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Old 02-05-2019, 08:41 AM   #1
BIG KAHUNA
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A pressuring dilemma

So today's newbee question is when changing location altitudes how does one compensate when reading and filing their RV's tires. My tires should be at 80 PSI cold, but they all read around 70 PSI cold. Do I just bring them up to 80 PSI and call it a day? I do have a tendency to overthink things!
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Old 02-05-2019, 08:47 AM   #2
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Inflate them to the 80 psi, but make sure they are cold or at ambient air temperature.....not as soon as you have arrived and been travelling.
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Old 02-05-2019, 08:49 AM   #3
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The way I understand it is that you adjust tire pressure WHEN COLD at the altitude you're physically at. If towing up or down to a different altitude, DO NOT make adjustments while the tires are hot. Once cooled, recheck pressure and adjust accordingly.

Honestly, tires will change pressure based on temperature much more than they will change pressure based on altitude (unless your tires are on a "high altitude aircraft). So, if the tires have "engineered in capacity" to function in the "greater pressure swings" of temperature changes, they also have the capacity to function with the smaller pressure changes caused by altitude changes.
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Old 02-05-2019, 11:56 AM   #4
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Since I installed a TPMS on my trailer wheels, I've become less fussy about the pressures.
My tires are supposed to be at 65PSI, so I set them at that by the compressor gauge. But, my TPMS says they're between 62 and 66. AND, even when cold, there's about 2-3 PSI difference between the shady side and the sunny side of the trailer. Then when we get out on the road, they go up to 69-70 PSI.
Now, I just look for a tire that seems to have unusually high or low pressure.
Having the TPMS has certainly educated me to the tire dynamics WRT temperature/pressure.
Just out of curiosity, how much difference in pressure does going up and down mountains make?
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Old 02-05-2019, 12:08 PM   #5
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This link is a good read on altitude versus tire pressure change....

https://m.tirerack.com/tires/tiretec...jsp?techid=167
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Old 02-05-2019, 10:31 PM   #6
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This link is a good read on altitude versus tire pressure change....

https://m.tirerack.com/tires/tiretec...jsp?techid=167
Excellent: So in summation, the difference between sea level and 10,000 feet is about 4.7 PSI. I get that much change and more just from the warming of the tires from driving. So "negligible" is the answer.
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Old 02-05-2019, 10:46 PM   #7
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The procedure for tire inflation is not complicated. It just says to inflate them to the recommended inflation pressures when they are cold. Cold meaning they have set there for about three hours.

Tire inflation pressures for RV trailers is critical. The loads on each tire position vary, sometimes to extremes. When using any tire pressure monitoring system you should insure its low warning goes off at the recommended inflation pressure.

Tire inflation pressures are directly proportional to the load they can carry.

The Michelin truck tire data book has the best description for tire thermodynamics. It's accounted for by each tire manufacturer.
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Old 02-06-2019, 04:08 AM   #8
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So I am a bit confused with all the recommendations I read on tire inflation. My tires say to inflate to a max of 75psi cold. So if I inflate to 75psi when cold, and they heat up during travel, aren't they over inflated, or does the few psi increase because of heat fit within manufacture expectations on the tire?

Or do I inflate (cold) to 71psi and leave room for expansion?
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Old 02-06-2019, 04:57 AM   #9
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CWtheman is on it, as usual.
Air the tires up to max pressure when cold. I prefer to do that in the mornings when they haven't rolled or had the sun shining on them. Get a good quality air gauge. I use a digital from Joes Racing. It always matches what the truck TPMS says. I've found the pencil pocket type aren't very good. Don't chase the pressures. The tires are designed to pressure up in use.
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Old 02-06-2019, 06:53 AM   #10
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Originally Posted by CWtheMan View Post
The procedure for tire inflation is not complicated. It just says to inflate them to the recommended inflation pressures when they are cold. Cold meaning they have set there for about three hours.

Tire inflation pressures for RV trailers is critical. The loads on each tire position vary, sometimes to extremes. When using any tire pressure monitoring system you should insure its low warning goes off at the recommended inflation pressure.

Tire inflation pressures are directly proportional to the load they can carry.

The Michelin truck tire data book has the best description for tire thermodynamics. It's accounted for by each tire manufacturer.
The one thing is disagree with you about, is the low pressure alarm point setting if using TPMS. If you set the low pressure alert at the same pressure setting as the recommended inflation number, there is a very high chance that your TPMS System is going to be in alert mode quite often. As the tire cools back off and gets to ambient temperature, the pressure would go back down to the psi that you set it at and trigger a "nuisance" alarm. I keep mine set about 3 psi below the pressure that I run in the tires. If it alerts, something has changed...I.e. colder ambient temperature....slight leak....elevation change..etc.
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Old 02-06-2019, 08:17 AM   #11
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I have always set the tire pressures at cold temp irregardless of altitude FYI the automotive industry considers cold temp to be room temperature ie 70 deg f or 20 C. If altitude was a big factor it would be stated on vehicle tire inflation decals
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Old 02-06-2019, 09:17 AM   #12
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Like has been said, inflate to recommended PSI when the tires are cold -- the recommended PSI as stated by the coach manufacturer -- not the maximum PSI shown on the sidewall. The tire is designed to compensate for the rise in pressure from normal heating from driving on them.

When I got my trailer tires installed, I asked the installer how many PSI they put in the tires. They told me something like 75. I told her that I wanted the recommended 80 psi in the tires. She said they put 75 in there so they wouldn't be overfilled when they warmed up. As Bugs Bunny says, "what a maroon!" I topped them up to 80 psi on my own before I left their establishment.
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Old 02-06-2019, 10:21 AM   #13
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The one thing is disagree with you about, is the low pressure alarm point setting if using TPMS. If you set the low pressure alert at the same pressure setting as the recommended inflation number, there is a very high chance that your TPMS System is going to be in alert mode quite often. As the tire cools back off and gets to ambient temperature, the pressure would go back down to the psi that you set it at and trigger a "nuisance" alarm. I keep mine set about 3 psi below the pressure that I run in the tires. If it alerts, something has changed...I.e. colder ambient temperature....slight leak....elevation change..etc.

My argument is #1; in the RV trailer manufacturer's regulations world, the recommended tire inflation pressure set by them for the Original Equipment tires is the MINIMUM requirement for that application. #2; being 3 PSI below minimum in your application costs you about 100# of lost load capacity per tire.


RV trailer owners load their trailers differently. Their argument is they don't need the tires inflated to max pressure. Why not? It's not going to hurt the tires and the trailer was built to sustain max pressures. And, gaining load capacity reserves is a great preventative of early tire failures.
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Old 02-06-2019, 10:37 AM   #14
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Quote:
Originally Posted by Dblhack View Post
So I am a bit confused with all the recommendations I read on tire inflation. My tires say to inflate to a max of 75psi cold. So if I inflate to 75psi when cold, and they heat up during travel, aren't they over inflated, or does the few psi increase because of heat fit within manufacture expectations on the tire? YES!

Or do I inflate (cold) to 71psi and leave room for expansion? NO!

The PSI value on a tire's sidewall is not a recommendation. It is the amount of cold inflation pressure needed for the tire to provide its maximum load capacity, also shown on the tire sidewall.


The recommended inflation pressures presented on the vehicle certification label, tire placard and in the vehicle owner's manual is the correct inflation pressures for the Original Equipment tires and any replacements with the same size designation.
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Old 02-06-2019, 11:06 AM   #15
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Originally Posted by CWtheMan View Post
My argument is #1; in the RV trailer manufacturer's regulations world, the recommended tire inflation pressure set by them for the Original Equipment tires is the MINIMUM requirement for that application. #2; being 3 PSI below minimum in your application costs you about 100# of lost load capacity per tire.


RV trailer owners load their trailers differently. Their argument is they don't need the tires inflated to max pressure. Why not? It's not going to hurt the tires and the trailer was built to sustain max pressures. And, gaining load capacity reserves is a great preventative of early tire failures.
I wasn't suggesting or stating that I run the tires 3 psi under the recommended psi, I was saying that the low pressure set point for alarm from the TPMS is at 3 psi below the recommended pressure. Run the tires at 80 psi cold.....give me an alarm at 77 psi.
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Old 02-06-2019, 12:28 PM   #16
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I wasn't suggesting or stating that I run the tires 3 psi under the recommended psi, I was saying that the low pressure set point for alarm from the TPMS is at 3 psi below the recommended pressure. Run the tires at 80 psi cold.....give me an alarm at 77 psi.

From that statement it looks like you're allowing them to go 3 psi below minimum.
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Old 02-14-2019, 11:39 AM   #17
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If you are really concerned about inflation as altitude increases, inflate the tires to maximum pressure limits of the tires when cold using nitrogen and press on.

Nitrogen is what is used in aircraft tires where altitude and pressures are of great concern and works very well.
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Old 02-14-2019, 01:49 PM   #18
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Nitrogen is inert so it does not interact with the rubber in case of fire.
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Old 02-14-2019, 07:14 PM   #19
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I have always set the tire pressures at cold temp irregardless of altitude FYI the automotive industry considers cold temp to be room temperature ie 70 deg f or 20 C. If altitude was a big factor it would be stated on vehicle tire inflation decals
concours, do you have a link or other reference source regarding the 70F/20C? I've searched for what temperature the tire pressure ratings are specified at but haven't been able to find anything. I even contacted Goodyear but got a useless answer. I've assumed 60 or 70F but don't like making assumptions.

Here in AZ the Cold/Ambient temp can vary widely based on season and altitude. A cold January morning might be 40F while a "cold" July morning might be closer to 90F. According to Tire Rack the rule of thumb is ~2% change in tire pressure for each 10 deg change in temp. So for my 110 PSI tires I could see an 11 PSI diff between winter and summer. Or if I'm headed into the mountains from the desert, with say a 40F diff in typical "cold" morning temp, the baseline pressure difference is about 9 PSI.

While the over/under inflation may not be too bad if you're within 10-20F of the temperature the manufacturer used to spec the tire pressure rating, it can become significant when you get 30, 40 or even 50F from the temperature used to spec the tire.

As a retired engineer its always bothered men that tire manufacturers don't provide a temperature to go with their PSI specs. I guess the company lawyers just think that extra detail would confuse customers...?

Thanks
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Old 02-14-2019, 08:34 PM   #20
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Nitrogen is inert so it does not interact with the rubber in case of fire.

True but meaningless. I have never heard of a tire catching fire because the inflation "air" got too hot.


If a tire does catch fire because of a wheel bearing failure where the grease catches fire and spreads to the tire sidewall the N2 on the interior will have no impact till the tire failed because the sidewall burnt up.
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