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Old 11-08-2018, 02:22 PM   #1
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Setting tire pressures on cold days...

I am looking for a formula for adjusting "cold inflation" pressures for large tires. I have read that for smaller (car) tires the recommendation is 1 psi per 10 degrees F.

What is also not clear is what the assumed "cold" temperature is. Is assumed to be 65 F, for example. So if we are sitting at 25 F, what is the adjustment?
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Old 11-08-2018, 02:33 PM   #2
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https://www.michelintruck.com/assets...k_Sept2011.pdf

Check page 91 for cold wx adjustment.
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Old 11-08-2018, 02:49 PM   #3
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According to Michelin, under certain conditions, no adjustments are necessary...


"These adjustments are only necessary if the pressures
are verified and adjusted inside a heated garage with an
air supply that is also at the higher room temperature.
(No adjustment necessary if done outside.)"
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Old 11-08-2018, 03:55 PM   #4
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Thanks guys - that is a better explanation than I've come across. What it is saying is if my weight and tire combo calls for 90 psi, and the temperature drops to -10 F, I need to inflate my tires back up to 90 psi from what ever they have fallen to.

Conversely, as I head into warmer weather I may have to release air.
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Old 11-08-2018, 04:16 PM   #5
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I could be wrong, but what I got out of it was that if you are outside and checking your tire pressure at 40 degrees and the air compressor is at about the same temp. there is no compensating for temp.. If your gauge reads 85 psi and your weight dictates 90 psi, you need to add 5 psi. If it's 80 degrees out and your tire is at 85 psi the same 5 psi will need to be added to reach 90 psi. The compensation is only needed if you are checking pressures in a warm environment and then pull your rig out into a much colder environment.
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Old 11-08-2018, 04:35 PM   #6
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Here is my blog post that explains "Cold Inflation"
We are not in High school chem class so there is no adjustment need to get to "standard " temperature.


If you are running a TPMS, which you should be, you will soon recognize the normal range of temperatures and pressure variation in your tires.


I would not worry about adjusting inflation pressure is today's expected temperature is +/- 10F from yesterday. After all, you should be running +10% over the inflation you need to support the measured load.


Let's say your minimum needed is 90 psi. Adding 10% that means your cold inflation (cold meaning the tire was not driven on or in direct sunlight for the previous two hours) should be 99 psi so you round to 100 psi. All is good.


The next day's weather is 20F colder. That would mean the tire inflation will drop 4% ( 2% per 10F change)


So now your tires would be 96 psi but since you have to stay above 90 I would say there is no reason to worry about adding air. Just enjoy life and head out.


If the next day your weather went up 10F instead of down that means tour "cold" inflation would go up 2% from the 96 or to about 98psi. Again you don't need to do anything.


I would not worry about lowering the pressure until I saw pressures 10 psi above the pressure molded on the tire sidewall. When you get to that point you could bleed off a few psi but remember if the weather gets cold over the next couple of days you will be needing to add air again.



Don't get your shorts in a bunch about inflation. Just watch your TPMS and you will learn what is normal for your coach.
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Old 11-08-2018, 04:41 PM   #7
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Quote:
Originally Posted by Twomed View Post
https://www.michelintruck.com/assets...k_Sept2011.pdf

Check page 91 for cold wx adjustment.



Interesting chart but only applies if you are setting pressure in a heated garage and it is a lot colder outside. The adjustments would not apply if you were outside. I note that their table follows the 2% inflation for 10F change that I cover in my blog.
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Old 11-08-2018, 09:56 PM   #8
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Lots of places much colder than Akron , OH...didn't assume anything about garages or where the air compressor was located.

Just providing facts for the OP to apply.
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Old 11-09-2018, 07:56 AM   #9
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All these answers are correct but... I think you all are really overthinking this. There are millions of big rig trucks on the road and I'm going to bet that they don't worry about this stuff too often as long as their tires are kept aired up within an acceptable range.


I look at it this way. If my tires are rated for a max pressure of 120 psi for example and my load charts says I should be running 85 psi for example, I try to keep my tires inflated to about 90 - 95 PSI. I don't notice the difference in ride or performance. If the outside temp turns cold, my tires are still adequately inflated to handle the load. If the outside temperature rises, then I'm still below the max pressure rating of the tire. I've been RVing since 1982 and have never had an inflation issue. Just make sure you replace your tires every 6 years or so. That's the biggest safety issue.
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Old 11-09-2018, 09:57 AM   #10
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Thanks Tireman. I didn't save the link to your blog so was hoping you'd chime in.

I get it. Set the pressure where it needs to be regardless of the current temp. For me it has been an issue because I've had to load the coach and leave when it was -35 F out and the tire pressures were down considerably. No possibility to inflate them in a nice warm garage (which would have required some conversion and adjustment). By the time I got south of Utah the cold inflation temps were up to 60 F, and I did back the pressures off before heading over to Southern California.

BTW, you say "If you are running a TPMS, and you should...", I don't. I've always been fastidious about checking my tire pressures, including every day before we start driving and I carry a 150 PSI compressor so I can inflate the big tires if I need to. I never leave more than about a 2 PSI difference between tires. So far I've been fortunate and haven't had a tire fail in over 40 years.

Perhaps when I change out this set of tires I'll consider a TPMS, but it is a big job to pull eight wheels and tires unless you have to.
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Old 11-09-2018, 10:46 AM   #11
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You don't need to pull tires etc. for most TPMS systems. They screw on to your tire valves. They will give you a constant read on your tire's tire pressure but more importantly they will alert you to any quick air loss (beginning of a blow out situation) from any tire.
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Old 11-09-2018, 11:19 AM   #12
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Setting tire pressures on cold days...

Quote:
Originally Posted by distaff View Post
Thanks Tireman. I didn't save the link to your blog so was hoping you'd chime in.

I get it. Set the pressure where it needs to be regardless of the current temp. For me it has been an issue because I've had to load the coach and leave when it was -35 F out and the tire pressures were down considerably. No possibility to inflate them in a nice warm garage (which would have required some conversion and adjustment). By the time I got south of Utah the cold inflation temps were up to 60 F, and I did back the pressures off before heading over to Southern California.

BTW, you say "If you are running a TPMS, and you should...", I don't. I've always been fastidious about checking my tire pressures, including every day before we start driving and I carry a 150 PSI compressor so I can inflate the big tires if I need to. I never leave more than about a 2 PSI difference between tires. So far I've been fortunate and haven't had a tire fail in over 40 years.

Perhaps when I change out this set of tires I'll consider a TPMS, but it is a big job to pull eight wheels and tires unless you have to.


I commend your attention to your tire maintenance and pressure. There is one eventuality that your morning checks donít cover. On Highway tire damage. I overnighted at the Flying J in Black River Falls, WI and had been on the road less than an hour when my TPMS alarmed. Inside passenger side drive axle tire was low and falling. By the time I got off the interstate it was down to 80 psi. When I opened the door I could hear the leak.

A big lag bolt was driven into the center of the tread. About 4 hours & a new tire later we were back on the road with no damage except to my wallet.

At alternate scenario, using the morning psi check, the damaged tire would lose all pressure and itís mate blows out due to severe overloading and friction from the deflated tire beside it. Possibly taking out the tag tire behind it. Damage to the coach sidewall. Possible damage to other vehicles.

Of course all of the above is just speculation because my TPMS gave me a heads up to the psi loss.

Just another perspective but I donít drive without my TPMS functioning.
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Old 11-09-2018, 11:24 AM   #13
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X2 what Paul said.


RE checking your air. You might read my blog post where I told people to "Stop Checking Your Air"
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Old 11-10-2018, 07:25 AM   #14
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The points about the TPMS that were already stated are all valid reasons, but having TPMS on your toad as well is peace of mind if a tire goes down or blows on the toad you will know it, preventing real damage and/or a fire.
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