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Old 10-11-2022, 08:38 PM   #1
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Exactly what is "cold" for tire pressure?

One inflates the tires to a given desired pressure when the tire is "cold", which seems to mean "when the RV hasn't moved for a while". So I suppose that really means when the tire is at thermal equilibrium with the environment.

Well, the environment, here in Michigan, can be above say 90F on any hot day, or say around 50F on a cool one. Filling the tires to that given pressure is going to add, I would think, a lot more air on the cool day. Then one hits the road and the tires heat up a lot. More air, same temperature, more pressure.

After running down the road a while, do the tires get just as hot on a cool day as on a hot one? If so, is there a risk of overpressure? Should one adjust the desired "cold" temperature pressure according to the ambient temperature?

Or, to put it another way, how much pressure variation would be normal due to fluctuation in ambient temperature alone? If my SOP calls for filling my tires to 80 pounds in July or August, what's the right pressure for them in October or April?
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Old 10-11-2022, 09:04 PM   #2
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"cold" on the tire means not driven on for some period of time, say 12 hours or whatever. Doesn't matter if it December 15th or August 15th. You go out early in the morning and set all your tires to the desired pressure.

Now I will say that I "cheat" a bit. I don't feel the need to constantly be changing tire pressures every day because of weather changes. A few PSI below rating isn't going to be a disaster waiting to happen. So if I'm leaving NJ on a chilly day heading south into warmer weather I'll check to make sure my tires are within a few PSI of the required number. Then when I check the next day in warmer conditions I'll be closer or right on.

Thats me. I've read plenty of posts of people going out and inflating or deflating their tires every day of a week long trip. Too much effort for no gain to me. But theres nothing wrong with it if thats what you want to do.

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Old 10-11-2022, 09:38 PM   #3
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Cold means not driven in awhile, but it also means not sitting out in the sun. I note from measurements that the sunny side of the parked vehicle will have higher pressures than the shaded side.

I have done RV trips where the daily average temperatures could drop or rise 20, 30, or 40 degrees F from the start of the trip to the end, be it from changing weather or changing latitude. I have had to adjust tires on a morning mid trip to the new “cold” temperatures.
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Old 10-11-2022, 09:48 PM   #4
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Originally Posted by jimmarako View Post
"cold" on the tire means not driven on for some period of time, say 12 hours or whatever. Doesn't matter if it December 15th or August 15th. You go out early in the morning and set all your tires to the desired pressure.

Now I will say that I "cheat" a bit. I don't feel the need to constantly be changing tire pressures every day because of weather changes. A few PSI below rating isn't going to be a disaster waiting to happen. So if I'm leaving NJ on a chilly day heading south into warmer weather I'll check to make sure my tires are within a few PSI of the required number. Then when I check the next day in warmer conditions I'll be closer or right on.

Thats me. I've read plenty of posts of people going out and inflating or deflating their tires every day of a week long trip. Too much effort for no gain to me. But theres nothing wrong with it if thats what you want to do.

Jim M.
Jim, I also cheat but a little more than you. I fill my tires to 80 psi. I. Live in Florida so most times I go to cooler temps so my pressure usually lower in the mornings before I leave on my next leg.

I have scaled my 5th wheel at a CAT scale and know the RV itself weighs 10K pounds. I assume the driver side is heavier than the passenger side as all the kitchen appliances are on that side. So, I just assume 6K pounds on that side and 4K pounds on the other. That is probably overkill but I think itís a safe number.

Since I know the one side is about 6K pounds I use that number along with load/inflation data for my GYE tires. From that table I know the tires can get down to 70lbs and still hold 6300 lbs. I have TPMS and will pull out of the campground if the pressure is 70 lbs or above. I know after being on the road for a short period the pressure will rise to 80+.
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Old 10-12-2022, 07:20 AM   #5
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To my opinion "cold" pressure is when temperature of gascompound in tire is the same as outside the tire, and that is when no external factors, and enaugh time after driving to get that equilibrium.

And my opinion is that its for a certain temperature.
Porche writes 68 degrF, and I saw info from tiremanement organisation for fleetowners 15 degrC is 60 degrF I think.
Made a list for cold pressure filled at 70 degrF, and behind that the degrF/psi change.

My idea is that when 100degrF outside you must not lower to recomended pressure even if its above the referencepressure of tire, what most think is the maximum cold pressure allowed.

Example 80 psi filled at 70 degrF becomes 86psi at 100degrF, and if you then let out air, you get more deflection so heatproduction, at that 100 degrF cooling down of tire is worse because of the lesser temperature differences. So tires can overheat then, and this is allowed only to happen ZERO times in tires use. The little cracks that are made then, dont dissapear, and crack further in time by the mechanical forces, untill mayby after 3 years that far that tire blows or treath separates, then the possible extremer situations at that moment are blamed.

The other way, if 40 degrF outside, pressure drops to 75 psi, and then to prevent overheating, you dont have to highen up the pressure to 80 psi, but may do so for roadhandling and fuelsaving.

Here the list I mentioned in the beginning, made going from cold 70 degrF but if you use it for 60 degrF, it wont give dramatic differences..

In next post I will give text I used in other topic on other forum, in wich I was burned down for my idea of not having to highen up at 40 degrF.
Filledcold/ change/psi
20 psi/ 15,5F/psi
21 psi/ 15F/psi
22 psi/ 14,5F/psi
23 psi/ 14 F/psi
24 psi/ 13,5F/psi
25 psi/ 13,5F/psi
26 psi/ 13 F/psi
27 psi/ 12,5 F/psi
28 psi/ 12,5 F/psi
29 psi/ 12 F/psi
30 psi/ 12 F/psi
31 psi/ 11,5 F/psi
32 psi/ 11,5 F/psi
33 psi/ 11 F/psi
34 psi/ 11 F/psi
35 psi/ 10,5 F/psi
36 psi/ 10,5 F/psi
37 psi/ 10 F/psi
39 psi/ 10 F/ps
40 psi/ 9,5 F/psi
42 psi/ 9,5 F/psi
43 psi/ 9 F/psi
45 psi/ 9 F/psi
46 psi/ 8,5 F/psi
49 psi/ 8,5 F/psi
50 psi/ 8 F/psi
53 psi/ 8 F/psi
54 psi/ 7,5 F/psi
58 psi/ 7,5 F/psi
59 psi/ 7 F/psi
63 psi/ 7 F/psi
64 psi/ 6,5 F/psi
70 psi/ 6,5 F/psi
71 psi/ 6 F/psi
77 psi/ 6 F/psi
78 psi/ 5,5 F/psi
86 psi/ 5,5 F/psi
87 psi/ 5 F/psi
96 psi/ 5 F/psi
97 psi/ 4,5 F/psi
109 psi/ 4,5 F/psi
110 psi/ 4 F/psi
126 psi/ 4 F/psi
127 psi/ 3,5 F/psi
148 psi/ 3,5 F/psi
149 psi/ 3 F/psi
177 psi/ 3 F/psi
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Old 10-12-2022, 07:25 AM   #6
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Now here the text I used in another topic.
Dont burn me down on the exact temperatures, see them as an indication. But this already answers that tire inside stays cooler at lower ambiŽnt temperature.

Why pressure should be calculated back to 70 degrF

That pressure of tires is related to the temperature of gascompound in tire, is fact.
But that for instance you need for 100 degrF ambiŽnt temperature higher cold filled pressure, then for 70 degrF, and for 40 degrF can do with lower pressure then for 70 degrF, is a conclusion I took, and is the discussion here.
I am going to explain how I came to this conclusion.
I once had a long telephone call with a man from Vredestein , and he explained that goal of pressure determination and maxload of tire, is to not overheat them.
He explained that the molecule-structure of natural rubber can be compared to spagetti, and if you push your nail in it the print remains . Then the “ spagetty-strings" slide over each other.
Tires are vulcanised at the end, and this makes sulfurbridges between the “ spagetti “ wich makes the rubber flexible, so when you push your nail in it , the print directly dissapears, the rubber goes back to its original shape.
The rubber needs a certain amount of sulfurbridges, but to much makes the rubber hard, and if then bending of the rubbber, by the deflection and flexing back every cycle, gives small cracks, wich dont disapear ( logical)
When rubbers temperature goes above a critical temperature, to many sulfur bridges are created,and rubber hardens. Wich temperature this is I dont know, but the vulcanising proces is at 170 degrC is 340 degrF.

Rest is my Einstein way of thinking, and because my average IQ is probly half of that of Mr Einstein, change is 2ce as much that my theory is proven wrong once.

The temperature of the rubber is created by the balance between heating up a second and cooling down a second.
When you begin driving at cold pressure in ambiŽnt temperature of 70degrF, in-and out-side-tire air and all the rubber of tire is 70 degrF. The heat-transport then is zero. In basics Maxload is calculated for reference-pressure ( E-load 80 psi) and reference-speed ( P- and LT tires mostly 99mph/160kmph) so when you drive that speed constant with maxload on tire and reference-pressure cold filled in tire, no overheating of any part of tire. I practice a bit more complicated.

Yust for the example say you then drive 99mph constant speed, with referencepressure of 80psi ( E-load) and maxload on tire. In the beginning cooling down is marginal, because temperaturedifferences between rubber of tire and in and outside tire gascompound is still zero, but driving long enaugh rubber yust below its critical temperature in the middle of the thickest parts of tire, and at the edges , where rubber meats the gascompound, lets say 300 degrF.
Inside tire gascompound I state as 140degrF. Outside the tire always air 70 degrF

Then temperature difference between edges of rubber and inside tire gascompound 300-140=160 degrF., and between outside tire air 300-70=230 degrF. Also the inside tire gascompound is cooled down mostly trough the rimm to the air outside the tire with only 140-70= 70 degr temperature difference so 160/70 = 2.28 times as much cooling down by the better transport trough the metal rimm.
So coolingfactor then inside tire 160/230=70% of outside tire. In and outside tire Together 230+160=390 degrF
The rubber temperature then stays in balance ,so heating up factor also 390 degrF.

But the deflection , so heatproduction a second at that speed, then is lesser then at cold pressure, because temp in tire 140degrF, wich gives higher pressure. 80 psi filled at 70 degrF becomes 92.9 psi at 140 degrF.

Now situation ambiŽnt temp 100 degr F.
Temp of rubber still max 300degrF.
Temperaturedifference rubber and outside tire air 300-100= 200 degrF
Temperaturedifference rubber and inside tire gascompound 70% of outside 200= 140 degrF.
Together200+140=340 degrF worth of cooling capacity
So temperature inside the tire 300-140= 160degrF.
So at 70degrF ambiŽnt temperature 390degrF worth of cooling capacity.
At 100degrF ambiŽnt temperature 340degrF worth of cooling capacity.
This has to be compensated by lesser heatproduction a second by lesser deflection of tire is 340/390=87.2 % of the heatproduction at 70degrF ambiŽnt temperature.
Now if 80psi filled at 70degrF
At 100F/85.4psi
At 140F/ 92.5psi
At 160F/ 96.1psi
So 92.5/96.1=96.25% of the surface on the ground then at 70degr F driving 99mph with maxload on tire. Rule of tumb I determined gives 0.9625^2= 92.64 times the deflection . If heatproduction goes lineair with the deflection ( I hope and not more), this gives 92.64% of the heatproduction.
Cooling down is 87.2% , so heating up is even still more then cooling down, if 300degrF is the critical temperature of rubber .

If you then lower the 85.4psi cold pressure at 100degrF ambiŽnt temperature to 80psi, as is stated to be allowed because cold filled is at ambiŽnt temperature, the pressure becomes at 160degrF 90.2psi instead of 96.1psi ,wich gives more deflection so heatproduction a second, so rubber of tire temperature goes above 300degrF, and it hardens and cracks.
In fact , because at 100degr heating up factor is 92.64% and cooling down factor is 87.2% at 100degrF ambiŽnt temperature of the 70degrF situation, you should even pump the tires up a little to give lesser heatproduction a second., so heating up factor goes also to 87.2

Now ambient temp 40degrF.
Outside tire 300- 40= 260 degrF worth of cooling down factor.
Inside tire 300- ( 70% of 260= 182 degrF)= 118 degrF inside tire gascompound
So cooling down factor 260+182= 442 degrF
This is 442/390= 113.3% of coolingdown factor then at 70 degrF.
Then at 40 degrF 80psi filled at 70 degrF becomes
74.6 psi cold and at 118 degrF inside tire temp 88.6 psi
92.5psi/88.6 psi = 1.044 times more surface on ground gives 1.044^2= 1.09x more heatproduction.
So when colder tires rubber stays cooler then 300degrF when driving 99mph with maxload and reference-pressure on tire. So for safety not needed to fill up the cold pressure of 74.6 psi to 80 psi , with difference to hot temps, that you may do so for riding quality and fuell saving.
So I think the lists for filling higher at 70 degrF in a heated garage , to get the 80 psi in this example at for instance 20 degrF is because at those extreme cold ambiŽnt temperatures the deflection gets to much so for instance snake-bite.
Then its not anymore to prefent overheating, but for riding -quality.
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Old 10-12-2022, 07:56 AM   #7
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Old 10-12-2022, 08:14 AM   #8
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Here is a simple article from Tire Rack on "cold" psi.

https://www.tirerack.com/tires/tiret....jsp?techid=73
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Old 10-12-2022, 08:49 AM   #9
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I keep my RV stored at a storage location away from my home (Damn HOA!). I keep my tires at 65 PSI, as per GY tire recommendations. I have aftermarket external TPMS sensors.

When I go to pick up my trailer, the first thing I do is to turn on my TPMS station in the truck. Usually by the time I get all hooked up, the pressures are picked up the base station, though sometimes not. Usually within about 30 seconds of the tires rolling, they're picked up. I also visually check the tires, and as long as they look okay, I roll out. I have about a 20 minute drive to my house, and the first five minutes are at a speed of about 25 MPH, so I'm fine with the visual check to get started. The TPMS sensors are very accurate compared to manually checking... So I don't manually check, unless I need to add or remove air. I rely on the TPMS for my readings.

I use a 5 PSI fudge-factor. As long as I'm within 5 PSI, I don't add or remove air.
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Old 10-12-2022, 10:34 AM   #10
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replace the word 'cold' with ambient .
am∑bi∑ent
adjective: ambient
relating to the immediate surroundings of something.
"the tire is checked at ambient temperature, before the vehicle is driven"

before driving look at your pressure ....
If it is LOW / HIGH adjust it to be the correct pressure.
32lbs Pressure at 30 degrees is the same as 32lbs Pressure at 70 degrees,

the amount of air required to achieve the pressure changes with temperature...
as you drive the tires will warm up ... pressure will get a bit higher
BUT the tire manufacturer have taken this into account and are happy..

-------------------------------------------------

The only time this could become a problem ....

If you live in Florida and on the same day go into the mountains ---> COLD PLACE
(How I wish I could do that)


Your tire pressure may go down, but you as a good driver know this and...

get out the portable compressor PLUS your recently calibrated pressure gauge and dutifully adjust your pressure after letting your tires cool down to ambient temperature.
----------------------------------------------------------------


Just be thankful they decided to use pressure as the means to inflate tires
what a drama it would be if it was.... " by weight"
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Old 10-12-2022, 10:45 AM   #11
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which reminds me
before hurricane Ian .. I added some pressure to the yukon just in case had to hookup the trailer and P off in a hurry... I like to add about 10pounds to the rear just to stiffen them up a bit while towing.



Does anyone know where I put my pressure gauge

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Old 10-12-2022, 10:52 AM   #12
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The whole purpose of checking tire pressures before driving is to insure there is sufficient pressure to carry the load. If you start with low pressure on a cold day you will be driving on underinflated tires, period. Expecting the pressure to rise to a proper pressure as tires heat up is foolish thinking. How long will it take to warm enough to provide that minimum load carrying pressure specified by the Manufacturer and posted on door post? Tire is overloaded until it does and there's no way of telling how much damage may or may not be done during that driving time.
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Old 10-12-2022, 01:48 PM   #13
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The whole purpose of checking tire pressures before driving is to insure there is sufficient pressure to carry the load. If you start with low pressure on a cold day you will be driving on underinflated tires, period. Expecting the pressure to rise to a proper pressure as tires heat up is foolish thinking. How long will it take to warm enough to provide that minimum load carrying pressure specified by the Manufacturer and posted on door post? Tire is overloaded until it does and there's no way of telling how much damage may or may not be done during that driving time.
Exactly, thatís why I know the minimum safe load/pressure for my tires is about 65 lbs. I usually cold fill to 80psi but if I go to a cooler climate and the pressure drops to 70psi I know that my tires are good regardless if they get back to 80psi.

If below 70psi, time for me to add air.

My SD F350 has tires that are rated at 80psi cold but on the tire sticker it states 60psi for front tires and 80psi for rear tires. Tires only need to be filled to the safe air pressure for the load.

The 80psi in my rear tires allows a load of 7500lbs. My rear axle with no RV only weighs 3500lbs. I sometimes reduce the rear tire pressure to make the ride around town not as rough.
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Old 10-27-2022, 07:10 PM   #14
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Exactly, thatís why I know the minimum safe load/pressure for my tires is about 65 lbs. I usually cold fill to 80psi but if I go to a cooler climate and the pressure drops to 70psi I know that my tires are good regardless if they get back to 80psi.

If below 70psi, time for me to add air.

My SD F350 has tires that are rated at 80psi cold but on the tire sticker it states 60psi for front tires and 80psi for rear tires. Tires only need to be filled to the safe air pressure for the load.

The 80psi in my rear tires allows a load of 7500lbs. My rear axle with no RV only weighs 3500lbs. I sometimes reduce the rear tire pressure to make the ride around town not as rough.

I agree with Titan,. You should never run inflation lower than the minimum needed to support the load on your tires.
I understand the desire to not "mess" with changing PSI each day, that's why I suggest that people run a "Cold" inflation that is at least 10% more than what is needed for the measured load with +15% to +20% a better choice as long as you do not exceed the max inflation rating of your wheels.
In my Class-C, that is really light in weight I run Fronts at +20% and my rears at +25%.
Since PSI will change by about 2% for each change in temperature of 10įF a +15% change in inflation means you can have a change in temperature of 70įF before you need to be concerned with your tire inflation which is extremely unlikely.
Some people want to overthink tire inflation IMO and create long charts as running a nice "cushion" means I almost never have to adjust my tire inflation.


IMO making tire inflation a "task" with calculations and multi-page charts will likely end up making it a chore, and I don't know anyone that likes "chores".
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Old 10-27-2022, 07:35 PM   #15
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Some people want to overthink tire inflation IMO and create long charts as running a nice "cushion" means I almost never have to adjust my tire inflation.


IMO making tire inflation a "task" with calculations and multi-page charts will likely end up making it a chore, and I don't know anyone that likes "chores".
We used to call this "Paralysis through Analysis".




As you alluded, eventually nothing gets done.
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Old 10-27-2022, 08:14 PM   #16
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I am surprised there is not a single viewer of this thread who has received even a basic Physics education.
So here is the abbreviated Cliff Notes version of why we have a "cold" pressure measurement and what it means. When Napoleon (yes it's THAT old a concept) endowed his civil service with a need to bring science to the fore to Make France Great, one of the things his scientists lacked was a reference point to define pressure which was really important for making and using artillery and sundry other weapons. So they did 2 important things, one was to define a meter which required a temperature because of the expansion of materials and the other was to define pressure which also required a temperature. That temperature is 20 deg C or 68F. And that's what we mean by "cold". Of course if it's Summer day in Tempe AZ, the tires can never be " cold" and similarly in Flynflong Canada, they can never be a standard cold. The rule of thumb is about 1 psi per 10 deg F adjustment. Some do this and some don't just as some don't check their tire pressure assuming a kick will do and others make decimal point changes.
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Old 10-27-2022, 09:20 PM   #17
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I am surprised there is not a single viewer of this thread who has received even a basic Physics education.
So here is the abbreviated Cliff Notes version of why we have a "cold" pressure measurement and what it means. When Napoleon (yes it's THAT old a concept) endowed his civil service with a need to bring science to the fore to Make France Great, one of the things his scientists lacked was a reference point to define pressure which was really important for making and using artillery and sundry other weapons. So they did 2 important things, one was to define a meter which required a temperature because of the expansion of materials and the other was to define pressure which also required a temperature. That temperature is 20 deg C or 68F. And that's what we mean by "cold". Of course if it's Summer day in Tempe AZ, the tires can never be " cold" and similarly in Flynflong Canada, they can never be a standard cold. The rule of thumb is about 1 psi per 10 deg F adjustment. Some do this and some don't just as some don't check their tire pressure assuming a kick will do and others make decimal point changes.
Funny, I still remember my college physics classes.

The fault with your presentation is that it really has no application in the tire world. The constant in a tire application is the load. In order to support the load in a manner that reduces tire flex and heat buildup the air pressure has to be adjusted to meet minimum liad carrying ability------regardless of the temperature.

STP is usually used when comparing different data sets. For example comparing the CFM spec's of several compressors where the discharge is more dense if it's cool/cooled or hot.

BTW , I'm pretty sure Tireman9 also remembers his college physic's classes.

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Old 10-28-2022, 12:04 AM   #18
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After some research on tire pressure etc. I believe "Cold" tire pressure is at a outside temp of 68 F and tires not driven for at least two hours, then inflate to the required pressure.
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Old 10-28-2022, 12:39 AM   #19
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I agree with Titan,. You should never run inflation lower than the minimum needed to support the load on your tires.
I understand the desire to not "mess" with changing PSI each day, that's why I suggest that people run a "Cold" inflation that is at least 10% more than what is needed for the measured load with +15% to +20% a better choice as long as you do not exceed the max inflation rating of your wheels.
In my Class-C, that is really light in weight I run Fronts at +20% and my rears at +25%.
Since PSI will change by about 2% for each change in temperature of 10įF a +15% change in inflation means you can have a change in temperature of 70įF before you need to be concerned with your tire inflation which is extremely unlikely.
Some people want to overthink tire inflation IMO and create long charts as running a nice "cushion" means I almost never have to adjust my tire inflation.


IMO making tire inflation a "task" with calculations and multi-page charts will likely end up making it a chore, and I don't know anyone that likes "chores".
I never advocated running my tires at a pressure less than what is recommended as a safe tire load limit specified for my tires. Plus you know these tire load limits are numbers that already have an added safety margin included dictated by the engineering group and company lawyers.

I know what my RV weighs and I stay at a safe level above the recommended inflation. No need to make it 80psi just to have it at 90psi 30 minutes down the road.

There was a tire contributor on here who once said checking and filling tires increased the possibility of getting a contaminant in the valve causing a small leak.

I must have missed them but I have been RVing for 7 years and spent 100s of nights on the road and Iíve never seen a fellow camper charging their tires in the morningÖ just saying. Maybe theyíre idiots like me..lol.
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Old 10-28-2022, 07:05 AM   #20
Norwood Auto Italia
 
Join Date: Aug 2020
Location: Montana & TX
Posts: 1,234
Quote:
Originally Posted by TitanMike View Post
Funny, I still remember my college physics classes.

The fault with your presentation is that it really has no application in the tire world. The constant in a tire application is the load. In order to support the load in a manner that reduces tire flex and heat buildup the air pressure has to be adjusted to meet minimum liad carrying ability------regardless of the temperature.

STP is usually used when comparing different data sets. For example comparing the CFM spec's of several compressors where the discharge is more dense if it's cool/cooled or hot.

BTW , I'm pretty sure Tireman9 also remembers his college physic's classes.

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Well you are right of course because there is no science in the RV and vehicle world that applies. Just the hearsay.

Every aircraft manufacturer specifies an adjustment to tire pressures to accommodate a given load based on temperature at rest. In the tropics we routinely inflated slightly more for the given load and slightly less in the Antarctic (there was always a table supplied in the POM) and mostly if it was a psi or 3 we ignored it. But STP applies to all air density unless you know of some air ( and not N2 or He) that is impervious - so why are vehicle tires different for a given load?

I still retread and service aircraft tires and see very little difference in fundamental construction.
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