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Old 04-10-2013, 07:19 AM   #1
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Tire pressure advice is made with use of a formula, wich I got hold of in the European version once, and went running with.
The whole tire-pressure story went a little out of hand, and now I frequently google for it , and got much to know about it and maximum load and pressure of tires.

In another topic I already placed some PDF's but here as a seperate topic.

But many people rather look it up in a list given by the tire-makers.
Those lists are made with different versions of the formula in Europe and America, and mostly lead to to high loadcapacity for the pressure given.
That is why I made my own lists with my own formula wich is a combination of that used by the tire-makers , and that of an American J.C.Daws.
My formula always leads to lower loadcapacity for a sertain pressure then that of the American and even of the European calculation, so is saver , and to my opinion, takes care that the deflection of the tire stays the same over the whole range of pressure/load combinations, wich is the goal of all the calculations.

I will give the link to a map on my skydrive of hotmail-adress, with the same username as here, where different made PDF files for the different reference-pressures , given per tire, axle single and axle dual.
In America these lists are given only per tire, so you have to do times 4 for dual load and times 2 for singleload ( because 2 or 4 tires on an axle) .
Best is to use the PDF for single and dual, the number of tires on an axle is done for you.

Look on the tires sidewall for something like this
" maximum load xxxx lbs AT yyy PSI ( Cold) then this is the referencepressure.
Most tires for RV's give it like this, only normal car tires and XL give only maximum pressure of the tire. The yyy is called the reference-pressure and is lower then the maximum pressure of a tire. That means that you may go over that reference-pressure, and that is adviced for higher speed and better riding quallity. TRA allows 10 psi above for LT tires and even 20 PSI for truch-tires. For low speed even extra load above maximum load is supported by TRA ( Tire and Rimm Assosiation ).

Now busy with a manual of how to use these lists , because it can still go wrong , though the calculation is secure.
If you read something wrong , or you estimate the loads to low, there still can be to low pressure , so to much deflection of the tire, and you still get damage to the tires and the missary that goes with it.
So these list are made, so that part of the determination of the pressure is right.

And those lists are mostly hard to find, now you have them at one place.
If you cant find your loadindex in one of the lists , give me a meassage and I will make a list with yours on it .

for example I uploaded one for 65psi , wich begins with 65psi for the reference-pressure then 95li138 for the loadindexes it covers, and last singleaxle that it is gives loadcapacity for axle ( so 2 tires) single load.
So you can pick the list you need by the name of it .
Yust planted the whole series of single axle here .
Attached Files
File Type: pdf 65psi95li138singleaxle.pdf (75.5 KB, 24 views)
File Type: pdf 80psi98li141singleaxle.pdf (75.9 KB, 22 views)
File Type: pdf 95psi100li143singleaxle.pdf (75.7 KB, 18 views)
File Type: pdf 110psi100li143singleaxle.pdf (76.0 KB, 12 views)
File Type: pdf 120psi110li153singleaxle.pdf (75.9 KB, 12 views)
File Type: pdf 50psi95li138singleaxle.pdf (75.9 KB, 20 views)

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Old 02-24-2014, 02:22 PM   #2
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I guess my curiosity still stands-

1) what European formula are you referring to? Is this some standards agency? Some proprietary formula for a company? Do you have a link or documentation for it?

2) what tire/wheel engineering background do you have? Have you worked with/for a manufacturer? Meaning - what makes you an expert in this field?

I am not intending to be, sound or act harsh- I've seen you post advice and you're obviously very passionate about this stuff. I'm just curious as to where the info/your expertise originates from.

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Old 02-25-2014, 02:20 AM   #3
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I already explained some things in the opening post of this topic, but here some history.

My profession is Optician-contactlens-specialist in Holland , and began in 2005 with an old motorhome ( Called Camper in Holland) . Had 3 in a row, but because of motor-problems with last one stept over to Caravan ( Travel-trailer).
End 2007 on Dutch camperforum a discussion about the high pressure that is adviced for motorhomes of 5.5bar/80psi all around.

My idea was that if you only had 80% of maxload on a tire , you also need 80% of the "maximum pressure( later found out its the reference-pressure Pr) "
Mailed a Tire maker of Dutch Origen about it , and after second mail they sended me a copy of page 14 of the standards manual 2007 of ETRTO ( European Tyre & Rimm Technical Organisation, America is TRA= Tire and Rimm Association).
On that page for different speedcodes of normal car tires formula's for calculating pressure for a sertain load , wich could be brought back to one , in wich Pr has to be highened up with a system for higher speed then 160km/99m/h and then multiplied with the camber factor wich is calculated out of the camber angle.
for motorhomes the Speed and camber angle is not an issue so the basic formula can be used.

I declared this formula holy and was allowed to make an article on the Dutch camperforum about it.
Later learned myself Excell to make spreadsheets for it, and translated a few to English to go worldwide with it.

Reacting on American Fora , and Googling for Tire/Tyre-pressure I gathered much information .
Stumbled on an article of IR J.C.Daws about an alternative formula in wich the deflection of the tire is important. The names Padua and Rhine are mentioned in it. He compares it to the formulas used in Europe and America and comes to the conclusion that European can do for normal and XL tires but gives to high loadcapacity for the pressure for C-load and up.
The American formula even worse and is not enaug for normal car tires and XL.
The official formulas can be brougt back to one universal formula in wich only the power is chanched and different in the American and European system.
TRA swiched over as late as 2005 to the Power used in Europe for all kind of tires , but only for standard load and XL,reinforced/Extraload. I ask myself why they did not do that for C-load and up at the same time.
For C-load and up there is still a difference , so American lists , that are made with the Old formula lead to too much loadcapacity for the pressure and so to much deflection wich can lead to tire failure and the missery that goes with it.

To be continued.
Batery of lapptop is empty .
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Old 02-25-2014, 04:23 AM   #4
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So now I am on a wire , and we go on.

The formula that Mr Daws describes is linear and asumes a load to be carried by the construction of the tire ( explanation I give to it) and the rest is logical pressure times surface story.

I contacted Mr Daws to ask for discussion about this and he provided me with usefull information in time.

One thing is how the official formula is determined, and next is partly my conclusion.
1928 the basic formula for diagonal tires was a aproaching formula determined by measuring a large number of different tires , and I assume they put a series of different pressures in it , and loaded the tire until it gave the deflection that it would have when Pr and maxload on it, and put it in graphics.
That graphics line gave a curve and not a straith line.
Also they probably did not go back to zero pressure , I think a range between 100% and 50% of Pr.

so this all made me conclude that there has to be a curve in the graphics because measurements dont lie.
This curve can be get by putting a power in the formula , and that is what the official formula has .
Mind there are 2 ways of calculating. one is to determine loadcapacity for a sertain pressure, the other one is calculating pressure for a sertain load.

I determined my own universal formula in wich, and a construction load ( Lc)as Mr Daws described formula , and a power as in the officiall used formula.
By picking the righ power and Lc , you can make the official formula's and that described by Mr Daws of it,

Lc is the load that is carried by the construction of the tire at the actual deflection .
X is the power used and I made a spreadsheet in wich you can put your own power and Lc to compare 3 different calculations and/or tires.
Determined by the article of Mr Daws that a power of 0,95 and an Lc % of maxload,depending on the Pr, then for 35 psi 6% and for instance 70 psi 3% , to give a save loadcapacity so the deflection of tire stays the same over the whole range of pressure /loadcapacity.

This link leads to a map on my One-drive ( former Skydrive) that belongs to my hotmail adress with same username as here. ( so jadatis) .
This map I specially made to put pictures and spreadsheets in to support my discussion with Mr. Daws.
A lot of spreadsheets with my own formula and at one tab the compare part and a little history of used powers in time.
That power thing is a bit dificult.
But power 2 means Squaire and power 1/2=0.5 means root.
Power 1 is the number itself. On sciencetific calculators there is a button X^y for it and ^ is the symbol for power
Example 3^2 = 3x3=9
3^1= 3
ETRTO uses Lc zero and Power 0.8 for every kind of tire since decades.
TRA used to use for normal car tires/P-tires with Hight/Width division above 50% power of 0.5 even wich lead to to much deflection,But stepped over to the 0.8 power in 2006 saying it was for globally using the same system , but in real it was because 0.8 power is better .

Hope you now have an idea of why I allow myself to be as pigheaded to construct my own formula , because its saver.
Can be that I have to adjust it in time in its power and Lc , but I think my formula gives sooner to low loadcapacity then to high, wich is never wrong.

In next post I will give a picture of different calculations, to show what difference it gives and also a "on the road"methode.
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Old 02-25-2014, 04:29 AM   #5
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Here the picture of the different calculations .
Then notice that the European calculation comes close to that of Mr Daws and mine, but the American old one goes way off, for P-tires and XL.
Even for C-load tires and up the American formula is far off and the European one is beginning to loose it, in the lower pressures.
The Lc part of Mr.Daws formula is somewhat more complicated, but I simplified it by taking one fixed Lc of 100lbs/45kg and use that also in the "on the road methode". So a little adjustment of mine to make it easy , and dont influence the outcome to much.

Thats the reason why Eur stops at 21 psi and the USA at 26 psi in lists for normal car tires.
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Old 02-25-2014, 08:05 AM   #6
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Old 02-26-2014, 07:48 AM   #7
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Thanks! I'm going to save your posts, too!

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I don't know when we'll be able to go camping again...
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