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Old 07-26-2017, 11:14 PM   #1
Fid
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THOR #3851
Determining the Correct Tire Pressure

My 2015 Thor Miramar 34.3 has the Michelin XRV tires. When I bought the RV with about 6K miles on it all tires had 80 psi +- 1 psi. I have researched enough to know that I needed to get a fully loaded weight (full tank of gas, full fresh water, gray and black tanks, full gear and full passengers) at all 4 corners of the RV which I have done on a CAT scale using the three tires off the scale on one side and three on the scale on the other side.

My weights were:
Front left 3,860
Front right 3480
Rear left 6,740
Rear right 6,660

According to the Michelin RV Inflation Tables my 80 psi should be about correct.

However, I have noticed that the plaque on the driver side inside wall lists the pressure as 90 psi.

I am assuming the Thor recommendation on the plaque is a general recommendation. Am I right to follow Michelin's recommendation or should I be running 90 psi?

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Old 07-27-2017, 01:17 AM   #2
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I would go with what the tire manufacturer says. You could even put in an additional few pounds of air you should be ok.
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Old 07-27-2017, 02:18 AM   #3
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I agree 80×/- psi should be good, the door sticker may have been the original tires, I would think Michelins would've been an upgrade.
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Old 07-27-2017, 01:37 PM   #4
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yep, I'd go with the load inflation tables....but make sure you are looking at the correct table for the type and size of tire.

I wish I could find a cat scale that I could get off on one side. So far everyone where I've tried, the sign posts are too close to the scale pad. So far I've only been able to get axle weights
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Old 07-27-2017, 04:36 PM   #5
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If you check the instructions on the Cat scale web site you will see that the design is intentional. They don't allow weighing one side at a time. https://catscale.com/how-to-weigh/
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Old 07-27-2017, 05:51 PM   #6
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yeah, that's what I figured.
Hoping the Fid will chime back in with how he did it.

I talked to a cat weighmaster once, thinking that maybe I could put my steer axle on the drive platform and my drive axle on the trailer platform, and thereby miss the sign, but she told me she thought (wasn't sure) that it wouldn't work with noting on the steer platform.
They were busy so I didn't want to mess around trying it at the time.

Seeing that video, I didn't know they had an app so you can weigh without having to try to explain what I'm doing....so maybe that's something to play with.... as a work around.

I hope to find a non-cat branded scale some day.... just make things easier.
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Old 07-27-2017, 07:44 PM   #7
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Originally Posted by Fid View Post

.....cut.....

However, I have noticed that the plaque on the driver side inside wall lists the pressure as 90 psi.

I am assuming the Thor recommendation on the plaque is a general recommendation. Am I right to follow Michelin's recommendation or should I be running 90 psi?

I wouldn't quite call it a "general recommendation". I don't think they have option to specify any lower pressure.

Manufacturers must specify required tire pressure to meet the chassis' Gross Vehicle Weight Rating (GVWR), including the front and rear axle ratings used to obtain that GVWR.

In your case I believe the motorhome is probably built on a 22,000-pound F-53 Ford chassis, which has 8,000-pound front and 15,000-pound rear axle ratings. Standard tires are 22.5-inch while 19.5-inch are optional.

With standard 22.5-inch tires, it takes 90 PSI to carry the maximum axle loads safely, which is why Ford and Thor include it in specs.
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Old 07-27-2017, 09:31 PM   #8
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THOR #3851
Thanks for the responses. I will be comfortable with using the Michelin Tire Pressure tables.

As far as weighing each corner, yes I was aware that CAT says that they do not allow weighing with two tires off the platform and two on. According to their website, it is to "prevent damage to your vehicle as well as our scales" (see Motorhome section here).

However, I had heard that other had done this all four corner weight successfully on the CAT scales so I decided to give it a try.

Yes the cross brace that holds up the CAT Scale sign is in the way but I was able to put the driver's side wheels off the scale on the concrete area between the scale and the yellow curb and pull forward far enough to just get my front right wheel on the front axle weight plate of the scale. My right rear wheel was on the trailer plate of the scale with nothing on the rear axle plate of the scale. If you have a shorter RV your rear tire will probably be on the rear axle plate of the scale.

I pulled up to the cross brace post as far as I could go to get my wheels in the positions I just described. After the weight, I was not able to pull through the scales without backing up first, so I had to remove my toad from the RV before I did this weight so I could back up the RV.
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Old 07-27-2017, 11:02 PM   #9
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Tire Pressure

I would put more than the 90 pounds recommended.
I went through a front set of G670's in 13,000 miles using the recommended pressure that Thor set.
The dealer where I bought the RV said they had done a wheel alignment but apparently not. He recommended that seeing the tires maximum pressure is 120 pounds I should be running around 105 to 110 pounds.
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Old 07-28-2017, 12:09 AM   #10
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My wife thought one of our tires looked a bit low when we picked our Vegas up. The guy at the dealer checked them all and they were all around 80 pounds. He said that was the correct pressure.
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Old 07-28-2017, 03:12 AM   #11
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I did that also. Started at 80 pounds then went to the Thor's recommendation of 90 pounds. As I said the right front tire was totally gone at 13,000 miles from not being aligned properly and the left one was almost there with definite signs of under inflation. The tire shop said for the 19.5" tires you need to be over 100 pounds. Running mine at 110 now (maximum pressure for the tire is 120 pounds) and the motorhome drives down the road like a different unit.
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Old 07-28-2017, 12:14 PM   #12
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I would go with whatever Michelin recommends.
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Old 07-28-2017, 01:24 PM   #13
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If you ever get close to a location where these guys are weighing RVs it is worth the time and cost to have your unit weighed. They weigh each wheel (or set of wheels) and then give a great report on what each tire inflation should be. If you have a TT or 5er they will first weigh all points hooked up and then you weigh just the truck and the report gives exact tongue weight. I had it done a few years back at a rally and found that some tires were over inflated and some were under inflated. http://rvsafety.com/weighing/weighing-schedule
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Old 07-28-2017, 01:41 PM   #14
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thanks for that link! I may look up those folks in St loud the next time I'm down that way...
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Old 07-28-2017, 02:11 PM   #15
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Originally Posted by srobm View Post
I did that also. Started at 80 pounds then went to the Thor's recommendation of 90 pounds. As I said the right front tire was totally gone at 13,000 miles from not being aligned properly and the left one was almost there with definite signs of under inflation. The tire shop said for the 19.5" tires you need to be over 100 pounds. Running mine at 110 now (maximum pressure for the tire is 120 pounds) and the motorhome drives down the road like a different unit.
I assume you had it aligned as well... and would guess that has a lot more to do with the current 'drive down the road'...
Running at the MH placard pressure is 'safe' - it is calculated assuming max weight... Assumes of course that you are at or below the max weight!

Running less than that should be based on the ACTUAL axle weight of your rig loaded for travel (people, cargo, fluids) and the tire manufacturers charts. Running less than required for the actual weight certainly risks damage from under inflation...

Running at the tire's maximum - above the MH placard is not a recommendation I have seen... I expect a harsher ride at least. Less road contact - more likely to hydroplane, extended braking...

From one tire store:
Quote:
What happens if you inflate your tires to the max PSI?
  • The handling characteristics change
    Since tires inflated to the max can’t give as much on the sidewall, you might see superior cornering, but it could be at the risk of your braking threshold. One quick corner and your back end could slide out.
  • The life of your tire decreases. When your tires are inflated too much, the rubber rounds out at the top of the tire when you’re driving, and the center will quickly wear out. You’ll also reduce your traction and you could even cause a blowout.
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Old 07-28-2017, 03:09 PM   #16
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The right front tire wore out because of the poor alignment (asked dealer when I bought it if they had done a wheel alignment as per recommendations on this board and they say they had but did not do it) but the left front tire was worn down substantially on the outside edges from being under inflated. I am going to go with between 100 to 110 pounds to see how the unit handles. The wheel alignment helped. Also on recommendation from the tire store I went with Toyo tires instead of the Goodyear. He said they were a better tire and I had Toyo's on my half ton and they were great tires.
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Old 07-29-2017, 03:01 PM   #17
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I have the Ace 30.2 and my recommended pressure is 82 psi. Three different shops have said the same.
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Old 07-30-2017, 02:51 PM   #18
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I have the Hurricane 35C 80 psi looked really low for me when I did it. I'm running 95 now and seems much better will get to 110 hot which is fine
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Old 07-30-2017, 03:28 PM   #19
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So I am wondering, what ambient air temperature is the tire pressure measured at. Is that on the manufacturer's charts? Its not listed on the coach sticker?

I installed a TPMS and was surprised by the wide variation of tire pressures that result from tire temperature changes. I filled my tires to 82# as stated on the coach sticker at probably 65 F. But on a cool morning they may be as low as 78# and after running on the highway, up in the 90's. On a hot day they will exceed 100#. So.... if you "cold inflate" to 82#, but the tires are normally running 10# or more higher, aren't they technically "over inflated".

IMHO, seems like we may be obsessing a bit much on tire pressures.
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Old 07-30-2017, 03:35 PM   #20
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Originally Posted by thutchis View Post
So I am wondering, what ambient air temperature is the tire pressure measured at. Is that on the manufacturer's charts? Its not listed on the coach sticker?

I installed a TPMS and was surprised by the wide variation of tire pressures that result from tire temperature changes. I filled my tires to 82# as stated on the coach sticker at probably 65 F. But on a cool morning they may be as low as 78# and after running on the highway, up in the 90's. On a hot day they will exceed 100#. So.... if you "cold inflate" to 82#, but the tires are normally running 10# or more higher, aren't they technically "over inflated".

IMHO, seems like we may be obsessing a bit much on tire pressures.
I agree. I'm going to fill at the recommended pressure on the sticker and put it out of my mind.
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