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Old 12-09-2009, 06:25 PM   #1
Art-n-Marge
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Air Inflation considerations.

A quick check of the internet and I was not successful. It is a duh moment for me.

I have read many times that when checking air pressure that the temperature will affect the psi by 1 psi per every 10 degrees, but I don't know where "center" is. For example, if the psi is supposed to be 80 psi at 70 degrees, then the psi should be 79 at 80 degrees, 78 at 90 degrees, etc. The opposite for a decrease in temperature.

The 64,000 question is, at what temperature is 0 degrees for 80 psi. I am needing to check the tire pressure for my rig and with the temps dropping I will need to add 1 psi per 10 degree drop but where does this start?
 
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Old 12-09-2009, 10:32 PM   #2
Tom S.
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It's the other way around. Each ten degree increase will increase the pressure by one pound. So tires inflated to 80 psi at 70 degrees will be 81 psi at 80 degrees, and 79 psi at 60 degrees.

I don't know what the median point is, but I suspect if you use 80 psi at 70 degrees, you'll never have a problem. My reasoning for this is you are only talking a 3 pound increase between 70 and 100 (the highest ambient temps most of us will see). On the other end, even if you are in 20 degree temps, the tires will show 5 psi under inflation, but as soon as you start to move, friction will cause the tires to heat up, which will bring the temp, and therefore the psi, back to normal - or at least close to it.

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Old 12-10-2009, 01:40 AM   #3
LonnieB
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Art, I believe you are trying to turn tire air pressure and ambient temperature into an exact science. It's not THAT critical . As Tom S. points out, the variance over a wide range of temperatures is minimal and adjusting for it is not necessary. Set the psi at 80 when the tires are cool and go on your way.
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Old 12-10-2009, 02:48 AM   #4
8e3k0
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It's people like use here in the northwest where the temperature can vary 120 degrees from summer to winter, and yes those tires if you don't take care of them at -40 will look pretty low!!! Not really a problem with the large tires on trucks etc. but the little SUV/ car tires with less air space must be maintained on a regular basis when you have a drastic temperature difference. It has been in the -15 to -30 here so I wonder what the air pressure is on the Monty as it sits out back in that temp where they were 80 psi at about 65 degrees F earlier in November.
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Old 12-10-2009, 05:35 AM   #5
Art-n-Marge
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Okay, LonnieB - I will use 80 psi even when cold, since it never gets below freezing. Tom S. - yup, I had it backwards, it's about the molecules shrinking in the cold and fattening in the heat. When it's hot, more air is needed to compensate.

8e3k0 - you brought up an excellent point that where I live I don't like to RV when it's too hot, and the Monty isn't going anywhere and it doesn't get that cold here so the temp differences are not nearly as critical as Lonnie mentions. I have never driven in below 0 temps. 0 was the lowest (Grand Canyon was at 0 at 1 a.m. as I completed our trip in the tent trailer.

I met some folks who were really making a big deal about this. I just listened cuz I didn't know that much about it. The psi is just not that big a difference to make such a big deal. Thanks all!
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Old 12-10-2009, 08:00 AM   #6
TLightning
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I set the TP for the correct pressure for the correct temp when I start out. If it needs adusting during the trip, I do it.
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Old 12-10-2009, 03:16 PM   #7
Waynem
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I run 110 psi, and when I parked the rv, they were set at 110 psi. I put a gauge on it this morning and with the temerature drop from the high 70's to the low 50's the psi now read 104.

I'm going to check it again in the morning before we leave and adjust accordingly.

Or, do I run it up to where I'm going, 90 miles away, and check it there.

Better to check it cold, right?
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Old 12-10-2009, 03:34 PM   #8
illapah
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Another interesting observation, check your tire pressure in the sun. The ones in the sun will be higher than the ones in the shade.
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Old 12-11-2009, 01:52 AM   #9
ols1932
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If I get ready to travel one day and I check the tires and find out they are all 5 psi low, I air them up to where they should be at the cold pressure. The next morning, if the temp has dropped about 10 degrees, I find that the pressure in each tire has dropped down about 1 or 2 psi, I just leave it alone. If, when I get ready to start out and the pressure in the tires is a couple pounds up, I leave them alone. It appears that we can get into a nail-biting problem if we're going to get all concerned about the tire pressures each morning. Just be close to what they should be. That's been my experience and I haven't had a problem caused by pressures being up or down. Just don't move without airing up if you are 5-10 psi low.

I didn't get this from any tire expert, it's been my experience. Tire experts I have talked to all seem to agree with my approach.

Orv
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Old 12-11-2009, 05:34 AM   #10
farmboy
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In 20 years of aircraft maint. we set air press. at the time of the preflight inspection irreguardless of where we were at or where the flight would land. I did this on both coast(CA.&S.C.) and South Dakota. The chart was only for gross weight of aircraft.
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Old 12-11-2009, 06:09 AM   #11
Art-n-Marge
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Orv is doing what I expected based on what I've learned about the relationship with pressure and temperature. Based on that, I think in Waynem's case and if it were me, 104 is too low if running at 110. It seems to be 3 psi too low and that's what I would air up to, about 107-108. In this case I don't think going all the way back up to 110 would hurt especially if only going 90 miles.

By George, I think I am getting it. It's a consideration, not an exact science. I think I should only be concerned when there are large differences.
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