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Old 04-20-2016, 01:31 PM   #1
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Maximum Tire Pressures

It's been stated here several times that the RV tire pressures listed are the minimum psi to carry the stated load. If 120 psi is the minimum needed for the max rated load of 7830# on my 295X80/22.5 Michelin tires, what is the maximum cold psi allowed for the tire? A call to the company said that the 120 psi was the "Maximum" pressure for the maximum load of 7830#.

So my question is which is it? The company says the psi values are the amount needed to carry the load, but also that the 120 psi on the sidewall is the "Maximum Cold" psi. If that is the case, then the recommended psi values for an RV tire is the same as for any other tire and not different as so many here have claimed.
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Old 04-20-2016, 01:34 PM   #2
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Well here is what the major tire companies put on IN PRINT in their brochures/fact sheets along with quotes from major magazines:
Quote:
The pressure on the sidewall of a Michelin RV truck size 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. NHTSA defines a truck tire as those used on anything with a GVWR of 10,000#'s or more.

From the 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.
From our owners manual:
Quote:
Federal law requires that the tire’s maximum load rating be molded into the sidewall of the tire.
If you look there, you will see the maximum load allowed and the cold air inflation pressure required to carry that stated maximum load. Less air pressure restricts the tire to carry a lighter load.
I'm also thinking that whoever you talked to at a tire company was thinking of car type tires, (less than 10,000#'s GVWR) where it is the maximum cold pressure the tire should ever have.
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Old 04-20-2016, 02:48 PM   #3
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My rims also have a max PSI of 120. I guess that makes it a max!
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Old 04-20-2016, 03:39 PM   #4
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I gave the person the model of tire I have along with the size and load I was carrying which is 7300#. She confirmed that 110 psi was the minimum. I have always carried 115 psi cold and just started to wonder if the load was 7830 and the minimum to carry it is 120 psi, what is the maximum that could be used? It was there that she pointed out that the 120 was also the max, however that the pressure could increase up to 150-155 psi when running at 65 mph on a 100* day on asphalt. Maybe I need to get a second opinion. Increasing the cold psi isn't something I'm going to do, but just wondering???
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Old 04-20-2016, 03:42 PM   #5
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RV Tire Safety: Max Inflation

RV Tire Safety: Minimum Inflation
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Old 04-20-2016, 04:19 PM   #6
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Crasher, what does it actually say on the tire ?

Could be that things have changed, in tire markings, leading to the confusion.
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Old 04-20-2016, 04:43 PM   #7
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Thanks, "1bigmess," for the links. I thought the most helpful paragraph was:

"However, for most practical purposes, on TBR tires the pressure marking is typically considered the maximum pressure recommended in the tire while in ordinary service. Certain situations may permit cold inflation pressure higher than the marking, usually in consultation with a tire manufacturer for a specific product, application, and service."

Here's a statement that might satisfy everyone:

"The pressure listed on a manufacturer's inflation table is the minimum cold inflation pressure necessary to support the load listed in the table."

It follows that the highest pressure listed on the manufacturer's inflation table is the minimum cold inflation pressure necessary to support the maximum load- as "Mr_D's" citations attest. The sidewall markings of RV tires should reflect this. (It would be interesting to see if the tables and the markings match. I don't have any tires nearby with which to confirm or deny this hypothesis.)

Per the paragraph in blue, above, and in accordance with common sense, the tire manufacturer does not want us to exceed the maximum load on each tire. In his mind, this also means not exceeding the listed maximum pressure (i.e., "the minimum cold inflation pressure necessary to support the maximum load"). On the other hand, he doesn't want us running any load with under-inflated tires. (see Toyo's answer in "Mr_D's" post for a sly way of telling us to inflate above the table values, but without guidance on how much)

It appears that each tire owner is on his own to decide how much, if any, he wants to inflate his tires above the cold inflation pressures listed in the inflation table for his tires for the known loads on the tires. This is true at any level of known load, even the maximum, unless the manufacturer has specifically listed an absolute maximum cold inflation pressure value, or similar guidance, in his documentation.
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Old 04-20-2016, 04:49 PM   #8
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Or you could try to get in touch with the tire manufacturer and see what they say about your particular situation. No one will ever know more about the tires than the folks that made them (as long as you can talk to someone that knows about the tires and not someone reading from the attorney approved script).
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Old 04-20-2016, 04:56 PM   #9
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Quote:
Originally Posted by 1bigmess View Post
Or you could try to get in touch with the tire manufacturer and see what they say about your particular situation. No one will ever know more about the tires than the folks that made them (as long as you can talk to someone that knows about the tires and not someone reading from the attorney approved script).
Which is why what I posted are direct quotes from the tire manufacturers literature or known magazine sources.

And, yes, the max pressure on the wheels also comes into play, and again, it's the maximum cold pressure as the pressure rise due to use is taken into account when designing the wheels.
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Old 04-20-2016, 05:14 PM   #10
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2 years ago I got a new set of Michelin 295/80R22.5 XZA2 with a max PSI of 120 lbs. I had my coach weighted at the FMCA Rally in Albany,Or (4 corner wt) and I'm over wt in the rear by 1150 lbs and under in the front by 730 lbs so I moved some of the heavy things to the front bay when we got home. With a combined wt of 33,620 lbs FR 6,405lbs-FL 6,065lbs RR 10,195lbs-LR 10,955lbs I was told to run 105 PSI in the front and 100 PSI in the rear. They give you a print out of your coach wt's and the air pressure to run. This is the only way to get a true wt reading and know what PSI to run, money well spent.
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Old 04-20-2016, 06:33 PM   #11
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Crasher, what does it actually say on the tire ?

Could be that things have changed, in tire markings, leading to the confusion.
The markings on the tire states max load single is 7830# at 120 psi. No mention of 120 being the minimum or maximum. The wheels are rated at 131 psi max. Theoretically according to what Mr D has posted, I could run the tires at 131 psi cold. Not that I will, but this is what I read into the information provided.
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Old 04-20-2016, 06:59 PM   #12
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Quote:
Originally Posted by Crasher View Post
,,,If 120 psi is the minimum needed for the max rated load of 7830# on my 295X80/22.5 Michelin tires, what is the maximum cold psi allowed for the tire?,,,,
120 PSI. Your maximum and minimum pressure converged when you load a tire to its full design load capability.

Drop the load weight to 7570 and the minimum pressure is now 115 but if you wish you could air up to 120. Drop the weight all the way down to 5375 and the minimum is now 75 but still if you want you can inflate to 120. Think of 120PSI as your never exceed cold pressure while the minimum with vary with loading.
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Old 04-20-2016, 07:20 PM   #13
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120 PSI. Your maximum and minimum pressure converged when you load a tire to its full design load capability.

Drop the load weight to 7570 and the minimum pressure is now 115 but if you wish you could air up to 120. Drop the weight all the way down to 5375 and the minimum is now 75 but still if you want you can inflate to 120. Think of 120PSI as your never exceed cold pressure while the minimum with vary with loading.
That is what I was thinking when I started the discussion, but was looking for confirmation. If this is true, then there is no difference between the lighter LT tires that have similar load charts that I have used for years on my trucks and autos. I weighed my first Avalanche and ended up using 28-30 psi which is what the load chart called for even though the tire said 40 psi for max load, and the tires lasted 80,000 miles. So all tires, RV or otherwise, require a minimum psi to carry a given load and all tires have a maximum psi for any load up to it's max load rating.
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Old 04-20-2016, 07:22 PM   #14
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Ok, what is considered COLD air pressure? If it is prior to tire heating from travel you can be all over the map with the pressure before you start out if the sun is on some tires but not others in the morning (inside dual vs. outside dual). Plus, quite a bit of difference starting out at 30 degrees and driving into 100 degrees, or starting at 80 deg and driving into 30 deg weather.
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