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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Tire pressures to be requied for cars
Date: Thu, 07 Apr 2005 22:41:12 -0400
Message-ID: <>

On Thu, 7 Apr 2005 15:40:59 -0600, "Bob Hatch" <>

>"Neon John" <> wrote in message
>> On Thu, 7 Apr 2005 11:41:17 -0600, "Bob Hatch" <>
>> wrote:
>>>> I know how to use a tire gauge just fine
>>> So do I. How do you do it at 65 MPH?
>> Don't know how you do it where you're from but here in the Sunny South
>> we have enough sense to pull over and stop before gauging our tires.
>So in your part of the Sunny South what sort of device do you use to assure
>that your tires never, ever, under any conditions pick up a nail or some
>other piece of material that would cause the tire to lose air and over heat?

I use my brain, of course.  The first use of my brain is to realize
that worrying about never, ever, under any condition type of problems
is foolish since one can never figure out ALL those abnormal
conditions.  Once I realize that, I prepare for the most likely events
(spare tire, gauge, pyrometer, spare lug bolts and studs, etc) and
simply ignore the fanciful possibilities.

The second use of my brain is to realize that I stop at least every
hour to hour and a half to stretch my legs, pee, get gas and/or get a
snack.  While I'm stretching my legs I walk around the rig to look and
sniff.  As anyone who's ever actually had a roadside tire defect
knows, the warmer tire emits an odor long before anything drastic

 If I see or smell anything suspicious then I reach behind the seat
for ye olde tire gauge and ye olde infrared pyrometer.  A quick check
of the temps and pressures tells me what I need to know.  I can count
on one hand with fingers left over the number of times I've actually
had to do that.  I'd say it's been at least 2 years since I've had to
add air to any of our RV tires.

My walk-around look-and-sniff test enables me to detect other
incipient problems such as overheating fanbelts, leaking antifreeze,
oil leaking hitting the exhaust, transmission overheating and a bunch
of other things that no idiot gauge could possibly detect.

I would have thought this to be patently obvious to anyone who drives
over the road but I guess not.

>When one of the duals goes flat the other tire on the same side is

That, of course, depends on the rig weight, the tire class and the
wheel size.  In my particular case, the un-flat tire handles the rig
just fine as long as I don't over-do it speed-wise.

>When one of the duals gets under inflated by as little as 20%
>the tire starts to experience damage. When the tire that is carrying the
>bulk of the load is in fact overloaded, will overheat and will cause damage
>to the tire. It may not show within minutes, but the damage is done

Really?  So you mean to say that when I aim my pyrometer at the tire
and see normal temperatures, some little demon is inside the tire
doing "damage" anyway?  Wow, I would have never guessed.  Guess I need
to squirt some demoncide into the tires, huh?

I guess that as an experimentalist, my approach is a bit different
than most.  When I have a flat on the dual axle, rather than
panicking, running around waving my arms in the air or screaming for
road service, ruining the rest of the day and raising my blood
pressure in the process, I stop and evaluate the new system

I look at my log to see what weight is on the axle and do a little
mental figuring. I see that the tire isn't badly overloaded so I
decide to experiment a little.  I drive for a mile or two and check
the tire temperatures (both flat and inflated.)  Only a little above
normal so drive another few miles and check again.  Speed up a little
and check.  Speed up a little more and check again.  I have three
points so I can sketch a graph and project what the tire will do at
various speeds.  I pick a speed that will keep the tire tolerably cool
and proceed.

The result of this little experiment was that I finished my trip
without interruption, hassle or expense and without worrying.  I could
then have my tire repaired at my friendly local dealer that gives me
good tire prices and who doesn't turn the tire monkeys loose with the
air wrench to gall or twist off my wheel studs.  My experiment added
maybe 30 minutes to the trip.

I guess I'm just different.  I suppose the normal thing to do would
have been to panic, call road service, pay side-of-the-road ripoff
prices for the tire, get my blood pressure up and spoil the rest of
the day.  I think I'll remain not normal...


PS:  I highly recommend having an optical pyrometer in the vehicle.
Mine are those rat shack models made by Raytek that cost about $30 and
are quite accurate.  I put one in each of my vehicles. I've gotten in
the habit of shooting my tires when I stop for gas as part of the
walk-around.  One tire significantly hotter than the rest is probably
low on air.

It's also handy for scanning the radiator (for clogged tubes), coolant
hoses (blockages and internal swelling cause cool spots), the heater
box (AC air leakage causes cold spots), the battery (weak cells will
be hotter than the rest) and any number of other things on the rig.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Tire pressures to be requied for cars
Date: Thu, 07 Apr 2005 14:29:54 -0400
Message-ID: <>

On Thu, 7 Apr 2005 11:04:22 -0600, "Bob Hatch" <>

>"stuckinthemud" <> wrote in message
>> But not full size vans?  Isn't that odd?
>> Nate
>No, what is odd is that people are not equipping their vehicles, (TT's,
>fivers, MH, toads, MDT's, etc) with aftermarket devices. They are available.
>They work. They could save lives and property damage, but only a small
>percentage of RVers will take the time and money to be safe. That is odd.

If my grandmother was still alive she'd repeat her well-worn response
to this kind of stuff:  "I wonder how the h*ll I managed to live my
life without the federal government?"

I can just imagine the day when there'll be gauges to tell you when to
pee and take a dump!  NOT!

Why I don't want those things:

I know how to use a tire gauge just fine
I don't want another system to break
I don't want to waste my attention on false alarms.
I have better things to spend my money on.
My vehicles' handling degrades long before the tire is low enough to

Frankly, I'm amazed at people so mentally inert that they need such
gadgets to wake them up to a low tire condition.  I'm not sure that a
fatality resulting from such inertness would not be a good thing.
Cleansing the gene pool and all that.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Inflation pressure (was Re: Where are you getting your air?)
Date: Mon, 26 Mar 2007 19:12:47 -0400
Message-ID: <>

On Mon, 26 Mar 2007 11:22:00 -0700, "Kevin W. Miller"
<> wrote:

>Lon VanOstran wrote:
>> After 8 blocks your tires are no longer cold. You are supposed to
>> check and inflate them cold.
>> Lon
>How, exactly, do you define "cold"?

Could I be so off the wall as to suggest Doing The Math?

We know from Charles Law that
for a constant volume of an ideal gas (the tire volume is close enough
to constant and air is close enough to ideal) that the absolute
pressure varies directly in proportion to the absolute temperature.
That is, as the temperature rises so does the pressure.

First, let's see what we know.

Zero deg F is about 460 deg Rankin (absolute).
70 degrees is 460+70 = 530 deg R
100 degrees is 460+100 = 560 deg R.

zero psig is 14.7 psi absolute at sea level and standard barometer.
75 psi (just to pick a number) is 75+14.7 = 89.7 psia.

If we use 70 deg as our reference then the pressure at any other
temperature will be:

new pressure = reference pressure * (new temperatureDeg R/530deg R)

89.7 psia * (560 deg R/530 deg R) = 94.7 psia or 80 psig.

We see that when the temperature changes from 70 degrees to 100
degrees, the tire pressure will change 5 psi, from 75 to 80 psi or
about 6%.

I happen to know because I checked with an IR pyrometer and kept
records that the tires on my semi truck ran around 100 deg F when the
ambient was about 70 and the tires were properly inflated. Temperature
on a semi tire is a very sensitive indicator of underinflation.

For the anal retentives like Lon, let's see what happens during a
short drive to the C-store for air.  Let's say the tires heat up 10
deg during that trip.

70 degrees is 460+70 = 530 deg R
80 degrees is 460+80 = 540 deg R
75 psi is 75+14.7 = 89.7 psia.

89.7 psia * (50 deg R/530 deg R) = 91 psia or 76.5 psig.

1.5 psi change.  Reckon Lon could reliably and repeatably read 1.5
psig on his tire gauge?  Reckon it matters?  To state the obvious,
this same pressure change occurs when the ambient temperature goes
from 70 to 80 deg or when the sun warms the tires.

I wonder if the anal retentive types have a chart of temperature vs
pressure?  And pressure vs altitude.  After all, at 5000 ft the
atmospheric pressure is about 2.5 psi lower than at sea level.  Better be sure to stop and
let out that extra 2.5 psi :-)

Now I know that for some it's more fun to argue and sling insults but
the math provides the simple and direct answer.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Inflation pressure (was Re: Where are you getting your air?)
Date: Mon, 26 Mar 2007 21:15:07 -0400
Message-ID: <>

Just plug your numbers into the equation and do the math.  Your
message isn't clear but if your nominal pressure is 75 psi then the
example I ran gives the numbers you need.

I'm not a tire expert nor do I play one on the internet but from my
observations with both the semi truck and my MH, all a little extra
pressure does is maybe cause the center to wear first and give a
little rougher ride.  A little too little pressure can result in quick
failure, especially with AZ-like temperatures.

Fully loaded semi truck tires are the most sensitive tires I've ever
observed. The company spec (mfr's spec I think) was 110 on the steers
and 100 on everything else.  At 90 psi (cold, nominal 70 deg) a tire
would be heating significantly after a few miles.  Running for some
distance at 80 PSI would cause tread separation to start.  I'm judging
from trailers that I picked up that had 80 or less PSI in the tires
and were showing signs of tread separation.

My MH is a little more tolerant.  With the tires and wheels it came
with even the slightest underinflation would result in a blowout.  I
blew 5 in about as many years before I bit the bullet and replaced the
tires and wheels with something more appropriate to the load.

Based on that experience I run my tires a little over-inflated.  So
far on the MH I've not seen signs of abnormal center wear.


On Tue, 27 Mar 2007 00:32:47 GMT, "Anne Watson"
<> wrote:

>OK John read your entire message which I will not repeat.  How about in S.
>Arizona where the tires on the east side are at 100 already and the tires on
>the west   in the shade  are  at 70.
>My rule of thumb is  east tires 80 degress, tires on the west 75.  All bets
>are off driving down the road.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Duran Pressure Pro saves the day
Date: Thu, 26 Jul 2007 19:48:45 -0400
Message-ID: <>

On Thu, 26 Jul 2007 09:23:49 -0700, Don Bradner <> wrote:

>On Wed, 25 Jul 2007 17:01:33 -0400, Neon John <> wrote:
>>If you (or anyone else) wants to calculate the hot pressure precisely, use this gas
>>law formula
>>Pabsnew = Pabsold * (NewTempR/OldTempR)
>>Pabs = the absolute pressure - add 14.7 psi to the gauge reading
>>TempR = absolute temperature on the Rankin scale - add 461 to the Fahrenheit reading.
>>Old temp would be 70 deg + 461 = 531
>>Old absolute pressure would be 120 (in your case) + 14.7 = 134.7
>>For 90 deg F,
>>135 * (551/531) = 140 psi
>>Note that the temperature is the actual tire temperature.
>>I made up a table using Excel and used it daily along with an infrared pyrometer to
>>check my semi's tires.
>And you found that it actually worked? I'm aware of the rationale of
>the formula, but I've found that the actual results are far, far
>different than it would predict. The reason why is fairly obvious:
>Gay-Lussac's law requires a *fixed* volume. In an elastic container,
>the law fails. Expansion of the volume as pressure increases results
>in a lower slope for the rate of pressure increase.
>I do not see any way to properly calculate such a rate increase; I'm
>much happier seeing what actually happens.

Sure it works.  The change in volume is negligible.  The change in absolute pressure
is tiny so even if the carcass of the tire stretched (high pressure tires don't, as a
practical matter) it would affect the volume enough to matter.  If it did turn out to
matter then there are some fairly well known equations for computing the effect of an
elastic container.  My office-mate at TMI used that principle to develop the
procedure to measure the (somewhat mythical) hydrogen bubble in the reactor by
injecting a known volume of water into the system and measuring the pressure

As to another objection raised concerning the air temperature vs the tire
temperature, almost by definition the air surrounded by a container at a given
temperature is at that temperature once the temperature is stable.  The only issue is
how fast the tire surface cools outside vs inside.

My testing (including inserting a thin thermocouple through the valve stem on one of
my RV's tires and again on my trailer) indicates that this is not a problem if the
temperature is shot soon after stopping.  My testing also shows that the temperature
of the inside of the rim closely approximates the air temperature IFF there hasn't
been much brake activity to heat the wheel.  For the purposes of using an infrared
pyrometer, the wheel is less reliable because of the changing emissivity with dirt
buildup than the sidewall of the tire is.

That your toad tire monitors alarm when the temperature drops bothers me.  That
indicates that it isn't temperature compensated.  About the only sensor technology
cheap enough AND low power enough for that application is thermal conductivity or
maybe MMIC silicon.  Both types are highly temperature-dependent.  I just wonder how
accurate those "hot" readings are?

Just about every semi trailer that I picked up during my driving adventure had low
tire pressures.  The company provided no incentive to check tires and no penalty for
not doing so, so most drivers didn't.  I did NOT want to have to wait for the tires
to cool say, in Laredo in the summer, so I developed this procedure.

BTW, I'm curious as to how high the air pressure goes in your rig.  Both the trucks
that I had developed between 130 and 140 psi.  Slow to fill a tire to 120 psi, to be
sure, but with a locking chuck, that wasn't too much of a problem.

I made up a medusa with a 50 ft hose that connected to the red gladhand on the
tractor, equipped with 4 chucks that would let me fill 4 tires (one side of the
tractor drives or tandem) at the same time AND quickly equalize the pressures.  I
could fully fill the air system, connect up the chucks, press the emergency brake
knob to start filling, kill the engine and when the hissing stopped, the pressure
would be about right.  Bleeding off a little was then easy - pop the emergency brake
a few times until the gauge read the right pressure.

I found it to be easier on my back to attach the chucks to the tires, let the
pressures equalize and read the gauge than it was to gauge each tire individually. If
the tires needed air, hook up the air hose, turn on the valve and go make a cup of
coffee :-)

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