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Old 03-21-2018, 12:13 PM   #1
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Tire Inflation, Again!

Yeah, I know. Covered to death, right? I had my RED 33aa weighed yesterday and found out that we're riding a little lighter than anticipated. With over 1/2 tank fuel, full propane, but no water aboard we were at 9340 on the front, 18K even on the back. According to the Michelin book, that's 80psi on the front, 85 on the back, with a bit of reserve. But that seems very low.

Any insights?
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Old 03-21-2018, 01:22 PM   #2
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Quote:
Originally Posted by TexasSprinter View Post
Yeah, I know. Covered to death, right? I had my RED 33aa weighed yesterday and found out that we're riding a little lighter than anticipated. With over 1/2 tank fuel, full propane, but no water aboard we were at 9340 on the front, 18K even on the back. According to the Michelin book, that's 80psi on the front, 85 on the back, with a bit of reserve. But that seems very low.

Any insights?
You may find the answers to your questions in chapter four of the reference below.

https://www.ustires.org/sites/defaul...TruckTires.pdf
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Old 03-21-2018, 02:43 PM   #3
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I recently had my 2018 33AA weighed by RVSEF with fuel tank full, LPG full, DEF full, 1/3 water, and empty gray/black along with an average amount of clothing for an extended trip. Our weights were 9800 front and 18900 rear. They recommended 95# front and 100# rear which included what they felt was a conservative and safe reserve. It all depends on how you will load and pack for the type of trips you will do.
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Old 03-21-2018, 03:02 PM   #4
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You may find the answers to your questions in chapter four of the reference below.

https://www.ustires.org/sites/defaul...TruckTires.pdf
"Inflation pressure recommendations may also be determined based on the tire manufacturer’s specifications, which define the amount of inflation pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.
However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall"


Interesting. So ignore the tire manufacturer's recommended weight/PSI chart unless the PSI above the motorhome manufacturer's PSI?
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Old 03-22-2018, 06:59 AM   #5
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Originally Posted by TexasSprinter View Post
Yeah, I know. Covered to death, right? I had my RED 33aa weighed yesterday and found out that we're riding a little lighter than anticipated. With over 1/2 tank fuel, full propane, but no water aboard we were at 9340 on the front, 18K even on the back. According to the Michelin book, that's 80psi on the front, 85 on the back, with a bit of reserve. But that seems very low.

Any insights?
Niel what tire is on your coach ?
Is it Michelin X line 275/80 R 22.5 G or H rated ?

If so according to the Michelin web site vs your book your pressures do not match the web site recommendations. Over at the Tiffin RVnetwork web site the tire pro recommends adding 5% more pressure to weight rating.

https://www.michelinrvtires.com/refe...tion-tables/#/
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Old 03-22-2018, 07:56 AM   #6
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Start with your axle weights, divide by two, and multiply that by 1.05. This will account for side to side weight differences. Use this number to look up your inflation pressure. Once you have that pressure number, add 5 psi to it as a safety/fudge factor.

It really doesn't matter what other owners inflate their tires to, you need to inflate them to the load you are carrying in your motorhome.
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Old 03-22-2018, 08:29 AM   #7
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Try those recommended pressures and see how it drives. If it seems a bit squirrely, add 5 PSI at a time to give the sidewalls a bit more stiffness. I have had a similar issue in the past, and that solved my problem, while keeping the better ride that lower pressures gives.
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Old 03-23-2018, 01:01 PM   #8
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Originally Posted by Sonic View Post
"Inflation pressure recommendations may also be determined based on the tire manufacturer’s specifications, which define the amount of inflation pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.
However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall"


Interesting. So ignore the tire manufacturer's recommended weight/PSI chart unless the PSI above the motorhome manufacturer's PSI?
That's because tire manufacturers do not set recommended cold tire inflation pressures, vehicle manufacturers do. In between what is recommended by the vehicle manufacturer and tire max is the optional area.

For replacement tires using different inflation charts, the standard is to set a recommended tire inflation pressure that provides, at the minimum, the amount of load capacity the OE tires provided at certification label inflation pressures.

It's all in that reference. You just have to dig it out, so to speak.

RVs using large commercial tires must use inflation standards in accordance with the final stage manufacturer's recommended cold inflation certification. Normally FMVSS.
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Old 03-23-2018, 01:49 PM   #9
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Quote:
Originally Posted by FastEagle View Post
That's because tire manufacturers do not set recommended cold tire inflation pressures, vehicle manufacturers do. In between what is recommended by the vehicle manufacturer and tire max is the optional area.

For replacement tires using different inflation charts, the standard is to set a recommended tire inflation pressure that provides, at the minimum, the amount of load capacity the OE tires provided at certification label inflation pressures.

It's all in that reference. You just have to dig it out, so to speak.

RVs using large commercial tires must use inflation standards in accordance with the final stage manufacturer's recommended cold inflation certification. Normally FMVSS.
I'll take another look. It's confusing because the Michelin inflation charts specifically state recommended cold tire pressure.
"Michelin displays tire loads per axle end in the load and inflation tables. We recommend weighing each axle end separately and using the heaviest end weight to determine the axle's cold inflation tire pressure. For control of your RV, make sure tire pressures are the same across an axle, while NEVER exceeding the maximum air pressure limit stamped on the wheels."
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Old 03-24-2018, 06:55 PM   #10
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I don’t get it? Your weight is constantly changing as you use up fuel and water. So why not just air up to above the minimum and a little below the maximum recommended and call it a day? I roll with 100 to 110 lbs all the time and have never had a problem.
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Old 03-25-2018, 09:56 AM   #11
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I only checked as 1) I wanted to know the weight, and 2) the front end doesn't track quite as well as I would like, so I wanted to see if I could reduce pressure up there just a bit.
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Old 03-25-2018, 10:39 PM   #12
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What? You are joking right? Nope!
Remember, the vehicle manufacturer, not the tire manufacturer, determines' the correct tire pressure for the tires on your vehicle. That’s a direct quote from NHTSA.

Here is the paragraph from The FMVSS standard, 571.120.

S5.3.1 Tires: The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR.

That information is on all vehicle certification labels, tire placards and in the vehicles owners’ manual.
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Old 03-26-2018, 05:35 AM   #13
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Remember, the vehicle manufacturer, not the tire manufacturer, determines' the correct tire pressure for the tires on your vehicle. That’s a direct quote from NHTSA.

Here is the paragraph from The FMVSS standard, 571.120.

S5.3.1 Tires: The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR.

That information is on all vehicle certification labels, tire placards and in the vehicles owners’ manual.
OK semantics...
Vehicle mfg specify tire press based on info / charts provided by the tire mfg and the vehicle RATED loads...
Which in many cases is significantly different than actual.

You won't go wrong using vehicle mfg pressures so guess that's the answer.
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Old 03-26-2018, 06:14 AM   #14
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just try 95 or 100 psi and see which gives you the best ride. For me 110 gives to ruff a ride. of course I'm not to much into over teching things. Dave
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