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Old 10-02-2019, 09:49 AM   #15
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For example lets pretend that you are running with 10,800 pounds on an axle / 5,400 pounds per side, and the inflation chart shows that 70 psi will support 5,440 pounds, 75 psi will support 5720, and 80 psi will support 6,000 which pressure do you use?
I'd use 80 psi, i.e. about 10% above the value shown in the tables. The tables show the minimum inflation for the load, but I want a "safety margin" to cover the possibility of additional load (after the weigh in) or changes in temperature after I fill the tires. I do that to avoid repeated psi adjustments every time conditions change a little. It's NOT because I don't trust the tables.

Several replies reference the tire placard values, but that's not what Isaac-1 asked about. The placard values have already had somebody's notion of a safety margin added, based on a guestimate of the actual load. That's a different method of setting pressure and not relevant to scaled weights.
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Old 10-02-2019, 02:00 PM   #16
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Several replies reference the tire placard values, but that's not what Isaac-1 asked about. The placard values have already had somebody's notion of a safety margin added, based on a guesstimate of the actual load. That's a different method of setting pressure and not relevant to scaled weights.
I think Forest River sets the tire pressure placard to the GAWR limits.

My Forest River GT5 placard says my "people + cargo + water + fuel + tongue weight" can never exceed 2,838 lbs. My GVWR is 22,000 lbs so my empty weight is 19,162 lbs.

My placard says to put my Michelin XRV 235/80R22.5 tires at 90 lbs psi on all tires.

  • The Michelin Inflation Charts for RV Usage PDF says that my tires can each carry 3,980 pounds as a single tire at 90 lbs psi (the front tires).

  • My front axle GAWR is 8,000 lbs so the two tires at 90 lbs psi each can carry a total of 7,960 lbs also known as "close enough".



  • The Michelin Inflation Charts for RV Usage PDF says that my tires can carry 7,510 pounds as a dual set at 90 lbs psi each (the rear tires).

  • My rear axle GAWR is 15,000 lbs and 7,510 * 2 = 15,020 lbs. also known as "close enough".

So it looks like Forest River, at least on my model year and model, has set the tire placard so they will support the maximum GAWR limit of each axle.

If I am misunderstanding how this all works, PLEASE let me know because I'm betting my life and my family's life on it.

I just had it weighed last week with everyone and everything in it and with full water (82 gallons) and full fuel. My GCWR is 26,000 lbs (4,000 lbs above GVWR) so I'm getting very close to the GAVR and GAWR. I usually travel with about 1/3rd water and I've drained the tank to that level to lose about 450 lbs off the rear axle.

I keep each tire at 95 to 98 lbs psi when cold. If I get below 95 lbs psi I bring them up a bit if the temps are going to stay stable or go lower.

Ray
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Old 10-02-2019, 02:10 PM   #17
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Going a bit over as others mention is a reasonable thing to do. It helps when atmosphere pressure lowers or when ambient temperature drops. Makes it less likely you will need to add air. Also gives cushion should you lose some air.
Do try to keep from getting a harsh ride (relative speaking). In my case it is quite noticeable if pressure is a lot more than required.



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Old 10-02-2019, 07:18 PM   #18
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Why use a tire chart if you don't trust it??? That's like your coach taking 5 quarts of oil and you add 6 as a safety measure. Not a great analogy, but you get the drift.

In regards to the placard.....when a coach is built, a GAWR is determined and air pressures decided. So someone buys a coach and lightly loads it, why would you fill the tires way over what it needs.....you're just asking for a rough ride and possibly poor handling.

Weigh your coach when it's loaded, check the weight and round up to the heaviest wheel on each axle for the correct pressure......using the chart as intended.

I want the best and softest ride from my coach and carefully watch wear to make sure the tires are properly inflated. Once I know they're fine, no need to mess with the pressure.
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Old 10-02-2019, 07:34 PM   #19
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A simple search of this site will demonstrate that there are as many "opinions" as there are belly buttons on this board, maybe more. The tire manufacturers don't publish inflation tables because they don't have anything else to do. They publish them to inform you of the correct inflation for your load. Use them, it's to only sensible thing to do. I'm sure there are those that think I'm wrong. They are entitled to their opinion.
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Old 10-03-2019, 10:44 AM   #20
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Originally Posted by Dutch Star Don View Post
Why use a tire chart if you don't trust it??? That's like your coach taking 5 quarts of oil and you add 6 as a safety measure. Not a great analogy, but you get the drift.

In regards to the placard.....when a coach is built, a GAWR is determined and air pressures decided. So someone buys a coach and lightly loads it, why would you fill the tires way over what it needs.....you're just asking for a rough ride and possibly poor handling.

Weigh your coach when it's loaded, check the weight and round up to the heaviest wheel on each axle for the correct pressure......using the chart as intended.

I want the best and softest ride from my coach and carefully watch wear to make sure the tires are properly inflated. Once I know they're fine, no need to mess with the pressure.
Unlike oil, the air "capacity" changes with ambient temperature. Following the charts means the tire's weight-bearing capacity changes with ambient temperature.

When you do a "set it and forget it" you're following the manufacturer's chart for that ambient temperature (assuming the tires themselves are in a cold state) and assuming your tire gauge is 100% accurate, which it's not. If you fill the tires at 80 degrees ambient in the afternoon and there is no direct sun on any tire and leave the next morning when it's 50 degrees your pressures and your load-carrying capacity will be somewhat less. Tomorrow we're having a high of 72 and a low of 41.
  • Per the various articles I should only see a 3 to 6 lb psi drop for a 30 degree drop in temperature.

  • I know from experience I'll see the 6.

  • With my tires at 90 and the TPMS low pressure alarm set to 10% below that (81 lbs psi), my TPMS sensor is only a few degrees from triggering and that assumes the sensor accuracy is perfect and my tire gauge is perfect, which they're not.

  • When the TST 507 sensors are new they are only guaranteed to be accurate +/- 3 degrees.

That's why I add in some extra pressure safety margin. I know the tolerances of my tools when they were new and I want to use the manufacturer's chart as the base.

And because if the TPMS is beeping when we start off on a cold morning I'm pretty sure my DW is not going to accept a response of "Don't worry, honey. As soon as we get up on the interstate the tires will warm up and it will stop beeping."

And she and I shouldn't take that attitude. IMHO, of course.

Ray
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Old 10-03-2019, 11:47 AM   #21
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Originally Posted by NXR View Post
Unlike oil, the air "capacity" changes with ambient temperature. Following the charts means the tire's weight-bearing capacity changes with ambient temperature.

When you do a "set it and forget it" you're following the manufacturer's chart for that ambient temperature (assuming the tires themselves are in a cold state) and assuming your tire gauge is 100% accurate, which it's not. If you fill the tires at 80 degrees ambient in the afternoon and there is no direct sun on any tire and leave the next morning when it's 50 degrees your pressures and your load-carrying capacity will be somewhat less. Tomorrow we're having a high of 72 and a low of 41.
  • Per the various articles I should only see a 3 to 6 lb psi drop for a 30 degree drop in temperature.

  • I know from experience I'll see the 6.

  • With my tires at 90 and the TPMS low pressure alarm set to 10% below that (81 lbs psi), my TPMS sensor is only a few degrees from triggering and that assumes the sensor accuracy is perfect and my tire gauge is perfect, which they're not.

  • When the TST 507 sensors are new they are only guaranteed to be accurate +/- 3 degrees.

That's why I add in some extra pressure safety margin. I know the tolerances of my tools when they were new and I want to use the manufacturer's chart as the base.

And because if the TPMS is beeping when we start off on a cold morning I'm pretty sure my DW is not going to accept a response of "Don't worry, honey. As soon as we get up on the interstate the tires will warm up and it will stop beeping."

And she and I shouldn't take that attitude. IMHO, of course.

Ray
This is another aspect of tire pressure that some just go WAY over the top on. I've actually read where some guy will start in 80 degree weather, set his tire pressure, drive through a snow covered pass, set his pressure again and a day later, end up in 80 degree weather and change pressure again. This is crazy. Have you ever watched a tire pressure monitor. You tires can go from 80 psi to 125 psi, just during the heat of the day.

Let's ask this question.....how many here will drive their car from their warm home to the mountains to go skiing for a day. Do you add air to your tires before you leave the mountains at the end of the day. No, because that is crazy.

Millions of people drive all over this country in their cars and RV's EVERY day and only check the tire pressure twice a year for normal leak down. How is it though some can get so crazy about everyday pressure? How many blowouts have you heard about from someone not adjusting their pressures between elevations?????

OMG......set your tire pressure, buy a TPMS if you have concerns, and go enjoy your coach.
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Old 10-04-2019, 06:49 AM   #22
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I set my pressures at 5 psi above the charts to compensate for variation in morning temperatures. I find this usually is enough to keep me from hitting the listed minimum inflation during a typical trip. Before leaving i verify pressures via tpms
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Old 10-04-2019, 09:11 AM   #23
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I set my pressures at 5 psi above the charts to compensate for variation in morning temperatures. I find this usually is enough to keep me from hitting the listed minimum inflation during a typical trip. Before leaving i verify pressures via tpms
The engineers from the tire companies should be aware that their end users go up mountains and start their trips in the morning when it's cool. Hence they recommend using their published charts and inflate first thing (before driving) at AMBIENT temperature (they expect the day to have fluctuating conditions).

Very smart practice to verify your pressure before starting with your TPMS. I do this also - at chart pressures.
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Old 10-04-2019, 12:23 PM   #24
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The engineers from the tire companies should be aware that their end users go up mountains and start their trips in the morning when it's cool.

All well and good, but tire engineers don't know whether I will recheck the pressure every morning, or whether my days journey will take me to a substantially different elevation or climate.


Nor do the inflation tables compensate for the 500 lbs of fuel I take on shortly after hitting the road, nor for the 100 lb piece of driftwood my wife fell in love with and stuffed in a bay. That's my responsibility.



I put more than the minimum psi in my tires so that I don't have to worry whether I'm still in the same load range I was at the last tire check/fill.
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Old 10-05-2019, 01:08 PM   #25
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Originally Posted by Gary RVRoamer View Post
All well and good, but tire engineers don't know whether I will recheck the pressure every morning, or whether my days journey will take me to a substantially different elevation or climate.


Nor do the inflation tables compensate for the 500 lbs of fuel I take on shortly after hitting the road, nor for the 100 lb piece of driftwood my wife fell in love with and stuffed in a bay. That's my responsibility.



I put more than the minimum psi in my tires so that I don't have to worry whether I'm still in the same load range I was at the last tire check/fill.
Gary, as always, thanks for sharing your experience and practices. And, yes, please use your equipment the way you prefer.

I've spoken to three separate Michelin reps over the course of several months while learning about my (new to me) MH after getting confused/doubts from other's responses to the inflation questions.

Every single time the reps from the company that makes my tires (not my salesman, not a service professional at my RV dealer and not the salesman at the tire shop) say - get four corner weights when fully loaded to determine your forecast load. Set your tire pressures to the chart while at ambient temperature before driving more than a mile (anywhere you happen to be). Drive where you wish from that locale. Never remove air from a hot tire. Good practice includes checking tires and pressure before driving each day. All other factors (altitude, heat from road/speed, temperature) are all factored into the recommended pressures from the charts. Their tires go millions and millions of miles every year.

Here's a good read for our fellow members;
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Old 10-05-2019, 06:45 PM   #26
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Gary, as always, thanks for sharing your experience and practices. And, yes, please use your equipment the way you prefer.

I've spoken to three separate Michelin reps over the course of several months while learning about my (new to me) MH after getting confused/doubts from other's responses to the inflation questions.

Every single time the reps from the company that makes my tires (not my salesman, not a service professional at my RV dealer and not the salesman at the tire shop) say - get four corner weights when fully loaded to determine your forecast load. Set your tire pressures to the chart while at ambient temperature before driving more than a mile (anywhere you happen to be). Drive where you wish from that locale. Never remove air from a hot tire. Good practice includes checking tires and pressure before driving each day. All other factors (altitude, heat from road/speed, temperature) are all factored into the recommended pressures from the charts. Their tires go millions and millions of miles every year.

Here's a good read for our fellow members;
Interesting PDF, thanks for sharing it. This statement especially: "Tires that have been in use for 5 years or more should continue to be inspected by a specialist at least annually." (bold emphasis is mine)

The "In use" versus "Bought with an old date code" thing people debate so much.

Others:

"Note: When a vehicle is stored, tires should be inflated to the inflation pressure indicated on the sidewall."

"If you use a dressing product to “protect” the tires from aging, use extra care and caution. Tire dressings that contain petroleum products, alcohol, or silicones will cause deterioration or cracking and accelerate the aging process.

In many cases, it is not the dressing itself that can be a problem, but rather the chemical reaction that the product can have with the antioxidant in the tire. Heat can add to the negative reaction. When these same dressing products are used on a passenger car tire that is replaced every three to four years, it is rare to see a major problem. However, in most cases, RV tires may last much longer due to limited annual mileage, and the chemical reactions have much longer to take place."

"Check with the motorhome chassis manufacturer for the correct alignment specifications. Michelin recommends, for optimized radial tire life and performance, that the “toe-in” setting should be as close as is practical to zero, within the motorhome manufacturer’s specifications. The caster should be set to the maximum positive or minimum negative setting within the tolerances specified by the manufacturer."

Ray
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Old 10-06-2019, 10:18 AM   #27
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Gary, as always, thanks for sharing your experience and practices. And, yes, please use your equipment the way you prefer.

I've spoken to three separate Michelin reps over the course of several months while learning about my (new to me) MH after getting confused/doubts from other's responses to the inflation questions.

Every single time the reps from the company that makes my tires (not my salesman, not a service professional at my RV dealer and not the salesman at the tire shop) say - get four corner weights when fully loaded to determine your forecast load. Set your tire pressures to the chart while at ambient temperature before driving more than a mile (anywhere you happen to be). Drive where you wish from that locale. Never remove air from a hot tire. Good practice includes checking tires and pressure before driving each day. All other factors (altitude, heat from road/speed, temperature) are all factored into the recommended pressures from the charts. Their tires go millions and millions of miles every year.
Nothing there is at odds with anything I wrote. If I could get a new 4-corner weight everyday and was willing to adjust each day for weight & temperature differences, I would do exactly that. But I don't re-weigh daily, nor do I care to fiddle with tire pressures every morning, so I increase the psi a bit to cover eventualities. A quick glance at my TPMS readings on the morning of departure tells me if I'm in the desired range and off I go, confident that my tires are never underinflated for conditions and load.
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Old 10-06-2019, 01:09 PM   #28
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...But I don't re-weigh daily, nor do I care to fiddle with tire pressures every morning, so I increase the psi a bit to cover eventualities. A quick glance at my TPMS readings on the morning of departure tells me if I'm in the desired range and off I go, confident that my tires are never underinflated for conditions and load.
Precisely, and how I do it as well.

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