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Old 12-05-2022, 09:22 AM   #1
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33.5 Actual weights and tire pressure

Finally got my 33.5 weighed with 1/3 tank fuel and 1/3 tank water with jeep in tow, 2 adults, 2 kids packed for normal trip:

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Full water and fuel would probably put it slightly over GCVW. Need to re-weigh with full tanks. Definitely have cargo capacity left on the GVWR but the Jeep is heavy eating into that.

At some point need to get 4 corner weigh and see if its balanced side to side.

Front axle is light like I suspected, although fuel tank is up front and will change that when full. Probably 50 gallons down at the time so about 350 lbs there.

Tires are Michelin XZE 255/7022.5 and came from dealer pumped to 120 psi like sticker says. Tire chart from Michelin bottoms out at 80 psi for 4190*2=8380 lbs on front axle and 85 psi looks close for 8220*2= 16440 lbs dual rear.

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I am currently running 90 psi all around which seems better for ride and wander than 120 psi for sure, TPMS complaining all the time until I can get it reprogrammed once I settle on good pressures. Will probably try 80 psi and leave 90 in rear and see how that goes. Large contact patch in front should help with push and wander. It did seem to get better when I filled up fuel again later putting more weight on front axle, which I think is a big part of handling issues, this was similar on my gas coach. I am thinking judicious ride height adjustment could shift weight to front more improving handling, just have to watch rear driveline angles.
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Old 12-05-2022, 09:57 AM   #2
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Your psi numbers of 80 and 90 may cover the current weight of the rig but they do seem a bit low for covering different situations. The addition of full fuel, water, and other stuff over time will definitely add weight and put you close to the tire's capacity at those pressures.

Imo, adding around 10% to your numbers, up front for sure, might be better.
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Old 12-05-2022, 01:15 PM   #3
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My 2014 33.2 with a half tank of water and full fuel tank weighs 7900 front and 17650 rear. Towing a Jeep weighing 4200 so total is 29,750 just under the 30,000 GVWR.

I have the same Michelin XZE 25570R-22.5 tires as you and I run 90 front and 100 rear.
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Old 12-06-2022, 08:00 AM   #4
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Then I made you a pressure / axleload list with even tighter formula then that of the list you gave and gives 90% of the calculated loadcapacity per axle, so you dont need to devide by 2 or 4, and maximum reserve, with still acceptable comfort and gripp.

That last is not totally true, I determined that to be, for loadcapacity calculated for 99mph, and your tires max speed is 75mph ( try to check that).
So pressure can even be higher, but this gives higher pressure then in that list you gave.
I dont search for the lowest safe pressure, but the highest for comfort and gripp.

In front of cold psi for single load axle, and behind for dualload axle.

When you weighed per axle-end, I can make a list for 95% used. If you then take heavyest side on the axle, you can do with lesser reserve.
I determined that the lightest side gives discomfort and bad gripp, if real weight is below 85% of loadcapacity for the pressure.
So if you have other ideas about the reserves, write it, and I put other percentages in the spreadsheed, I made for it.

Now you "ONLY" have to determine the axleloads acurately, and when it changes yust search the new axleloads back in the list.

Your 7920lbs front can do with 95 psi, and rear drive 16400 lbs with 110 psi. Then still acceptable comfort and gripp.

Here it is :

255/70 R 22.5 H-load
Maxload single a tire 5510 lbs AT 120 psi
Maxload Dual a tire 5070lbs AT 120 psi

AxleSingle90%/cold psi/AxleDual90% used
2766 lbs/ 30 psi / 5090 lbs
3179 lbs/ 35 psi / 5850 lbs
3589 lbs/ 40 psi / 6604 lbs
3996 lbs/ 45 psi / 7354 lbs
4401 lbs/ 50 psi / 8099 lbs
4804 lbs/ 55 psi / 8841 lbs
5205 lbs/ 60 psi / 9579 lbs
5605 lbs/ 65 psi / 10315 lbs
6003 lbs/ 70 psi / 11047 lbs
6399 lbs/ 75 psi / 11777 lbs
6794 lbs/ 80 psi / 12504 lbs
7188 lbs/ 85 psi / 13229 lbs
7581 lbs/ 90 psi / 13952 lbs
7973 lbs/ 95 psi / 14673 lbs
8364 lbs/ 100 psi / 15392 lbs
8754 lbs/ 105 psi / 16110 lbs/
9142 lbs/ 110 psi / 16825 lbs
9530 lbs/ 115 psi / 17539 lbs
9918 lbs/ 120 psi / 18252 lbs/ referencepress
10304 lbs/ 125 psi / 18962 lbs
10689 lbs/ 130 psi / 19672 lbs
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Old 12-06-2022, 10:44 AM   #5
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Quote:
Originally Posted by jadatis View Post
Then I made you a pressure / axleload list with even tighter formula then that of the list you gave and gives 90% of the calculated loadcapacity per axle
What formula did you use for 90%? 8380 * 0.9 = 7542 not 6794 at 80 psi of single.

This is the recommended safe pressure from Michelin I would imagine there is a safety margin in that calculation, but definitely can't hurt to add another 10% margin other than possible loss of contact patch and ride harshness, it will reduce sidewall flex which can potentially improve handling in its own way.

Michelin also makes the point that the tires need to be inflated for the heaviest side with equal psi. So getting a 4 corner weigh is important to do this right if I want to run close to minimums.

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My goal is to try and find the minimum safe pressure to see how it effects handling, going from 120 to 100 to 90 has so far improved harshness and more importantly front end wander which is my main focus to make long haul driving pleasant. It is much better than from the dealer after a Josam alignment and Safe-T-Plus install but still could be better IMO and I am trying to do process of elimination to see what actually works.
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Old 12-06-2022, 01:24 PM   #6
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Then I made you a new list to show what I done.
In front of psi, 90% of the loadcapacity, and behind 100%, and given per tire and only single load. So this list you can compare with the Michelin list you have, and you see that my calculation leads to lower loadcapacity then the Michelin for the lower pressures. The pressures of Michelin list I put behind with M.

Then you see that at front of psi is exact 90% of behind psi.

I checked the calculation of Michelin list and its made with the formula for LT Tires officially used in US. Newer lists for trucktires are nowadays calculated with the official European calculation used for all kind of tires, and since 2006 also in US for P-tires.

My calculation is based on an article of IR J. C.
Daws with an alternative better calculation.
Mayby my calculation is too tight, but I am using it for years now. If I would change it, I would get reactions that old lists of mine comes to different values as the new.

Mayby they builded in some reserve, but I would not bet on it. Most reserve is because you probably dont drive the 75mph Max speed of tire ( or RV tires sometimes 87mph) for wich the maxload is calculated.



90% 1tire/ cold psi / 100% 1 tire/ M is Michelin list
1383 lbs/ 30 psi / 1536 lbs
1589 lbs/ 35 psi / 1766 lbs
1794 lbs/ 40 psi / 1993 lbs
1998 lbs/ 45 psi / 2220 lbs
2200 lbs/ 50 psi / 2445 lbs
2402 lbs/ 55 psi / 2669 lbs
2602 lbs/ 60 psi / 2892 lbs
2802 lbs/ 65 psi / 3113 lbs
3001 lbs/ 70 psi / 3335 lbs
3199 lbs/ 75 psi / 3555 lbs
3397 lbs/ 80 psi / 3774 lbs/ 4190M
3594 lbs/ 85 psi / 3993 lbs
3790 lbs/ 90 psi / 4212 lbs/4550M
3986 lbs/ 95 psi / 4429 lbs
4182 lbs/ 100 psi / 4646 lbs/4895M
4377 lbs/ 105 psi / 4863 lbs
4571 lbs/ 110 psi / 5079 lbs/5205M
4765 lbs/ 115 psi / 5294 lbs
4959 lbs/ 120 psi / 5510 lbs/5510M
5152 lbs/ 125 psi / 5724 lbs
5344 lbs/ 130 psi / 5938 lbs
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Old 12-06-2022, 01:50 PM   #7
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Quote:
Originally Posted by jadatis View Post
My calculation is based on an article of IR J. C.
Daws with an alternative better calculation.
So your are using a more conservative formula then reducing load capacity by 10% on top of that?

Any link to information on this more conservative formula? Not turning much up searching IR J. C.
Daws other than other posts by you Would like to understand what's "better" and why one should tack on another 10% reserve on that. At some point you are going to over inflate losing significant contact patch and compromise handling, otherwise we would just always inflate to max psi.

Quote:
Originally Posted by jadatis View Post
Mayby they builded in some reserve, but I would not bet on it. Most reserve is because you probably dont drive the 75mph Max speed of tire ( or RV tires sometimes 87mph) for wich the maxload is calculated.
Would be odd for a large company like Michelin with its exposure to liability to instruct us to inflate to chart based on measured weight while leaving no safety margin.

I drive typically 65mph with a very occasional 70 mph pass, tires increase in pressure 15-20psi once warmed up, at 90 psi cold they seems to hover around 107psi hot.
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Old 12-06-2022, 02:05 PM   #8
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Here the pdf, Mr Daws died february 2017.and his site is not in the air anymore.
I contacted him after hovering over that article, and he gave me much usefull information in time.

That the reserves are low, is proven by ST tires, wich gave manny failures in the past.
Those are calculated in maxload in the larger sises for 65mph, a bit overloading and /or higher speed, brings overheating. This is only allowed ZERO times in tires use, beginning internal crackes, tear further in time, untill mayby after 3 years that far that tire blows or treath separates.
Then that once overheating is long forgotten, and other things are blamed.
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File Type: pdf tirepressureatreducedloads.pdf (427.2 KB, 5 views)
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Old 12-07-2022, 07:31 AM   #9
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I advice you to determine the axleloads acurately, and you did by weighing.
Once you weigh fully loaded per axle-end, I can make you a 95% axle- end list and search then heavyest side back.

Then use the pressures searched back in my made list, and give feedback after a while, if comfort and gripp and roadhandling is acceptable or not.

What have you got to loose, to high pressure is almost never bad for the tire. But to low gives that overheating, and that is ireversible.

Reserve is needed for next.
1. Pressure loss in time.
2. Inacurate reading of pressure.
3 In case of axle weighing R/L unbalance on the axle.
4. And what I might have forgotten.

But not for ambiŽnt temperature change, that aranges nature itself.
In short, I assume average ambiŽnt temp 70 degrF, then if 40 degrF outside the cooling down of tire material is more, because of the more temperature differences between tirematerial and in and outside tire air.
Then by the lower pressure more deflection, so more heatproduction, but compensated by more cooling down.
Then not needed to highen up the pressure for safety, but may do so for roadhandling and fuellsaving.

At 100 degrF outside, the other way around, lesser heatproduction and cooling down but still in balance, so no overheating.
Then certainly dont lower the pressure, otherwise more heatproduction.
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Old 12-07-2022, 08:53 AM   #10
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Quote:
Originally Posted by jadatis View Post
Here the pdf, Mr Daws died february 2017.and his site is not in the air anymore.
I contacted him after hovering over that article, and he gave me much usefull information in time.
Thanks very good info, what formula are you using? I built a simple spreadsheet based on the basic formula and could get numbers close but not exactly the same with a n = .94, Mr Daws seems to recommend using n = 1 for a simple model with reserve built in. Are you using a more complex stiffness model?:

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Michelin is not exactly using this model either, but seems similar to n = .6 - .7 although does not line up exactly

I understand ambient temp changes and cold pressure will go down and ambient goes down requiring adding air. I am not new to this I ran my gas MH 66,000 miles working out much of the handling issues and keeping tire pressure in check with no blowouts or tires issues, even with lots of abuse including off-road. This new motorhome seems more sensitive to pressure changes and handling, these large Michelins do seem like much better tires than the much smaller Goodyears.

I will not be adding or removing much gear as its packed how I like, however as mentioned I need to weigh with full fuel and water and get 4 corner weights.

TPMS reported no significant temperature changes at highway speed after lowering pressures, they seem to hover around 130F regardless, my understanding is 195F is the danger zone where damage starts to occur.
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Old 12-07-2022, 09:37 AM   #11
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I checked your sise in the Michelin pdf to be calculated with power 0.65, and then mayby in kg and kPA and rounded, conversed to psi and lbs and again rounded.
Not a verry sciencetific aproach.

That power is translation of dutch macht, but I think in english is exponent.
In any case power 2 is square and power 0.5 ( 1/2) is root, and power 1 is lineair calculation.
So correct me if power is the wrong word for it.
I see you using the ^ sign for what I call power rightly.

The closer to 1 to calculate loadcapacity for a certain pressure, the lower the loadcapacity.

If you want spreadsheets, I made for it, mail me at my hotmail.com adres with username jadatis ( combine yourselfes, spamm machines cant this way... I hope)

But I explained mr Daws calculation as a loadcapacity at zero measured pressure with the valve closed, and the rest lineair calculation.

My calc also a 5% for 36 psi zeropressureload, and the rest with power ^ 0.95 calculated.
A power he adviced to use for trucktires when wanting to use the power-equation alone.
So mayby a too tight formula.

I also checked the 315/80 R 22.5 in the pdf of Michelin, and it was calculated with power ^0.82, a power Mr. Daws adviced to use for personscar tires with referencepressure 36 or 35 psi.

So stange different used calculations in same pdf of Michelin. I Think these last mentioned trucktires are used by fleetowners, who can also pay expensive lawyers, but dont need to, because if tirefailures often and not solved, they buy tires elswhere.
The individual RV owner can be influenced by so called research that 50+% drives with wrong pressure, so they blame themselves if a tire fails.

Further discussion possible by mail, then I can attach spreadsheets and more.
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Old 12-09-2022, 06:32 AM   #12
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I have been looking in the desighners guide of ETRTO, wich I downloaded for free from pdfcoffy.

In there for standard load personscartires loadindexes.
For the 265/75R16 they give loadindex 116
Is 1250kg/ 2756lbs maxload, and also gives a power to use of ^0.65 to calculate the maxload for XL/ reinforced/extraload.
Earlyer I already deternined the power^0.585 for that, a power introduced in 1928 to calculate pressure for diagonal tires.
If I then do (80/36)^0.585 x 2756, I come to loadindex 132 for the D-load tire you have, and it gives 123 loadindex.
This would influence the to natures law needed pressures to prefent overheating.

Then maybe even 35 psi would give no overheating for your new E-load tires.
Going to study it further in the desighnersguide.
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Old 12-09-2022, 07:28 AM   #13
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Wrong topic posted, sorry, cant remove it anymore.
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