destroys tires, not minor levels of overinflation. Water vapor's
partial pressure in the mix is too low, even with a saturated
atmosphere at the compressor inlet* or with liquid present in the
tire, to have an important effect on the issue at safe operating
temperature ranges. Your assertion is still just as absurd as it has
been all along. (See below for a lengthier explanation of why water
vapor doesn't matter.)
>and rust
>will be inhibited.
Automotive wheels have their interiors painted, and rust from that end
isn't a problem. In nearly 40 years of dealing with car repair
issues, I have never seen an original equipment tubeless-tire wheel
that rusted from the inside out. It a steel wheel rusts, it's more
likely to be due to road salt or environmental salt spray, and it
starts at the outside edge (where the paint gets scraped off by a
variety of things) and works *in*; they simply do not rust from the
inside out. Ergo, rust from water (liquid or vapor) inside the tire
is not an issue.
>I still probably wouldn't pay much, if anything, for nitrogen, but it's
>just one advantage of buying tires at Costco. Costco uses nitrogen for
>their own benefit in terms of warranty-related costs, they aren't
>charging for the nitrogen.
You keep making this assertion of cause without a shred of real
evidence that this is really the case. Such evidence would be actual
figures showing that Costco had achieved a statistically significant
reduction in warranty expense as a result of the procedural change,
which given the relatively short time that they've been doing it, is
unlikely to be possible to prove. Even if someone at a Costco tire
center has made the assertion that what you state reflects the
*expectation* behind their policy, Costco's accountants almost
certainly wouldn't have been able to measure it in the time that has
elapsed. The reality is more likely to be that the policy was adopted
as a result of a decision taken solely on the basis of what someone
perceived to be its value in the marketing of their services.
If, over a period of several years, Costco can show a statistically
significant system-wide reduction in warranty expense which can be
directly attributed to N2 fills, then the claim can be verified.
Until then, its worth is unproven.
* In the absolute worst case real-world scenario, water vapor could
comprise no more than 4% of the air coming into a compressor. In most
instances, it's around 1%. It is interesting to note that the act of
compressing the air and then cooling it in the tank causes some of the
moisture to condense out. Ergo, unless the compressed air source is
from a compressor whose tank is so full of water that liquid is being
driven out of the supply valve, the air coming out of the compressor
will have less water vapor than the air that goes in, for any
environment where the water vapor is at a high enough level that it
might be able to condense inside the wheel to begin with. If the
water vapor partial pressure is below that level in the environment,
then the vapor is essentially completely irrelevant. But suppose that
the air coming from the compressor is at that real-world-impossible
4%-water-vapor point, and that for some unlikely reason none of it
condensed in the compressor's tank; the delivered vapor's *complete*
condensation later could therefore cause no more than a 4% drop in
pressure in a tire inflated with that mixture. 4% of 32psi is less
than 0.5psi. Most tire pressure guages are not accurate enough to
measure that difference.
Most tires are operated below their maximum rate pressure, and small
amounts of accidental overinflation generally will not even take them
to their rated pressure. For tires that are inflated cold to their
max rated pressure, however, operation beyond it as a result of
thermally-induced pressure change in a tire inflated with air, even
when moisture is present inside the tire, will not cause a problem;
the tire engineers have already taken this possibility into account.
The effects of water vapor on tire inflation pressure in service are
of absolutely no importance in automotive tires.
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