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Old 03-31-2013, 04:51 PM   #1
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Tire Pressure Sanity Check

I just weighed my coach today, the front axle is 10,400 and the rear is 18,240. I have Michelin XZE2 275/80-22.5. According to the Michelin pressure chart for my weights I should be running 85 psi front and 80 psi rear. The tire pressure sticker in the coach says 125 front and 95 rear which is what I have been running since getting the new tires a month ago but the ride is definitely harsh.
Anyone have any thoughts on the recommended pressure on the Michelin chart? I definitely don't want to run under inflated.

Glenn
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Old 03-31-2013, 05:03 PM   #2
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Glenn, I went through the same thing years ago with the tire pressure after weighing it, I set the tires up according to the charts and drove it and it seemed to want sway back and forth (soft tires) and did not drive good, counter steering all the time. So I added 5 lbs. of air starting with the steer tires, drive it and then add to the rears till it drove good. I do not remember off the to of my head as to the tire pressures I run but I have it wrote down on my tool bay door.
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Old 03-31-2013, 05:08 PM   #3
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I see your pressures as 90 and 85 according to this chart.


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And I would add at least 5 lbs to each for good measure.
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Old 03-31-2013, 05:33 PM   #4
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Quote:
Originally Posted by Clifftall View Post
I see your pressures as 90 and 85 according to this chart.


Attachment 36021

And I would add at least 5 lbs to each for good measure.
I see it the same as Cliff including the + 5 lbs.
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Old 03-31-2013, 08:15 PM   #5
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The problem with Rvs is - and as described in the Michellin instructions - that RVs are rarely loaded evenly from side to side and more than 10% variation side to side is quite common - so it is pretty much essential to weigh each corner with the rig in full running loading, and use the heaviest end of an axle to determine the correct pressure and use that pressure for each side.

Failing that - because a lot of weighbridges have kerbs each side to make 4-corner weighing impossible, some advocate weighing the axle, diving by 2 and then adding around 15% and using that load to check the pressures.

(I'd bet a lot of RV manufacturers would never load their units up ready for a trip and then weigh them for the tyre placard simply because they would have to admit they are running either overweight or badly balanced)
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Old 03-31-2013, 08:54 PM   #6
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A sticker on the side of my MH above the tires on all four corners says that you should use the Manufactures cold pressure that is marked on the sidewall of the tire....Mine say 125 lbs each tire no matter front or back, I did this and you are right the ride was a bit harsh so I dropped to 110 all the way around and its just right. I am going to watch the wear pattern and if it looks like they are a bit underinflated then I will go for a stiffer ride...........I also drove many trucks in my long life and we always ran the Manufactures cold air pressure plus 5 lbs..........These stickers are on the side of my Holiday Rambler and they do not make any mention of what brand of tire you are running.......... That my story and I am sticking to it.......

Mike
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Old 03-31-2013, 09:00 PM   #7
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Only real way is to weigh all four "corners" and consult your tire manufacturers charts. The pressure given on the MH sidewall is for the unit loaded to it's maximum and is usually wrong anyway.
ON TRUCK size tires the cold pressure on the sidewall is the MINIMUM required to support the maximum weight rating of the tires. Same with the tire charts, it's the MINIMUM cold pressure to support the weight.

Quote:
From page 2 of the 06/07 Michelin RV Tire Guide: "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 the Firestone/Bridgestone RV tire guide:
Quote:
Bear in mind that these are maximum ratings. The sidewall of the tire shows maximum load and minimum inflation pressure for that load.
From the GoodYear RV Tire 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.
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Old 03-31-2013, 09:20 PM   #8
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Places to weigh outside of truck stops.

large Farms
Seed corn companies
feed mills
grain elevators
farm chemical outlets
some concrete plants
weigh stations
any more you know of?
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Old 04-01-2013, 06:42 AM   #9
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Weighing each corner certainly makes sense. Now I just need to find a scale that I can maneuver on just one wheel at a time. The two scales that I know of near me sit above grade and have rails along the side, no way to place one side on and one off. For the moment I'll try using the +5 pressures and see how she drives.
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Old 04-01-2013, 09:36 AM   #10
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Quote:
Originally Posted by Tony Lee View Post
...
(I'd bet a lot of RV manufacturers would never load their units up ready for a trip and then weigh them for the tyre placard simply because they would have to admit they are running either overweight or badly balanced)

Id bet youre correct. These rigs are not built on the same specs as a passenger car or van. Well as least from a driving perspective.
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Old 04-01-2013, 11:02 AM   #11
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I have the exact same problem as the OP. If you go by the Michelin tables, I should only be running 70 lbs in the fronts and back of my Vista (75-80 if you add the "good practice" rule), yet the driver side placard says to run 90 lbs all the way around. After searching and searching, I saw in Michlin's RV brochure/pamphlet that while the tires should be aired up according to weight, NEVER to go below the RV manufacturer's recommended weights. After reading that, my simple math told me that the 90 lbs on my placard was the max front and rear weight ratings for my 22,000 lb F53 chassis. Since the tire manufacturer is telling me not to run lower than the placard, I just run 90 all the way around, and have not had any problems. Is this right or wrong... ? Don't know, but I do know that I am following all of the written recommendations if I have a problem.
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Old 04-01-2013, 11:20 AM   #12
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That 90 lbs on the placard is relevant if you have the chassis maxed out at ( in your case ) 22,000 lbs. weights lower than that should go by the tire chart with a few extra lbs for good measure.
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Old 04-01-2013, 11:35 AM   #13
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My MH comes with a sticker that says 120 PSI steer, 85 PSI drive & 75 tag. There are 2 factors that went into this...

1. While Spartan rates the front axle at 14,600#, Newmar de-rates it to 14,200#.

2. Tire selection by Newmar is the 305/80Rs.

The big issue with the front axle is because of the tire selection. TECHNICALLY, the tires at 120 PSI are rated to carry 7830# each or a total of 15,560#. Even @ 115 PSI they do 7570# each for a total 15,140#. So, theoretically, the tire selection should support the full 14,600#. So, my working theory is that Newmar went conservative on the entire line of DSDPs in 2006 because of the wide variety of slide options. Because of this they fired up the Ouija board, checked with lawyers and engineers and played the safe game. Hey, I understand liability issues so I don't fault them.

I think a somewhat related side bar to this would be the concerns of running the front axle tires all the time at 120 PSI. the biggest concern being potential for cold tire pressures increasing with warmer seasonal weather and folks not dialing them down properly. That is another theory.

We just bought it in January and haven't had it weighed yet nor have we carried any water or otherwise loaded it up much yet. So I do run it about 2-3 PSI below the stickers because I know that we are not loading the tires anywhere near the max weights. This week we will de-winterize, fill the tanks, add a few # of supplies and get it weighed. That should tell us how well it is balanced from side to side.
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Old 04-01-2013, 11:41 AM   #14
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Big problem because each stage in the manufacturing process adds their own CYA requirements and they get compounded at each stage. Then the owners get their turn.



Even after the chassis manufacturer and the powerplant manufacturer and the MH manufacturer have done their best to confuse and cloud the issue, the dealers and the owners get their chance to add to the confusion by adding all sorts of after-market bling like extra solar panels and 8 extra batteries and a 64" TV and washing machines and driers and basements full of toys - all of which make the placard a total work of fiction even if the manufacturer had got it right in the first place.
Then of course the hapless owner poses a question on a public forum and gets as many different advices as there are replies.

This unscientific approach contrasts with that of the tyre manufacturer which sells a the-buck-stops-here product with the final responsibility for vehicle handling, comfort and safety and who has been manufacturing and testing tyres for a hundred years. If they say weigh each corner and air them up according to their scientifically and real-world-testing derived figures, why would anyone bother going by a bit of stamped aluminium fixed to the door pillar 10 years ago.
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