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Old 10-30-2016, 05:23 AM   #1
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Tire Pressures For Michelin

I just weighed my 2006 National Dolphin and the total weigh was 21,400 lbs. My max weight or GVRW is 22,00 lbs. I was not able to weigh all four corners or even one axle at a time due to weight scale limitations in this area. So for now, and because I am leaving tomorrow on a long trip South, I have to go with the overall weight. I spent a considerable amount of time a few days ago airing up all 6 tires and all 6 are currently at 90 PSI.
The Michelin tire chart says that at 90 PSI the max total weight recommended is :
Single: 4,140 lbs Dual: 7,530 lbs for a total of: (11,670 lbs x 2 = 23,340 lbs)
The chart says for 85 PSI the max weight is: 22,410 lbs and for 80 PSI the max is: 21,820 lbs.
My question for those with much more knowledge and experience than I have is:
Is it OK to leave all 6 of my tires at 90 PSI or am I better off lowering pressures to 85 or even 80 PSI to match the chart, and is there any advantage to doing this? Also, is there any danger or possible damage involved with running at the higher pressure of 90 PSI based on my current weight of 21,400 lbs versus my max of 22,00 lbs?
Thank you for any suggestions or advice.
Dave
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Old 10-30-2016, 05:53 AM   #2
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Dave-

If you don't have four corner weights, the conventional wisdom says you should inflate the tires to the values listed on the label in your coach- if the size and rating of the tires listed there match the size and rating of the tires on the coach. If they don't, then you have two choices:

a) guess, using the tire inflation charts and adding 10 psi (in your case, that means using the 90 psi), or

b) inflate to the pressure listed on the sidewall of the tire, which usually matches the highest pressure listed in the tire chart. NOTE: All pressures listed in the tire chart, including the highest one, are the minimum pressures required to carry the weight listed at those pressures. Adding 5 to 10 psi to the charted weights is common practice for many.

If you have four-corner weights, you can manipulate the pressures downward, using the charts and whatever margin above those values that you want to add.

I suggest that as your coach is close to its GVWR, you should get four-corner weights while you're down south, where suitable scales are supposedly more common. Your axles will appreciate it.
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Old 10-30-2016, 05:54 AM   #3
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Without knowing the weight on each wheel, or at a minimum each axle, I'd opt to keep my pressures at the higher end (not exceeding max cold pressure). Underinflated tires are a blowout waiting to happen!
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Old 10-30-2016, 08:42 AM   #4
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There will be a placard on the wall near the driver seat giving recommended tire inflation. It will assume near max weight (GVWR) and be safe for your travel. Use it.
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Old 10-30-2016, 10:56 AM   #5
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Yep, what others said if you can't scale it.
next time your out traveling, look for a truck route - there will be some big fueling stations that will also have scales avail.
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Old 10-30-2016, 11:09 AM   #6
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Quote:
Originally Posted by CarlAda View Post
Without knowing the weight on each wheel, or at a minimum each axle, I'd opt to keep my pressures at the higher end (not exceeding max cold pressure). Underinflated tires are a blowout waiting to happen!
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. NHTSA defines car tires as those rated for vehicles under 10,000#'s GVWR.

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.
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Old 10-30-2016, 11:30 AM   #7
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Quote:
Originally Posted by l1v3fr33ord1 View Post
NOTE: All pressures listed in the tire chart, including the highest one, are the minimum pressures required to carry the weight listed at those pressures. Adding 5 to 10 psi to the charted weights pressures is common practice for many.
I don't usually quote myself, but "Mr_D's" oft-made post on "maximum pressure" gives me an opportunity to correct an error in my prior post, as above. I believe that this corrected statement is a more general way of expressing "Mr_D's" information.
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Old 10-30-2016, 12:43 PM   #8
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Good info Mr. D!!!
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Old 10-31-2016, 09:48 PM   #9
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I had the same questions before my trip 2 weeks ago.

I have a 2005 Newmar Kountry Star on a W22, rated at 22,000lbs
I have new Michelin XRV 235/80-22.5 tires. The coach sticker said
to set the tires to 100psi front/85 rear. ( I think the original tires were
Goodyear) I set the tires to these pressures and went to the CAT scales
at a Pilot travel center.

Here were my weights: front axle 7040 lbs
rear axle 13220 lbs
trailer weight 3600, included my tow dolly and
2015 Honda Accord

Total weight of coach 20,260 lbs including all our stuff, full gas, and 2 people and the tongue weight of my dolly. I was very happy to be 1700 lbs under max capacity.

Using the Michelin load chart, the front needs a minimum 75 psi and
the rears a minimum of 75 psi.

So I am 25 Psi over minimum in front, and 10 psi over minimum in the rear.
Any issues with keeping what I am using?? I was thinking about bumping the rears up to 90 psi for added safety.

Thanks,
Russ
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Old 10-31-2016, 09:51 PM   #10
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I would set them for 85-90 max (at least in front). You will get a better ride and maybe a little more even tire wear/contact patch.
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Old 11-01-2016, 07:33 AM   #11
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Quote:
Originally Posted by RMFUN View Post
I had the same questions before my trip 2 weeks ago.

I have a 2005 Newmar Kountry Star on a W22, rated at 22,000lbs
I have new Michelin XRV 235/80-22.5 tires. The coach sticker said
to set the tires to 100psi front/85 rear. ( I think the original tires were
Goodyear) I set the tires to these pressures and went to the CAT scales
at a Pilot travel center.

Here were my weights: front axle 7040 lbs
rear axle 13220 lbs
trailer weight 3600, included my tow dolly and
2015 Honda Accord

Total weight of coach 20,260 lbs including all our stuff, full gas, and 2 people and the tongue weight of my dolly. I was very happy to be 1700 lbs under max capacity.

Using the Michelin load chart, the front needs a minimum 75 psi and
the rears a minimum of 75 psi.

So I am 25 Psi over minimum in front, and 10 psi over minimum in the rear.
Any issues with keeping what I am using?? I was thinking about bumping the rears up to 90 psi for added safety.

Thanks,
Russ
Because you only have front and rear total weights, and not the weight for each wheel position, you don't have all of the information needed to use the Michelin chart. Your weight/pressure calculations at this point include the implied assumption that each axle weight is evenly distributed from side to side. Because one side of each axle is almost certainly carrying more weight than the other (though you don't know how much), it would be prudent to add a "fudge factor" to cover this uncertainty. Then, since the weights calculated are minimums, most people would add an additional fudge factor to cover that. But even at those pressures, you are apparently well over in the front, and right on in the rear.

Assuming that your calculations are right (I checked the Michelin chart and got the same #s), I think you would be pretty safe with 85 lbs. all around. Going to 90 would be fine too, but there's no more reason to increase the rears than the fronts.
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Old 11-01-2016, 09:10 AM   #12
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You adjust your air pressure to the level needed to carry the load placed on the axle. No, you don't need 4 corner weights to do this just the axle weights. What other people inflate their tires to only applies to their coach and their measured weights.

I understand that you are weighing on a single platform scale. You easily got your total weight and you could, if needed, only weigh the front or backend on that single platform scale. That and some simple math could have gotten you axle weights. If you can get back to that scale try weighing the front end, the total, and then the rear end. If you can't do that, look on the web for CAT scales and find the location nearest you (usually at truck stops). CAT scales have three platforms and will weigh your axles individually in a single pass and for about $10.

You can then, with the use of a calculator and tire inflation tables determine what is correct for your coach. The calculation is simple:

Axle weight + 5% (this compensates for side to side differences) = Corrected axle weight. Divide Corrected axle weight by two for wheel weight and use your tire tables for the proper inflation pressures. Add 5 PSI to number for a safety/fudge factor.

Your dual axle is the same as the inflation table usually lists dual tire pressures as a pair. If it doesn't, then divide by 4.

Once you have done this, you have determined the correct tire pressures for your coach and your load.
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Old 11-01-2016, 02:54 PM   #13
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Quote:
Originally Posted by luvlabs View Post
You adjust your air pressure to the level needed to carry the load placed on the axle. No, you don't need 4 corner weights to do this just the axle weights. What other people inflate their tires to only applies to their coach and their measured weights.

I understand that you are weighing on a single platform scale. You easily got your total weight and you could, if needed, only weigh the front or backend on that single platform scale. That and some simple math could have gotten you axle weights. If you can get back to that scale try weighing the front end, the total, and then the rear end. If you can't do that, look on the web for CAT scales and find the location nearest you (usually at truck stops). CAT scales have three platforms and will weigh your axles individually in a single pass and for about $10.

You can then, with the use of a calculator and tire inflation tables determine what is correct for your coach. The calculation is simple:

Axle weight + 5% (this compensates for side to side differences) = Corrected axle weight. Divide Corrected axle weight by two for wheel weight and use your tire tables for the proper inflation pressures. Add 5 PSI to number for a safety/fudge factor.

Your dual axle is the same as the inflation table usually lists dual tire pressures as a pair. If it doesn't, then divide by 4.

Once you have done this, you have determined the correct tire pressures for your coach and your load.
Roger, it's fine to say you don't "need" four corner weights to calculate tire pressure, but if you don't have them, you end up adding a fudge factor (5% in your case; I used 5 lbs., which amounts to about the same) to compensate for what you don't know. I'm sure in most cases that would be fine, but there have been cases where particular coach models/floorplans had significant side-to-side differences due to placement of heavy items (refrigerators, battery banks, etc.).

It would certainly be good to know if you have such a problem, not only to properly calculate tire pressures (which are always set the same for all tires on a given axle, based on whichever position has the greatest load), but also so that you might be able to compensate some by how you load your coach.
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Old 11-01-2016, 04:52 PM   #14
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Yep, its good to pad it a little just incase you get to some cooler air,, just to give you that margin of safety
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