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Old 10-22-2021, 09:24 AM   #43
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I recently noticed my cold pressure has decreased in all tires (by TPMS) by almost 7 pounds in the mornings with temps in the 40's. But within 10 driving miles they are back to normal pressure.
My question is, should I add more air at cold ambient temps to bring pressure to cold normal settings?

NO!!!!!!!!!
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Old 10-22-2021, 09:34 AM   #44
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NO!!!!!!!!!
Why would you advise to not add air to bring the tires to the recommended cold pressure?
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Old 10-22-2021, 10:11 AM   #45
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Yes, it is simultaneously, the maximum cold pressure and the minimum pressure recommended to support the maximum load. In most cases. Correct me if necessary, but does the wording on the tire not say “Max cold pressure” or Max cold psi”, and is that not what it means? And shouldn’t a basic understanding of the the term “cold” which has been explained ad nauseum, make it easy to understand that pressure will increase as the tire warms, due to warmer temperatures, sun exposure or rolling down the road?

Oddly, on one of my passenger vehicles, equipped with Michelin tires, the minimum recommended pressure to support the maximum load of 1477lbs, is 36psi. Yet the sidewall shows a maximum cold pressure of 44psi.

Determining correct tire pressure should be one of the easiest things anyone could do. It doesn’t require anecdotal stories or complicated math. For a vehicle owner, it really is simple.

Not all tires have the same wording on the tire sidewall except for the "Maximum Load".


Passenger car placard pressure is a recommendation from the vehicle dynamics and fuel economy engineers based on hundreds of miles testing and evaluating to find the right balance of ride, handling, noise and fuel economy. The pressure on most passenger cars provide 20 to 30% excess or reserve load capacity.
I have never heard of any RV company doing similar tire evaluations and from the numbers we see most select the smallest tire that will meet the DOT requirement for the tires to be able to support 100% of the GAWR with zero reserve load capacity.


Yes it would be nice if people read the literature and understood what "cold" inflation means but hardly a week goes by without someone asking how to calculate the pressure "correction factor" to set the pressure to the tire industry standard temperature.


The tire wording is suppose to tell people and mean " The max load capacity for this tire is xxxx Lbs when inflated to yyy cold pressure, The load capacity will not be increased even if you increase the cold inflation above yyy cold pressure"
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Old 10-22-2021, 10:23 AM   #46
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Ok, i think i see what you are trying to do ... calculate needed inflation at a given temperature to give a "target" pressure should the tire cool down (or heat up) to 70 degrees. (Ideal gas law stuff). BUT, this presumes that the inflation charts are premised on "cold" being defined as 70 degrees. My understanding has always been that "cold" really means "ambient", regardless of whether it is 100 or zero degrees. This also makes sense from a mechanics perspective, as it is the pressure inside the tire that actually supports the load, and you will need that given pressure regardless of what the temperature is.

Thought experiment - assume your approach that you need say 75 psi at 70 degrees. But the ambient temperature is 30-40 BELOW zero (F). This approach would have you drastically reduce the pressure in the tire, to a point where it is virtually fully flat. Just does not make sense (at least to me).

Tireman - are you able to confirm/refute whether the inflation tables are premised on a 70F ambient?



The "Load & Inflation tables" are NOT based on any specific temperature number. The tables are based on a tire inflation pressure when the tire is at air AMBIENT temperature. Not artificially cooled or warmed




While there have been some tables developed and published for tire inflation when a tire is mounted and inflated in a heated workshop but will be driven in extreme cold ambient. Think Winter in Alaska This is done to prevent the need to park the vehicle, in the cold for a couple hours and then go and adjust tire inflation on big military vehicles. So unless you are inflating the tires on your RV in a heated garage at say 65F and intend to drive outside and head down the highway where it might be -40F I don't see the need for you to even have these charts.
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Old 10-23-2021, 06:13 AM   #47
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Why would you advise to not add air to bring the tires to the recommended cold pressure?
You said it. cold pressure. If you have a leak yes but if they are all the same no. I use the tire manufacturers load pressure table for my tire. so right now at 40 my tires are in the 60's i'd say but after I get rolling they warm up and run about 90 to 100 depending on the air temp.
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Old 10-23-2021, 08:20 AM   #48
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You said it. cold pressure. If you have a leak yes but if they are all the same no. I use the tire manufacturers load pressure table for my tire. so right now at 40 my tires are in the 60's i'd say but after I get rolling they warm up and run about 90 to 100 depending on the air temp.
Yes, I said “cold”. “Cold” is key. Take a look at the load and inflation table that you are using. See if the word “cold” appears on that document. There may even be a definition of “cold”.

“Cold” is a variable, a moving target. In your case, right now, “cold” is 40F. That is the temperature at which your tires need to be inflated to the recommended pressure.

The tires are no longer “cold” after you’ve rolled down the road and ambient temperatures have increased. Those are exactly the wrong conditions to determine correct tire pressure. If that is how you are determining correct tire pressure, you’re doing it wrong.
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Old 10-23-2021, 09:47 AM   #49
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Originally Posted by 1Motorhome View Post
I recently noticed my cold pressure has decreased in all tires (by TPMS) by almost 7 pounds in the mornings with temps in the 40's. But within 10 driving miles they are back to normal pressure.
My question is, should I add more air at cold ambient temps to bring pressure to cold normal settings?
Apart fom my pigheaded idea of calculating it back to 70degrF, we now can calculate back what pressure you filled cold at about 70 degr F.

40 degrF ambiënt temp drops almost 7 psi.
70-40= 30 degr/7psi is 4.3 degrF/psi.
Looking back in my list, 97psi / 4.5degrF/psi
126 psi/ 4 psi/ psi
So your used pressure is in between 97 and 126 psi. To be more presise I gamble on 110 psi.

Not because its needed, yust because it is able.
What pressure do you use?

I am now working on a long story, first in WORD , to explain why I still think the pressure needs to be calculated back to 70 degr F.
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Old 10-23-2021, 09:53 AM   #50
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If the pressure needed to be calculated back to 70f don’t you think the manufacturers would provide info to do so. Set it to the proper PSI cold. Done.
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Old 10-23-2021, 10:07 AM   #51
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If the pressure needed to be calculated back to 70f don’t you think the manufacturers would provide info to do so. Set it to the proper PSI cold. Done.

Quite likely we are discussing two different ways to arrive at the same inflation PSI.


One complex, one simple. Doesn't mean either is incorrect, but give me the easy one!
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Old 10-23-2021, 10:44 AM   #52
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Just a thought to share, I usually always add about 6~7 psi more pressure than listed (on the door jamb, never exceeding the tire max psi, least not very much if any) prior to the first cold snap when performing my Fall maintenance. Otherwise when the temps drop with the first significant fall\winter cold front my newer vehicles complain about low pressure in the tires (tpms) and my older vehicle's tires need more air as well, but with no tpms to warn me.
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Old 10-23-2021, 10:52 AM   #53
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....when the temps drop with the first significant fall\winter cold front my newer vehicles complain about low pressure in the tires (tpms) and my older vehicle's tires need more air as well, but with no tpms to warn me.



Yup, your tires "read" the Ideal Gas Law and followed it to the letter.



Probably saying PV=nRT in their sleep.
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Old 10-23-2021, 11:57 AM   #54
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Quite likely we are discussing two different ways to arrive at the same inflation PSI.


One complex, one simple. Doesn't mean either is incorrect, but give me the easy one!
Yet some folks don’t understand or follow the simple method. The last thing that needs to be introduced is some futile mathematical gymnastics based on an irrelevant data point.
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Old 10-23-2021, 12:40 PM   #55
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Originally Posted by wvabeer View Post
You said it. cold pressure. If you have a leak yes but if they are all the same no. I use the tire manufacturers load pressure table for my tire. so right now at 40 my tires are in the 60's i'd say but after I get rolling they warm up and run about 90 to 100 depending on the air temp.



If I understand it you are depending on the heat from driving to get the tires to the needed minimum pressure. THIS IS ABSOLUTELY THE WRONG APPROACH.
When parked for a short time you do not need to adjust pressure. If parked for a long time say more than a few weeks over the winter you should inflate to the tire sidewall pressure to minimize tire "flat spotting".


You should ALWAYS ensure your tire inflation is correct BEFORE the tires are warmed by the sun AND BEFORE the tires are warmed from driving.
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Old 10-23-2021, 12:41 PM   #56
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If the pressure needed to be calculated back to 70f don’t you think the manufacturers would provide info to do so. Set it to the proper PSI cold. Done.

Correct
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