Registered to this forum to inform you about tire-pressure calculation.
I dont have an RV at the moment, and live in Holland ( Europe) , so excuse me for some strange words once in a while. Wont introduce myself in a seperate topic. Hovered over the 5 pages of this topic so might overread some good advices.
End of 2007 got hold of a copy of page 14 of the standards manual 2007 of the ETRTO ( European Tyre & Rimm Technical Organisation = European TRA), in wich formula and extra addings to calculate a minimum pressure for a load on a tire.
This formula is used to make advice-pressures for cars with radial tires.
Declared the formula holy and learned myself Excell to make spreadsheets for it.
Translated a few from Dutch to English to go worldwide with it, and react on many fora I found by googling for tire/tyre-pressure.
Found out that this formula was basically the same in America, but with an other power in it, wich lead to lower pressure for the load or the other way around higher loadcapacity for the same pressure.
This gives more deflection at lower pressure/loadcombinations the the tire can stand.
For the tires that are mostly used on RV's ( C- to H-load) even the European Power gives somewhat to low pressure for the load.
This together with some tires that have to high maximum load given on the sidewall already gave problems in the Ford/Firestone-Affaire.
After that tire- and car-makers got nervous and adviced higher pressures then calculated up to the maximum of the tire.
What the tire-lobby tries is that they can describe the pressure, and we dumb users have to obide that, and dont use our own common sense.
I found an article of an American IR J.C.Daws with another way of calculating and comparison with the old way and different used powers in Europe and America. Endconclusion was that the European power used is adequate for Standard load tires and XL/reinforced/Extraload ( different names for the same kind of tire), but for C-load/6PR and up it even gives to low pressure for the load, or to high loadcapacity for the pressure.
So now about this topic.
I saw a link to a pressure/loadcapacitylist from Maxis here, and that is made wrong for different reasons.
1. the wrong power is used of 0.7 as is normal in American TRA system, it leads to to much deflection of the tire at lower pressure/load combinations.
2. C- to E load is placed in the same list, mind that the steps in loadcapacity is inconsequent around the reference-pressures (Pr further will come back to this what it is) of the loadratings( that C-to E-load)
For every loadrating a different list should be made because a stiffer tire ( E stiffer then C) needs a higher pressure for the same load.
They made a calculation for one of the loadratings( fi D load) and placed the maximum load ( further Lmax) of the other loadratings in that list.
You dont have to be an Ingenieur to understand that this is unlogical to laws of nature.
already saw good advices for determining the loads to search back in those lists ( weighing ) but if you then look in a wrong list , the carefully determined reserves gets lost for a part.
I even made my own univeral formula from wich the old and the new "Daws" formula can be made by filling in the right power and construction load ( my own input= part of the maximum load that is carried by the construction of the tiresidewall depending on the deflection) and use that formula with those settings as I think is right in my spreadsheets.
Also made Pressure/loadcapacitylists for the most used loadindexes.
If you look back in those lists , the calculation part of it is even saver then the European calculation.
With this all I think the determining of the pressure for a load is to laws of nature save, what laws and standards of country's and organisations say can be different, but those are yust what they decided.
for the question of topicstarter it means that if the 65 psi is the right calculated pressure for his load, the 80 psi tire needs higher pressure for that same load, because its a stiffer tire.
Sertainly the front tires dont need that high pressure mostly, the front axle is seldomly used to the GAWR ( gross axle weight rating).
That 65 and 80 psi is called the reference-pressure ( Pr ) and is not the maximum pressure of the tire( Pmax) . Even TRA allows 10 psi for LT tires extra and 20 PSI for trucktires. Mind the stemms then must be HPrubber stemms ( up to 90 psi) or metal ( more then 145psi) cold .
to make this long first post complete 2 links , one to the map with PDF's of pressure/loadcapacity lists , and the other to my map with RV-tirepressure-calculator,still under constuction.
https://skydrive.live.com/?cid=a526e...E092E6DC%21904
for the loadcapacity.
and for the calculator
https://skydrive.live.com/?cid=a526e...E092E6DC%21793
From these maps you can navigate my complete public map of skydrive that belongs to my hotmail adress with the same username as here, like in a forum.
a lot about tire-pressure.
To use a spreadsheet first download by
Rightclicking , then choose download from dropdownlist, not use the first 2 items like open in Excell or web-app. After download and eventual virus-check , open in Excell or Open Office Calc to use it.
The PDF's can be leftclicked too but then opened in the browser , if you want to have it on your computer, download like described above.
Greatings from Holland, and if wanted, to be continued.