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Old 09-11-2022, 09:05 AM   #1
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Newbie Tire Pressure Question

Hello,

I know there has been many discussions regarding Tire pressure but I thought I would ask again just to be sure. I believe the tires on my unit 32SA (9k steer axel & 15.5k drive axel) Michelin XRV 255/80R22.5 are rated to carry more weight than the axel ratings, which I suppose is a good thing seeing that I would never exceed my axel ratings.

That said, the RV "sticker" recommends cold pressure at 100psi which looking at Michelin's chart for the tire indicates single weights 4,975lbs (9,950 axel) and dual 9,050lbs (18,100 axel)

My recent trip to Cat Scales I recorded 7,460lbs steer axle and 14,080lbs drive axel ( agreed, not a 4 corner measure, and not fully loaded ) but the Michelin guide suggests a 70-75psi at that weight. I don't think I would dare to go that low !

Perhaps the sweet spot should be around 90psi??? Or just keep it at 100psi and have a hard ride ????

Thoughts ?
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Old 09-11-2022, 09:08 AM   #2
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If 90 PSI cold (before driving) gives you a 10 PSI safety cushion over the Michelin chart's recommendation, which is the MINIMUM for the given weight, go with that.
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Old 09-11-2022, 10:19 AM   #3
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I used the Michelin charts on my 36ua. After about 15000 miles I noticed the steer tires wearing on both inside and outside edges. (low tire pressure) To bad because the ride was noticeably smoother at the lower pressures. Now run 10 over the charts. I did have front end aligned after purchase so alignment was not the issue Iím sure.
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Old 09-11-2022, 06:32 PM   #4
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When I purchased my 2021 Open Road 32SA all the tires were all set at 105 psi. It was a bone jarring ride. After weighing the coach, I added 5 psi to the Michelin suggested psi. The ride improved a LOT! I haven't noticed any wear yet.
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Old 09-12-2022, 05:12 AM   #5
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Thank you all very much for your thoughts, Yes the ride is hard and some adjustment is required before my teeth rattle out. I have started a spreadsheet to list all additional items being packed and their weights and will confirm the weights at the nearest cat scale on my next trip. I would rather err on the high side than the low side with the psi. I suppose a good TPMS would be a good investment to watch over things while on the road.

Hey Fio, I notice from your signature you are running the same rig with with Blue Ox and Patriot 3 system. I am looking at adding that flat tow system and wondered what you thought ? Could I send you a pm to get your thoughts on that system?
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Old 09-12-2022, 06:01 AM   #6
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High side error safe, low side not so much.
You should also figure a 10% side to side variation (and use the higher wt) when using axle wts and comparing to the chart.
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Old 09-12-2022, 09:21 AM   #7
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I recently weighed our new Phaeton 40IH and have concerns based on tires and the scale weight on steer axle. We were fully loaded & just filled up with diesel before driving on CAT Scales. The steer axle weight was 13,940 and drive axle 23,060.

All our tires, front and rear, are Michelin 295/80R 22.5 XZA2 Energy. The Michelin tire chart shows 13,880 LBS for dual, which is 60 LBS below our weight…if I’m looking at it correctly.

I had front tires at 115 PSI and rear at 110 PSI. I think the rear is okay but concerned about the front tires, both the tire weight capacity and PSI. My TPMS is off 4-5 LBS on the digital dash readout versus the actual measurement I’m reading when checking the tires. Assume there’s a way to calibrate them to a tighter tolerance.

Wondering if Tiffin should have placed 315’s on the front instead of 295’s?
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Old 09-13-2022, 07:19 AM   #8
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The 6940 lbs you are looking at on the weight chart is for one tire. Since you have four tires in the rear, their total weight bearing would be 27,760 lbs.
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Old 09-13-2022, 07:56 AM   #9
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Quote:
Originally Posted by Phaeton RVer View Post
I recently weighed our new Phaeton 40IH and have concerns based on tires and the scale weight on steer axle. We were fully loaded & just filled up with diesel before driving on CAT Scales. The steer axle weight was 13,940 and drive axle 23,060.

All our tires, front and rear, are Michelin 295/80R 22.5 XZA2 Energy. The Michelin tire chart shows 13,880 LBS for dual, which is 60 LBS below our weightÖif Iím looking at it correctly.

I had front tires at 115 PSI and rear at 110 PSI. I think the rear is okay but concerned about the front tires, both the tire weight capacity and PSI. My TPMS is off 4-5 LBS on the digital dash readout versus the actual measurement Iím reading when checking the tires. Assume thereís a way to calibrate them to a tighter tolerance.

Wondering if Tiffin should have placed 315ís on the front instead of 295ís?
Why would you want 315's on the front when the 295's will carry 15,660# @120psi, which is almost 1720# more than what you said the front weighs?? The 295 is more than enough to safely carry the stated weight.
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Old 09-13-2022, 05:33 PM   #10
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Tire info can be confusing because the owner is confronted with one pressure on the sidewall, and another on the sticker inside, and yet another on a tire chart.

Let me explain the first two. Anyone can feel free to correct me.

The pressure on the side wall is pressure at which the tire can carry it's maximum rated load. It's not a recommended operating pressure for a particular vehicle. Its a number from the tire manufacturer.

Sticker pressure. While this is the RV manufacturer's recommended tire pressure for the factory installed tires, it's not necessarily the best pressure for the RV. Why? Because for safety reasons, that number is for the pressure the tire can carry when the vehicle listed is at maximum weight. It's there because the manufacturer has no way of knowing, or controlling how the RVer will load the RV, so it opts for posting the pressure for the maximum weight the vehicle is designed to carry. It's an engineering number or a liability number so to speak. If the sticker listed the proper pressure for an unloaded RV (as weighed at the factory) the tires would be overloaded when the tanks were filled and all the RVer's crap was loaded. Same would be the case if the RV manufacturer guessed at an average cargo load. A wrong guess leaves the manufacturer liable should the owner load more, and inflate the tires to the recommended pressure on the sticker. By listing the pressure needed to carry max load, if an owner exceeds this load, they are clearly driving a overloaded vehicle.

So, what does an owner do? Well, many will just pick one of those two numbers and go, and the only thing that can happened is you'll get uneven tire wear, a harsh ride and possibly less than optimal handling, but honestly, most people, and tire places will fill the tires to one of those numbers and send you on your way. Again, because they can't guess how much crap you'll throw in your vehicle, and an over inflated tire that doesn't exceed the pressure on the sidewall cold, is safer than an underinflated one.

That's where a scale and inflation tables come in. The best way to know the best tire pressure is to get four wheel weights. Then its important to know the pressure on the table for that weight is the MINIMUM cold pressure needed to carry the weight. Cold is ambient temps before the vehicle is driven and before the tires have been warmed by the sun. Because it's the minimum pressure most people will fill either 5lbs, or 10% over as a safety margin, because if you start at the minimum, and lose any pressure, the tire is now running underinflated. Bad.

The next common mistake is to over think it. Do the weight, inflate to the correct pressure plus, and have a nice trip. Yes, you can check your pressures every morning and adjust to the exact pound, or you can just be sure to regularly check the pressure and adjust as needed when you experience a general change in the ambient temps. While technically any change in temperatures should call for a change in pressures, this rule ignores what a tire experiences throughout each day and each drive. The differences in cold morning and warm afternoon, going up and down mountain passes, a cold rain on a hot day, and tires on the south side of the vehicle vs the north, all effect tire pressures. The big takeaway is the goal is to not run your tires underinflated.

Oh, and did I mention that some wheels also have a mx pressure stamped on the rim? Yep, here's the three number for my rig:

Tire sidewall pressure: 130 max cold
Rim: 120 Max cold
Sticker 115 cold

Yet after a four wheel weigh, I run front 100 cold, and rear, 115 cold. That's chart number, plus. Note: I'm running slightly larger tires than the factory installed with a higher weight rating.

No wonder so many people get confused over tire pressures.

Hope this helps.
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Old 09-13-2022, 06:26 PM   #11
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To the OP. Some very informed people have put together information that all newbys should read and then read it again. It will answer most if not all of your questions. When you ask opinions on the forum, you might get good advise and some might not be accurate. Go to the "Stickie" below for correct information.

https://www.irv2.com/forums/f258/new...v2-201159.html
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Old 09-14-2022, 06:15 AM   #12
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Thanks again for all the great comments and advice. I will go with the "sticker" cold psi rating until such time I am fully loaded and can get the RV and four corners weighed, then adjust from there.
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Old 09-21-2022, 07:20 AM   #13
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Made a pressure/axleload list for you.

In that a reserve of 11% first added to make the axleload 90% of the loadcapacity for the pressure.
And single load in front of psi, and dualload behind it.
So dont add reserves yourselfes, and dont devide axleload by 2 or 4, its all done for you.

Your max speed of tire is probably 75mph, so even a bit higher pressure would still not give bad gripp or discomfort.
But I can also make a list in wich also reserve in speed used build in, with still acceptable comfort and gripp. So give the reserves and speed, or if you want it per tire( but why should you), I can make a list for that.

If you ever weighed per axle-end, you better use my made motorhome tirepressure calculator. To complicated to make a list for that.

Now you ONLY have to determine the real axleloads acurate, the most tricky part in it all

Then looking back your front 7460 lbs gives 85 to 90psi
And rear 14080lbs gives 90psi., so yust as you estimated, but to my opinion it can even be higher without bumping, read what I wrote about it.
255/80R22.5
Single......./ psi..... /........ Dual
2408 lbs/ 25 psi / 4447 lbs
2835 lbs/ 30 psi / 5234 lbs
3258 lbs/ 35 psi / 6015 lbs
3678 lbs/ 40 psi / 6791 lbs
4095 lbs/ 45 psi / 7561 lbs
4510 lbs/ 50 psi / 8328 lbs
4923 lbs/ 55 psi / 9090 lbs
5334 lbs/ 60 psi / 9849 lbs
5744 lbs/ 65 psi / 10605 lbs
6151 lbs/ 70 psi / 11358 lbs
6558 lbs/ 75 psi / 12108 lbs
6963 lbs/ 80 psi / 12856 lbs
7366 lbs/ 85 psi / 13601 lbs/ 7460 Front
7769 lbs/ 90 psi / 14344 lbs/ 14080 Rear
8170 lbs/ 95 psi / 15085 lbs
8571 lbs/ 100 psi / 15825 lbs/ GAWR R
8970 lbs/ 105 psi / 16562 lbs / GAWR F
9369 lbs/ 110 psi / 17298 lbs G-load
9766 lbs/ 115 psi / 18031 lbs
10163 lbs/ 120 psi / 18764 lbs
10558 lbs/ 125 psi / 19495 lbs
10953 lbs/ 130 psi / 20224 lbs
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Old 09-29-2022, 08:40 AM   #14
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Quote:
Originally Posted by KenJr View Post
I used the Michelin charts on my 36ua. After about 15000 miles I noticed the steer tires wearing on both inside and outside edges. (low tire pressure) To bad because the ride was noticeably smoother at the lower pressures. Now run 10 over the charts. I did have front end aligned after purchase so alignment was not the issue Iím sure.
Ken
Just had Michelin XZE's (245/70R19.5) mounted. Michelin Chart psi for my Cat Scale weights list 75psi both front and rear, ( which is btw., the minimum psi Michelin allows for this tire). I too was concerned about low psi wear so I bumped the rear's to 90 and steer to 85. The odd thing to me was that the tire center had put the max 120psi in all 6 tires and the ride wasn't noticeably different after lowering the psi.
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