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Tire Pressure Revisited

Yahoo Message Number: 2230
A month or so ago there was a lot if chatter about tire pressures. Although though I think they are critically important, I couldn't respond as I am too busy trying to retire (in 31 days, but who is counting) so we can travel full-time.

There is a high probability that tires are under inflated. We can read the tire pressure when we are at our campsite, but in many cases we can't get the high pressure required to inflate the tires at our campsite. By the time we drive to the nearest truck stop that can give us the kind of pressure we demand and read the new tire pressure, we find that our tires are hotter and the reading we got at the campsite doesn't help, right? Well, maybe the readings did mean something.
I read an article (in one of the RV magazine) that said in effect that if your cold reading were 5 pounds low when cold, the pressure would be about to 5 pounds low when your tire is hot. Using weights provided by A'Weigh We Go and the Toyo Tires Recreational Vehicle Truck Tire Warranty and Users Guide, I determined how much tire pressure I needed in a cold tire for all tires. Using the pasted spreadsheet below, I am able to come close to the pressure I need when filling a warm/hot tire (if the below comes out unintelligible, sent me a "personal" email and I will attach the spread to my return email to you).

(By the way, about a month ago someone talked about an inline air gauge and how to use Country Coach Air Compressor to fill the tires. I have looked in several stores and haven't been able to find an "inline air gauge." Can you tell me WHERE you got your inline air gauge?

Thanks,
Art

Tire Pressure - based upon A'Weigh We Go, October 2000.
Required Pressure   Left      Right
Front   114      109
Actual
Difference
Hot

Bring to

Required Pressure   Left Outside   Left Inside   Right Inside Right Outside

Rear   106   106   85   85
Actual
Difference
Hot

Bring to

Re: Tire Pressure Revisited

Reply #1 –
Yahoo Message Number: 2234
Quote from: , Art"
(By the way, about a month ago someone talked about an inline air

gauge and

Quote
how to use Country Coach Air Compressor to fill the tires. I have

looked in

Quote
several stores and haven't been able to find an "inline air
gauge." Can you

Quote
tell me WHERE you got your inline air gauge?

Art,

I bought my gauge at a Sheetz's Truck Plaza in Central Pennsylvania.
I have also seen them in Flying J's and other truck stops.

Dave G.
2000 Allure

Mechanicsburg, PA

Re: Tire Pressure Revisited

Reply #2 –
Yahoo Message Number: 2237
It makes a big difference as to the weather and its effect on the tires. I would be more inclined to either allow the tires to cool down for about an hour (maybe go and get lunch at a Flying-J) and then service the tires. Or, if the truck stop is not far away, use the pressure reading that I took in the campground as my basis and service from there.

BTW, I just uploaded to the file area the weight vs tire pressure chart for TOYO tires. I just received it from Country Coach via FAX. Copy was poor so I retyped it into an Excel worksheet. If anyone cannot read the worksheet or does not have Excel, drop me a line and I will convert it to PDF format and repost it.

Jim Hughes

Jacksonville, FL

2000 Allure 40' #30511

Re: Tire Pressure Revisited

Reply #3 –
Yahoo Message Number: 2238
Thank you Jim,

This is probably the first objective posting on this "tired" old topic.

Peter

2000 Intrigue 11066

Re: Tire Pressure Revisited

Reply #4 –
Yahoo Message Number: 2239
I just bought my 2000 Allure 11 days ago. I do carry a portable air compressor but doubt if it will fill the Toyo tires. How and where does one attach this inline air gauge??? Aren't there two air compressors (slide out and engine). Which one do you use and where would that be located? Thanks for the help.

On Wed, 03 Jul 2002 02:26:26 -0000 "dcgrazier2000" dcgrazier@...> writes:

Re: Tire Pressure Revisited

Reply #5 –
Yahoo Message Number: 2240
Jim

Thank you for the spreadsheet for TOYO tire pressure. What do the lettered footnotes(?)on the spreadsheet represent?

Thanks
Tom Royer
Allure #30858

Re: Tire Pressure Revisited

Reply #6 –
Yahoo Message Number: 2241
An excellent site for anyone wanting to get detailed info on this subject is:

http://www.toyo.com/rv_center/index_frame.html
Quote
>

There is a high probability that tires are under inflated. We can

read the

Quote
tire pressure when we are at our campsite, but in many cases we

can't get

Quote
the high pressure required to inflate the tires at our campsite.

By the

Quote
time we drive to the nearest truck stop that can give us the kind of > pressure we demand and read the new tire pressure, we find that our

tires

Quote
are hotter and the reading we got at the campsite doesn't help,

right?

Country Coach Prevost, Back in Business!!

Reply #7 –
Yahoo Message Number: 2242
It's OFFICAL!! Country Coach, that made an announcement earlier this year that it was ending its relationship with the Prevost conversion business, will continue on!! Many people felt that the last half dozen conversions completed by Country Coach (with the BuddyGregg Motorhomes interior decorators design input) have been nothing less the "Awesome"?. We were all wondering that it was to bad that right when Country Coach was getting their "act together"? with interior designs and building the best we have seen in years, they would end the bus line. With an excusive arrangement with BuddyGregg Motorhomes, the Country Coach Prevost Line will continue on!

Some great news!! See you at the next rally.

Robin McCracken
Sales

800-421-0031

Re: Tire Pressure Revisited

Reply #8 –
Yahoo Message Number: 2243
Tom

I asked the same question and here is the cut and pasted response form Kevin at Country Coach:

"The letter designation is for load rating. You can associate the letter of

the alphabet with its numeric position and multiple that by two, this will

give you the ply rating. For example, H is the eighth letter, multiplied by

two tells us the ply is sixteen. I asked why they didn't just use P, but was

told that the industry didn't want to confuse that with a Passenger tire

application. Please let me know if your need any further information.
Thanks

Kevin Kiscoan

DynoMax Chassis Service

Product Support Representative
1-800-452-8015, Extension 286"

Jim Hughes

Jacksonville, FL

2000 Allure 40' #30511

Re: Tire Pressure Revisited

Reply #9 –
Yahoo Message Number: 2244
Now here is what is from the TOYO website:

Reading the Inflation Table

Find your Tire Size on the left margin of the Table.

Determine if you need the "Single" inflation reading or "Dual" inflation reading (denoted with a "D" or "S" on the Table).

Find the corresponding PSI at the top columns to see the corresponding maximum weight capacity for that PSI. Note that every load range has a maximum rating. Do not exceed that rating, which is in parenthesis ().

Rated load capacities are listed for individual tires in a Dual or Single position. Multiply the steer axle ratings by 2 and depending on the configuration of the drive axles by 2, 4, 6 or 8.

Jim Hughes

Jacksonville, FL

2000 Allure 40' #30511

Re: Tire Pressure Revisited

Reply #10 –
Yahoo Message Number: 2245
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Oops. My cat just stepped on the key board!
Jim, During your investigation have you uncovered any "official" notes concerning temperature when filling tires?

I believe conventional wisdom says to check tire when cold. The assumption being that a hot tire will indicate a higher pressure than a cold one so filling to the prescribed pressure when cold creates a fail safe situation.
My question is this. We know tires when running will be 5 or maybe ten PSI higher than the PSI at cold.

But what is the effect of ambient temperature?
If a snowbird checks his tires cold in Upstate New York in November then drives to Florida what is the effect on tire pressure which experiences a forty degree swing?
Put another way, If the snowbird inflates to the maximum rated pressure written on the side of the tire, say 120, then heads south, what would his cold pressure be in Miami and maybe more important, what would be his running PSI at say 65 on the sunshine highway.

Will he be very over inflated?

The more I read the more confused I become.

David and Karen,

2000 Intrigue, 11062. G.O.R.

Re: Tire Pressure Revisited

Reply #11 –
Yahoo Message Number: 2246
"Jim, During your investigation have you uncovered any "official" notes concerning

temperature when filling tires? "

I'd like to jump in here with a question I have had and would like some response to:

The wheel pressure limit is stamped like 120 psi on the rim. Is this a cold pressure or a limit regardless of temperature? I suppose I could search Google but, being the 4th of July, I am lazy today.

Sincerely

George W Becker

gwb36@... 1999 Country Coach Intrigue #10700

Re: Tire Pressure Revisited

Reply #12 –
Yahoo Message Number: 2247

Tires, temperatures and the gas laws...
(No, not gasoline - more like the air we breathe)
There is an equation in physics called the General Gas Equation (references provided below for those who are interested). This equation is as follows:

Pressure1 x Volume1 Pressure2 x Volume2 =

Temperature1 Temperature2

usually abbreviated to

P1 x V1 P2 x V2

----------- = -----------
T1 T2

For a tire, the volume is fairly fixed and can therefore be dropped from the equation for the time being (because it is the same value on either side of the equation and is essentially independent of temperature and pressure, it doesn't matter what it's value is). Thus, we can write

P1 P2

---- = ---- written hereafter as P1/T1 = P2/T2 for convenience.
T1 T2

That is, assuming no significant change in volume and no significant change in the amount of air in the tire, the pressure divided by the temperature is pretty much a constant over the range of temperatures we are likely to encounter.
Talking of temperature, we have to be a little circumspect. The T1 or T2 is not measured in degrees Fahrenheit of even degrees Centigrade but in "Kelvins" in the metric system. A Kelvin is pretty much the same as one degree Centigrade except that it is based on what is called "Absolute Zero" instead of on the freezing point of water. The freezing point of water is zero degrees centigrade but is 273 degrees on the absolute scale of Kelvins. 32 degrees Fahrenheit (same as zero Centigrade) is actually 492 "Fahrenheit units" above absolute zero. (For interesting (?) facts on Absolute zero see http://www.sun.rhbnc.ac.uk/~uhap057/LTWeb/Absolute.html)

Now, remember your grade school algebra?
P1/T1 = P2/T2 is our equation and, since the two sides are equal (that's what = means!), if we do the same thing to each side, the two sides will still be equal. So, multiple each side by T2 and we get

P1 x T2/T1 = P2 x T2/T2

Again, your algebra memory tells you that any value (zero is not a value!) divided by itself is equal to 1. Thus

P1 x T2/T1 = P2 x T2/T2 can be written as P1 xT2/T1 = P2 x 1
and since any value multiplied by 1 is the same value (more grade school algebra) we can dismiss the x 1 off of the right hand side of the equation.

Now

P1 x T2/T1 = P2 so it stands to reason that P2 = P1 x T2/T1
We can now find the pressure, P2, for any temperature T2, given the original pressure and temperature P1 and T1 so long as we remember to add 460 to each of the Fahrenheit temperatures.
So, how hot do tires get? I have no objective data, but just laying a hand on a tire after a high speed run in an ambient of 80F suggests about a coffee drinking temperature which, for me, is about 120F. Those of you with monitors should have more realistic numbers. What does this mean in theory? If a tire is inflated to show 100PSI at 0F (460 absolute), then at 120F (580 absolute) the pressure would be

100 x 580/460 = 126 pretty scary, eh? At starting temperatures of 30F, 60F and 90F the corresponding pressures at 120F would be 118PSI, 112PSI and 105PSI respectively.
Alarming though this may be, there is a significant mitigator that is related to how tires actually support the vehicle. A tire is nominally cylindrical. That is a circle with some depth - the width of the tire - typically 10" to 12" for RV tires. Now a cylinder contacting a plane (tire on the pavement) is a line contact and (as we all remember from geometry) a line has no area.
Given a 30,000# RV with six tires, we can simplistically assign 5,000# to each tire. So how does the tire support the vehicle? It deforms. How much does it deform - simple. At 100PSI (Pound per Square Inch), 50 square inches are required to support 5,000#. With a 10" width, the tire deforms to produce a 5" long "flat" where it contacts the road and everything is copasetic.
Drop the pressure by 10% and the "flat" has to go to 5.5", increase the pressure by 25% (the tire that went from 100PSI at 0F to 125PSI at 120F) and the flat reduces to about 4" in length. (Interestingly, do the same arithmetic for a toad and you get about the same answer. 3,000#, four tires at 5" tread width and 30PSI is 750# per tire requiring 750/30 = 25 square inches contact for each tire, divide by 5 (the tire width) and you get a 5" flat again).
So, what about the mitigation of the pressure change with temperature. Two related elements come into play. First, as the pressure increases, the "flat" decreases and the volume of the tire increases. Second, as the pressure increases, the tire stretches and this also increases the volume. Looking back to the original General Gas Equation the effect of increasing the volume is to reduce the pressure and vice versa. For simplification, we ignored the volume since it is more stable than either the pressure or the temperature but, as the changing value of the "flat" implies, the volume is certainly not invariant. However, for the "flat" to change size at all implies that a pressure change has occurred and both the decrease in the flat size and the stretching of the tire are both effects of the increased pressure and both act to reduce, or mitigate, the final pressure.
Now, none of the above has taken into account the effects of ambient temperature on cooling, duals as opposed to singles in terms of air flow, road surface (melting blacktop compared to relatively cool whitish concrete), wet conditions with evaporative cooling in low ambient, under pressure tires producing more self heating and thus increasing their pressure more at operating temperature than a correctly inflated tire (at the expense of higher working temperature) and so on. Life is really complicated isn't it?
In the end reality is what matters. From the seemingly endless posts on this topic, almost everyone seems to run their tires between 100 and 120PSI. Common sense dictates that exceeding the maximum inflation pressure is probably not a good plan and running at 80PSI is just as foolish. So, how many problems have we encountered concerning tire failure? None that I recall. How much actual difference does plus or minus 9% on a nominal 110PSI pressure really make? Apart from the "Princess and the Mattress" types, who can apparently tell the difference almost by the individual PSI, I would suggest NONE AT ALL - especially bearing in mind that the pressure will vary by a large proportion of this amount on every journey you undertake.
There's my 2c (probably more like a nickel). Here is an address for more boring information on Boyles Law, Charles Law, Pressure Law and so on. Enjoy!
http://www.geocities.com/chemistryvillage/gases/gases.htm
Peter

2000 Intrigue 11066

Re: Tire Pressure Revisited

Reply #13 –
Yahoo Message Number: 2266
Keep in mind that the cold tire pressure is based on a temperature of 65 degrees.

Re: Tire Pressure Revisited

Reply #14 –
Yahoo Message Number: 2801

OK..... So now I have read this for the umpteenth time and maybe I finally get it. But here's a question.... Assuming that cold pressure is checked and is at 110F on dual rears, why would the temp on the inside tire on both sides rise enough to cause a temp alarm yet the outside tires remain relatively cool?

Dick May

2002 Intrigue, #11438

(And, yes, Damon, I still need to weigh and adjust the pressures.)
Dick May
2002 Intrigue, #11438, towing a
2014 Jeep Gr Cherokee ecoDiesel

Re: Tire Pressure Revisited

Reply #15 –
Yahoo Message Number: 2812
Just this weekend I experience an alarm on both inside rear tires (180 degrees). Pressure was greater than the outside tires, but not really as much as I would have expected for that temperature increase. Since the tire pressure was okay (not flat) and it happened to both inside tires within 30 seconds, I proceeded to the next rest stop 5 miles up the road for inspection. The surface of the tire did not feel excessively hot.
Conclusion. Although I was going uphill at the time of the alarm, 5- 15 miles prior I was heavier on the brakes that I should have been.
The heat from the drum brakes over which the inside tires are mounted, radiated to the inner aluminum wheel. The temp sensors are strapped to the wheel, so they were really measuring the extra radiated temp of the brakes and not the average air temp inside the tire. The outside aluminum wheel, when I poked by fingers into the holes, was also definitely hotter than normal conditions.

We took a lunch break. 30 minutes later the temp had dropped 50 degrees. Proceeded on the trip, which included some subsequent long
7-9% downgrades. Was more careful about the brakes and speed, and
the problem did not reoccur.
Herb

CC 2002 Allure #30690

Re: Tire Pressure Revisited

Reply #16 –
Yahoo Message Number: 2815
I've had the same alarms at the same temp without any excess use of the breaks. I have to think that there is something else that is not quite right.....

Dick May

2002 Intrigue, #11438
Dick May
2002 Intrigue, #11438, towing a
2014 Jeep Gr Cherokee ecoDiesel

Re: Tire Pressure Revisited

Reply #17 –
Yahoo Message Number: 2816
Herb,

You probably already know this but I'll tell you anyway. When going down a hill do not pump the brakes, use them stead to reduce spead and then release, also if you are using the Jake or pac brake do not let the rpms get to high or the transmission will up shift. Not a very good thing but the trans. is designed to detect a need for a higher gear when rpms are to high and it will go into a higher gear.

Bill G. 2001 Magna

Re: Tire Pressure Revisited

Reply #19 –
Yahoo Message Number: 2819
Dick

Could be tire pressure, we run 115 in the front and 105 in the rear and on the tag, and our temperature never goes above 130. Which I feel very comfortable with.

Bill G. 2001 Magna

Re: Tire Pressure Revisited

Reply #20 –
Yahoo Message Number: 2831
Thanks, Bill.... My pressures are still at delivery pressures but will be changed to the recommended pressures for the current weight. I still feel that the difference in temp for two tires next to each other at the same pressure should not be as great as I am showing. My hands aren't very accurate at measuring the temp but if the two tires on the same side are at the same pressure and feel about the same then Smart Tire should not report a 40-45 degree difference.

But what do I know........

Dick May

2002 Intrigue, #11438
Dick May
2002 Intrigue, #11438, towing a
2014 Jeep Gr Cherokee ecoDiesel

Tire Pressure Revisited

Reply #21 –
Yahoo Message Number: 18077
Hi Folks,

Now that Toyo has revisited their philosophy(liability?) on tire pressures (use max pressures, in my case 120 front and 110 rear), it might be interesting to look at the charts from the Toyo Recreational Vehicle, Truck Tire Warranty and Users Guide(1999) and Recreational Vehicle Tire Guide(2003). Both have the 6 point weighing procedure and Tire Inflation Tables.

Now for the math. The exercise below is for an empty coach from the factory and provides an example of how to calculate minimum tire pressures for a fully loaded coach.
First - looking at the side of the M111z-5 tire, single loading is 7200# at 120 psi and dual loading is 6320# at 110 psi.
If you divide 7200/120 = 60#/psi loading for the front tires and 6320/ 110 = 57.45#/psi for each rear tire.
Using my Weight Results sheet provided in the coach documentation folder (full fuel and water, full other fluids, empty gray and black tanks defines an empty coach), the LF tire has 4768# loading, RF has 4691, LR has 9405 and RR 8042. These pressures represent no additional weight for food, clothing, tools, etc and a 6 point weighting is recommended after your coach is fully loaded.
Starting with the LF, dividing 4768 by 60 = 79.47psi needed for that tire, RF is 4691/60= 78.18, LR is 9405/2=4702.5 loading on each tire, then dividing 4702.5 by 57.45 = 81.85psi and finally for the RR, 8042/ 2=4021, 4021/57.45=69.99psi.

Now, looking at the Toyo tire inflation tables, gives the following pressures - LF is 65psi (interpolated), RF is 62.5 psi (interpolated), LR is 70 and the RR is 50 psi (interpolated). The table does not show a recommended pressure below 70 psi.
For a comparison of the calculated (C) pressure requirements versus the Toyo inflation table (T) pressures remembering no pressures shown below 70.

LF 79(C), 70(T), 65(interpolated)
RF 78(C), 70(T), 62.5 "
LR 82(C), 70(T)

RR 70(C), 70(T), 50 "

My conclusions.

The Toyo inflation tables are lower than the calculated pressures in 3 instances and represent a safety concern.
The Calculated pressures are minimum pressures required for safety.
Making the tires pressures the same for the front and the same for the rear is logical, e.g., 80 for the front and 82 for the rear in this example.

I use 10 psi higher than the example above based on actual loading, using the above analysis to be absolutely safe. My coach is 32 foot long and not heavily loaded; the above analysis becomes more critical for 36 and 40 foot coaches if you use less than the new recommended pressures of 120 front and 110 rear for this tire.
Fred Kovol

2000 Allure 30593