Date: Mon, 30 Aug 2010 08:19:32 -0700
Reply-To: Don Hanson <dhanson928@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Don Hanson <dhanson928@GMAIL.COM>
Subject: Was: question: Sorry, but ...
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On Mon, Aug 30, 2010 at 4:57 AM, Mike S <mikes@flatsurface.com> wrote:
> At 01:52 AM 8/30/2010, Don Hanson wrote...
>
> minimize heat-generated pressure increase. As a tire comes up to racing
>> temp with atmosphere inflated inside, they can gain up to 7psi but with
>> nitrogen, they will only gain a few lbs....So when you filled the tires
>> with
>> nitrogen, you could start a race with almost the correct pressures, rather
>> than running a lap or so underinflated while the temperature caused the
>> pressure to increase.
>>
>
> BS. Air is 78% nitrogen to begin with. All gasses follow Gay-Lussac's Law
> extremely closely, unless at temperature extremes (think absolute zero and
> plasma heat). There are 2 legitimate reasons to use N2 in tires - the
> military uses it in some aircraft to eliminate a fire hazard from the oxygen
> in air, and race teams use it because a bottle is cheap, and can be used to
> fill tires without needing either a compressor or electricity (or they
> believe the BS the N2 salesman told them). A third reason to use N2 is so
> tire stores can make additional profit.
>
You may be right about the specific gas, Nitrogen, not really heating less
than atmosphere in tires....But Nitrogen-filled tires do gain less pressure
as they heat up. The numbers I cited in my original post were
collected from data over 5yrs of racing. The ~7lb pressure increase, that
is the maximum increase I recorded, at Portland International Raceway in
some 12.5 x 17" medium -compound Goodyear Eagle racing tires. Hoosiers (a
DOT tire, but for racing) in 350 x17 show a similar pressure increase as
they go from ambient temp to racetrack heat. This is proven fact.
No racecar can turn a competative laptime with under or over-inflated
tires.
In real life, I think the reasoning behind using compressed nitrogen to
inflate racing tires has a couple of components.
It is easy to carry a couple of nitrogen cylinders and the gas is
relativly cheap. My cylinders would last through about 1/2 a season,
perhaps about 15 sets of tires and 200 track sessions.
Second reason: Compressed nitrogen is dry. It has little moisture and it
may be dried as it's compressed into the cylinders or maybe it doesn't
absorb water, I don't know for sure.
Water vapor (humidity) is present in atmosphere. Water boils, changing into
steam at 212f. As the water vapor inside a tire turns into steam (race tire
inside temps regularly exceed 212f) it expands dramatically. You know,
like a steam engine?
So racers use nitrogen because it is dry, and it give a uniform rate of
expansion as it heats up. Any gas, including normal air, will expand a
little, as it goes from cold to hot. That is where you get a couple of lbs
increase in every tire as heats to operating temp and why the reccomended
tire pressures are marked "cold pressure"..But when the tire temp goes past
the boiling point you get another dramatic increase in internal pressure as
the water vapor from that compressed air turns into steam...more or less,
depending on what the relative humidity of the compressed air in the tire
might be.
My carefully recorded tire data shows that a nitrogen-inflated tire's
pressure increased an average of 4psi less than one filled from an air
compressor. It only took me a few races to learn that inflating my race
tires with compressed air left me a few seconds per lap off the pace in the
early lap of any race....or off the track in a spin, trying to keep up with
cars whos tires were properly inflated with nitrogen or running with
pre-warmed tires.
Don Hanson
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