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Old 04-22-2015, 05:06 PM   #1
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Tire pressure deviation

I'm trying to figure out tolerances for tire pressure deviation. We followed TST deviation ranges for set up.

But here is my story.

We installed Borg's on the rear duals and added TST TPMS to all 6 tires. Fast forward a week or so afterwards and we're loosing air pounds. So I tighten up the valves (Schneider?) and the TPMS after topping off and checking pressure with a high quality gage. 24-48 hours out, everything is holding. However yesterday pressures are down a couple of pounds in some of the tires. Their temps went from 68 degrees to 46.
I'm figuring the cold drop in air temp form mid / low 55 - 60 air temp to low 40s high 30s is the reason.

I'm keeping a log to chart each tire. But is it normal to experience a drop of a few pounds as temp dips?
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Old 04-22-2015, 05:21 PM   #2
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I' m not a tire expert but I believe that every 10 degree drop in ambient temperature, there will be at least a one (1) pound drop in tire pressures.
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Old 04-22-2015, 05:49 PM   #3
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you set your tires to the air temp you are at (cold pressure). if you stay there the only change will be if the temps rise or fall. If you drive any where expect the pressure to go up from the heat of driving if the place you went to is the same temp as where you were the temps will fall to what you set them at. If it is colder or hotter then you need to set the pressures again if you remain there any time.
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Old 04-22-2015, 05:59 PM   #4
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Tire pressure rises with temperature and altitude. On my car there can be a 10% variance between temp and alt.
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Old 04-22-2015, 09:12 PM   #5
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Quote:
Originally Posted by WDW View Post
I'm trying to figure out tolerances for tire pressure deviation. We followed TST deviation ranges for set up.

But here is my story.

We installed Borg's on the rear duals and added TST TPMS to all 6 tires. Fast forward a week or so afterwards and we're loosing air pounds. So I tighten up the valves (Schneider?) and the TPMS after topping off and checking pressure with a high quality gage. 24-48 hours out, everything is holding. However yesterday pressures are down a couple of pounds in some of the tires. Their temps went from 68 degrees to 46.
I'm figuring the cold drop in air temp form mid / low 55 - 60 air temp to low 40s high 30s is the reason.

I'm keeping a log to chart each tire. But is it normal to experience a drop of a few pounds as temp dips?
That is normal, and is also why you should not run the minimum air pressure for the corresponding load, as seen in the load/inflation charts. Run the mfgrs. tire pressure listed on the placard in the coach.
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Old 04-23-2015, 05:54 AM   #6
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The altitude does not influence the pressure that much, but warm and cold inside tire temp does more.
I have made a spreadsheet for it ,wich calculates it exact.
In part 3 you can even see what temp-chanche is when filled in old temp and old and new pressure.
Even the outside pressure can be chanched , given in millibar, standard set to 1013mb, then you only have to determine howmuch the pressure chanches by altitude.

Its more accurate then the rules of tumb, wich only work within a sertain rainch.
Cold pressure is when inside tire temp is same as outside, so if you know outside then you know inside. Warm is more difficult, how hot is it and howmuch has it cooled down already. Sunshine or severe braking can highen up the inside tire temp too.

Here the link to a map on my one-drive that belongs to my hotmailcom adress with same username as here, download first by rightclicking the pressurecalculationwithtemp spreadsheet, but also motorhomeRV spreadsheet there.
https://onedrive.live.com/?cid=A526E...e092e6dc%21793

But also I will include a picture of an example to show what you can do with it. First made it for races, but planned to review it in time.



Succes with it, but if you want me to help , ask here .
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Old 04-23-2015, 06:36 AM   #7
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Quote:
For every 10 degree drop in ambient temperature, there will be a 1 lb PSI change.

The effects of temperature and atmospheric pressure.

Air temperature and atmospheric pressure effect tire inflation pressure. If the outside temperature increases 10 tire inflation pressure increases approximately 2%.
Conversely, when the outside temperature drops 10, the tire inflation pressure lowers approximately 2%.

Tire inflation pressure increases approximately .48 psi for every 1,000 feet of altitude due to changes in atmospheri pressure. On the other hand, tire inflation pressure will decrease approximately .48 psi for every 1,000-foot decrease in altitude.
In other words, if there are changes in temperature or altitude during your trip, it’s important to check your tire inflation more frequently.
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Old 04-23-2015, 06:41 AM   #8
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nitrogen

if affordable fill them with nitrogen, not affected by temp.

Jim
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Old 04-23-2015, 07:26 AM   #9
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Why wouldn't nitrogen be affected by temperature? PV=nRT doesn't apply?
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Old 04-23-2015, 07:50 AM   #10
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Air is about 80% nitrogen, and the pressure of nitrogen-filled tires will still change with temperature. However, since the nitrogen used for tires is dry, the pressure may not change as much as with air.
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Old 04-23-2015, 07:54 AM   #11
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Quote:
Originally Posted by Triker56 View Post

The effects of temperature and atmospheric pressure.

Air temperature and atmospheric pressure effect tire inflation pressure. If the outside temperature increases 10 tire inflation pressure increases approximately 2%.
Conversely, when the outside temperature drops 10, the tire inflation pressure lowers approximately 2%.

Tire inflation pressure increases approximately .48 psi for every 1,000 feet of altitude due to changes in atmospheri pressure. On the other hand, tire inflation pressure will decrease approximately .48 psi for every 1,000-foot decrease in altitude.
In other words, if there are changes in temperature or altitude during your trip, its important to check your tire inflation more frequently.
I hope you do not mean to let air out as the day warms up?

I run semi from ND to FL. During the winter, I may leave ND at -20, and it could be 80 in FL.

I check the tires, and set the pressure in ND, with the tires at -20. Then forget them, other than making sure they are not losing pressure. Ignore the temp rise as you go south, as in a few days I will be going to cold again.

If however, you leave ND in Jan for FL, and will be staying there for several months, and DRIVING the vehicle, then I would readjust in FL. If you are just going to basically park it there, and drive a toad, I would leave them alone.
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Old 04-23-2015, 08:09 AM   #12
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Great Topic! Temp does affect the PSI, as it gets colder the PSI will condense and as it gets warmer the PSI will expand. If going from -20 to 80f I would definately check the PSI and adjust accordingly. It is very common to see the PSI rise as much as 20% when going from a cold tire in the morning, to a warm tire after driving for a while. When the tire cools down after driving it should go back to the same PSI as when you started.
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Old 04-23-2015, 01:29 PM   #13
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Air is about 80% nitrogen, and the pressure of nitrogen-filled tires will still change with temperature. However, since the nitrogen used for tires is dry, the pressure may not change as much as with air.
I have an Excell file with partial pressure of water and this makes that if temperature in tire rises from 18degrC/65 degr F to 45degrC/112 degr F , the partial pressure of water in tire rises from 0.02 bar to 0.096 bar. A rise of the pressure by only water in the tire would only be 0.076 bar wich is about a good 1 psi rise in pressure when going from standard cold to average warm tire.
So even the absense of water in nitrogen tires wont have such effect that its noticable. Another mith on Nitrogen filling de-mithed.
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Old 04-23-2015, 01:46 PM   #14
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It's also amazing how a DP exhaust heats up you toads front tire.
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