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Old 03-08-2016, 10:31 AM   #1
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Looking for help with Tire Pressures!!!

Just got my RV weighed today preparing for next trip... Need help in figuring tire pressures PLEASE..
The sheet says 6380lbs Steer axle and 12940lbs for drive axle.
It is a 2004 Newmar 35' with Firestone Tires 19" single rear axle.
Could not get side to side weight.
Tires says 4300lb max for duos. Inflate to 110lbs max
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Old 03-08-2016, 10:38 AM   #2
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You might check Firestone's inflation tables for your type of tire. http://commercial.firestone.com/cont...12-21-2015.pdf
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Old 03-08-2016, 03:10 PM   #3
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Need more information as to the tire model. from there you look at the table indicated in the above post for the weight on one tire (at least it is that way with the table I have for Michelin) Cross reference the tires make/size and weight and viola you have the recommended cold pressure (or whatever the manufacturer specifies in the sheet)/ Many people add 5 lbs of pressure for safety as more air (at least 5 lbs more) is safer than not enough.
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Old 03-08-2016, 03:17 PM   #4
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Well, at least you have the weights even though only total axle weight. If, like most RV's, one side is far heavier than the other you only have a close guess now.
And
The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire.

From page 2 of the 06/07 Michelin RV Tire Guide:
Quote:
"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."
From page 6 of the GoodYear RV Tire and Care Guide:
Quote:
"How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load."
From TOYO:
Quote:
Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:
Quote:
The maximum load capacity allowed for the size tire and load rating and the minimum cold air inflation needed to carry that maximum load are located on the tire’s sidewall.
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Old 03-08-2016, 03:22 PM   #5
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Quote:
Originally Posted by davismills View Post
You might check Firestone's inflation tables for your type of tire. http://commercial.firestone.com/cont...12-21-2015.pdf
And direct from those charts:
Quote:
(Pressure Listed is the Minimum for the Load)
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Old 03-09-2016, 07:39 AM   #6
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Duh!! Wrong make tire they are actually Goodrich ST230

The actual tire are Goodrich 245/70R 19.5
Thanks for all the response.

So I am assuming from the response that I am actually below max weight on each tire with a little more weight on the rear tires.
So does that mean I want to run more PSI in the rear tires?


The Goodrich tire chart shows 110 PSI
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Old 03-09-2016, 10:22 AM   #7
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Since were not able to get four corner weight, you really will not know for sure what PSI is correct. Very possible to have one side of an axle to be substantially higher then the other. For example we had a 98 Bounder, with 75 fresh water tank in the very back right corner of the RV. When full, the right rear corner tire was carrying substantially more weight then the left. And of course you know, that you always set the PSI the same for all tiers on an axle. Drives and Steers and appropriate Tag's always match each other from side to side for PSI.

I follow Tireman9's blogs recommendation, and add contingency PSI to the top of the Tire Manufactures specific tire PSI. In this case, I think I would add 15% contingency, instead of his recommended 10%.

I also recommend at your earliest connivence, eight getting true individual scale four corner weights. Or doing the straddle of the scale so you can do the math from side to side for the axle.

I've two 12r 22.5 ST230's on our coach, and plan to add 6 more (tag's) in April. FMCA program makes these good tires a solid value, for a good tire.

Best to you, have fun, be safe,
Smitty
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