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Old 09-16-2011, 02:20 PM   #1
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Tire Pressure

For 19.5 in. tires what is good practice in adjusting the 80 psi pressure for local ambient conditions.
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Old 09-16-2011, 02:29 PM   #2
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I would be more worried about adjusting for weight then ambient conditions. When you check your tires cold that is ambient conditions.
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Old 09-16-2011, 02:54 PM   #3
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What is a good practice for adjusting air pressure around the 80psi. When traveling from Fl to Nova Scotia late summer I experienced a 30 def change in ambient temp and a resulting 5 psi drop in tire air pressure.
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Old 09-16-2011, 03:49 PM   #4
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Not sure what chassis you're rig is on as weight determines what tire pressure to use. Have a friend with a 34 ft F53 chassis rig. For his 19.5 pressure he needs 95 lbs in front and 80 in the rears.
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Old 09-16-2011, 03:56 PM   #5
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Forest River recommended pressure for front and dual rears is 80 psi.
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Old 09-16-2011, 03:58 PM   #6
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I'm sure a search here would turn up the formula for temperature's impact on PSI but I don't recall it off hand. I think most folks just ignore the impact if they're just "passing through"... but if I'm going to be traveling for days in an environment which is significantly different than the one I first set my pressures at... I will adjust the pressures back to their proper PSI to correct the impact the current environment is having.

Good luck

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Old 09-16-2011, 04:43 PM   #7
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I had asked a similar question on another forum. I did a little research and found the following. "The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower)."

Based on the above information, I figure that airing my tires to 100 psi when the ambient temperature is 70 degrees will allow me to not chase tire pressures. I uses a TST TPMS and watch my pressures. I be darned if I am going to chase tire pressure based on ambient temperature. I have better things to do.

Here's the link if you want to read the entire article. Tire Tech Information - Air Pressure, Temperature Fluctuations
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Old 09-16-2011, 05:12 PM   #8
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Quote:
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Based on the above information, I figure that airing my tires to 100 psi when the ambient temperature is 70 degrees will allow me to not chase tire pressures. I uses a TST TPMS and watch my pressures. I be darned if I am going to chase tire pressure based on ambient temperature. I have better things to do.
That actually makes a lot more sense. If traveling from warm to cold just bump a few more pounds in before you leave. That can't be enough to harm the ride and gives some margin when the temps drop and PSI goes down.

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Old 09-16-2011, 07:59 PM   #9
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Quote:
Originally Posted by OneRVer View Post
I would be more worried about adjusting for weight then ambient conditions. When you check your tires cold that is ambient conditions.
There have been dozens of threads and thousands of post regarding tire pressures and it always comes back to ...
weigh the rv and set the pressure according to the weight.


On a recent trip from Texas to upstate New York I adjusted the pressure when I left (108 degrees) and had to add 10 pounds when I got up north with temps in the 50's. I did not let air out when I got home because winter had arrived-it was 95 degrees. I'll run this pressure during the cooler months and then readjust (if needed) next summer.

Weigh your rv and run those pressures.
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Old 09-16-2011, 10:14 PM   #10
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I'm not going to quote here, but let's get facts straight. You can find this information in any manufacturer's tire guide, or call them for information.

For every 10F degrees of temperature change there will be a 2% (Read that Percent) change in tire pressure. So for an 80 psi tire, 30 degrees of temperature change will result in 4.8 psi change in pressure. A tire gauge would most likely record that as 5 psi.

For every 1000 feet of altitude change there will be a 0.48 psi change in tire pressure, so at 2000 feet of altitude change you would see 0.96 psi, and a gauge would most likely read that as 1 psi of change. (Go up in the world and you increase pressure, go down in the world and you decrease pressure)

Make sure you combine the two when doing calculations.

Those are facts, pure and simple.

Without knowing the make, exact tire size, and the weigh of at least the axle, and preferably the weight of each corner of the RV, it would be impossible to tell you what your tire pressure should be to keep in consonance with the manufacturer's charts.

Running your tires at the maximum allowable tire pressure, which is most likely 80 psi on your tires for a single tire on an axle, is not in itself wrong. You would then have to gauge your tire each morning before a trip and adjust to the 80 psi. If I know the specifications of your tire, there may be an option for a fudge factor. I cannot even begin to give you those factors without knowing anything about your tires or weights.

Happy trails.
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Old 09-16-2011, 11:17 PM   #11
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Aw heck!
Okay, I'm just going to assume some information.

Just for the sake of explanation, let us assume that the tires are Goodyear 225/70R19.5, okay?

Lets start out with a simple chart.

For single tires per axle

PSI---------------70-------75-------80------85------90-------95---------100 (There's more but let's stop there)
Weight(S)------2895-----3040----3195---3315---3450---3640(F)-----3715 (F = Load range F)

Now, let us assume that the weight on your tire is 3040 per tire on a single axle (The psi setting has to be the same for all tires on the same axle)

So, let us assume that you inflate your tires to 75 psi. I mean, after all you just came back from the CAT scale and you know what your weight is. You drive home pick up the wife, 50 pound dog. The dog sits at DW's feet. Total weight is an additional 150 pounds on the passenger tire. (You didn't think I was going to make your DW heavy, did you?) You are 150 pounds overweight on the passenger tire. Even if you had distributed the weight evenly across the axle, you would still be 75 pounds per tire overweight. If you experienced a drop in temperature or altitude, you would be additionally overweight. (Remember, this is just an example - I actually like working with the 22.5 inch tires)

So do the fudge factor:

Inflate the tires to 80 psi, and the tires are now rated for 3195 pounds. Now you have a fudge factor of 155 additional pounds per tire, or a total of 310 pounds you can put in the RV distributed across the axle. See where this is going. No? Okay, now stop at Wall-world for the night, go in, buy some groceries, supplies, whatever, and make sure you go to the bathroom scale and weigh all that stuff, because you are not overweight on the tires. Don't forget that DW and Fido are using up 150 pounds of that, so you only have a margin of 160 pounds. Don't go to Mcdonald's every night and eat hamburgers. Don't fret - more fudge coming.

Inflate the tires to 85 psi, and the tires are now rated for 3315 pounds per tire. That is 275 pounds per tire, or 550 pounds across the axle. DW and Fido can be comfortably safe and do not have to have salad every night.

Now let us say that you are running at 85 psi, and when you checked the tires at that pressure the ambient temperature was 90 degrees (I'm in Texas). You are at sea level. You head North. The temperature drops to 60 degrees. That represents a 6% decrease in pressure. 85 minus 6% equals a 5.1 psi change, and the gauge will only show a 5 psi change. So you are at 80 psi the next morning when you get up. Oops, wait a minute, you gained 2000 feet in altitude. That is a 0.96 psi increase in pressure, so your actual tire pressure reading should be 81 psi. Look at the chart. Are you still within range of the tire manufacturers recommendations? Do you need to change the tire pressure by adding more air? Nope. Smile, be on your way. If you go to the other extremes, like from 90 degrees to 120 degrees you are going to gain 5 psi which will put you at 90 psi plus one for altitude of 0.96 psi so you are at 91 psi. That particular tire in this example goes to 110 psi at a max load of 3970(G Rated tire). Are you still within the parameters. Most definitely.

You can do the math for the duals, and remember their load rating for this specific tire is 3115 per tire. That is a total axle load rating of 12, 460 pounds. Your front steer tires, single tire on each end of a single axle at 85 psi is 6630 pounds.

I cannot express this enough. Never let air out of a hot tire that has not set for several hours and come to ambient temperature. The best time to check a tire is early in the morning. If the sun is hitting the outer dual, it will normally be at a higher psi than the inside dual. If both are on the shady side, the inside dual may be a tad higher that the outside dual, because the inside dual is getting heat from the RV. After about a mile down the highway they will both catch up with each other.

When to check tire pressure:
  • Before each trip
  • Every morning during long trips
  • Before you leave and when you return home on short trips
  • Before and after storing your vehicle
  • AT least once per month while the vehicle is in storage
The only way to know what the proper inflation for your tires are to weigh the vehicle, preferably at each corner, and use the manufacturer's tire inflation chart.

Happy trails. If I made a mistake in the computations, well, I'm not a tire engineer nor a math major.

Edited: Here is a link to the Goodyear Tire Inflation Loading web site.
Once there click on the blue "Download RV Tire & Care Guide."
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Old 09-17-2011, 12:11 AM   #12
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... because winter had arrived-it was 95 degrees. .
Sorry Jim... that's just not a common expression.

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Old 09-17-2011, 09:51 AM   #13
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Sorry Jim... that's just not a common expression.

Rick
After 90+ straight days of 100+ degrees and no measurable rainfall in months...it felt good to have a high of only 95.

Now back to the thread. No hijack intended.
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Old 09-17-2011, 10:40 AM   #14
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Wayne,

I think you have posted some good information. However, I don't think that OTR truckers constantly manipulate tire pressures. Perhaps they inflate to MAX cold tire pressure based on 70 or 80 degrees F. I don't personally know any truckers so I can't say for sure what they do. I have read a lot of posts, some by truckers that discuss whacking the tires with a baseball bat to see if they are okay. To me, whacking a tire with a bat is about the same as thumping a watermelon. Odds are 50/50 that the melon is ripe. Same goes with the proper inflation of a tire using a baseball bat for a gauge.

However, I agree that you must know what our coach weighs and you should set your pressures using your tire manufacturers inflation chart. To avoid the necessity to manually check the tires, purchase and install a good Tire Pressure Monitoring System (TPMS) and let it do the work.
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