Tire pressure plus sizing formula -- yea or nay?

[Thunder240]

I understand that lots of factors go into deciding what tire pressure to run, and that it's ultimately subject to personal taste. But since I'm plus sizing, I'd like to translate the recommended pressure with old tires into a "recommended" pressure with new tires. I'll then adjust based on the "recommended" pressure with new tires to suit my taste.

 

I read on a random website a rule of thumb, which supposedly is:

 

[sticker's Recommended P.S.I.] *([Original Tires Max Load Rating] / [Original Tires Max P.S.I.]) * ([New Tires Max P.S.I.] / [New Tires Max Load Rating])

 

But because I don't necessarily trust the source, I wanted to run it by the guys on this forum. The units work out, so that is good. But if the two ratios are both inverted, the units still work out. And if the factor by which you multiply [sticker's Recommended P.S.I.] was greater than 1 before inversion, it's less than 1 after inversion.

 

For example, I'm going from my OEM Turanzas in 205/60R16 (with max psi 44, max load 1356), to Conti ExtremeContact DW XL in 235/40R18. Using the above formula, the product of the two ratios is 1.033. But if the ratios are inverted, it's .968. This works out to about +1 psi using the original formula or -1 psi using the inverted formula.

 

Can anyone say whether this plus formula is reasonable?

 

Much thanks!

[jadatis]

https://skydrive.live.com/?cid=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21235

 

In this map the ultimate spreadsheets to re-calculate for other tires.

workes with the same formula and system that the European tire-makers use since decades, and the Americans swiched over to in 2006 .

It prooved to be save for normal and XL tires.

 

Registered to this forum to give you this information.

To open a spreadsheet first download it and then open in Excell.

For download , click on the line but not on the name of it , then in the right bar at download.

 

The calculation you write about can go wrong , especially in America.

[jadatis]

My first answer is not still aproved but , reading over your question, I see that you chanched from normal car tires to XL . For normal car tires ( P-tires or Standard load) the reference-pressure ( pressure to calculate with in the formula) is 36psi in Europe and 35 psi in America. For XL tires ( Extraload/reinforced all the same) the ref.press is 42 psi in Europe and 41 psi in America but exeptions of 44 psi can also give.

This is not the maximum pressure of the tire, as is written on these tires sidewalls.

Difference between reference- and maximum-pressure is used for higher speed and alignment ( camber angle).

So this also gives diferences in the calculation.

 

My first post was done in a hurry , so now I will write more what brougt me to tire-pressure calculation.

I live in Holland and got hold of the European formula in 2007, and went running with it.

Learned myself Excell to make spreadsheets for it. Translated a few to English to go worldwide with it. Discovered by reacting on several american fora , that the calculation there was different , and gave lower pressures , but later was upgraded to the European calculation, wich proved to be save for SL and XL tires.

Now I frequently Google for tire/tyre-pressure to see if I can spread my information to the world. Think I am the only non tire-specialist, who know all the ins and outs about it

 

Greatings from Holland

Peter

My E-mail is on hotmail com with the same username as here( combine it yourselves,)

[Thunder240]

Peter, welcome, and thanks for the post and for creating this resource. I'll download the spreadsheet and see what I can determine from it regarding reference pressure for my new tires.

 

Cheers!

[jadatis]

If you need help, just ask me here.

You need from the car the GAWR's ( Gross Axle Weight Ratings) front and behind.

They are mostly under the motorhood somewhere, or in one of the doorstyles.

Also the maximum technical car speed, or speed you sertainly wont go over.

The alighnment in form of the camber angle is seldomly needed.

 

From the tires, the maximum load and tire-kind for the reference-pressure, as it is called on the formula sheet. Also the letter that stands for the maximum to use speed of the tire . Example here 235/40R18 91V, then 91 stands for the loadindex = maximum load 615kg/1353lbs?, and V stands for maximum speed of tire 240km/h/150m/h?. Sorry , estimated the LBS so can be a little different.

In America the excactly calculated maximum load is written on the sidewall in the form "maximum load xxxx lbs AT yyy psi maxpress ( cold)" or a little different.

In my spreadsheet you have to round that down to the nearest loadindex in the dropdownlist.

Last is that for low aspect ratio tires , here the /40 , I have my doubts about if the calculation of the maximum load is not to high.

To give absolute savety for the tires , add 3 psi to the referencepressure,

So calculate with 44+3=47 psi for XL tire.

Goal of all the calculations is to keep the deflection of the tire the same , and the maximum load is calculated with a much more complicated formula, and calculates what the load is at the reference-pressure for the deflection that does not give damage to the tires at the reference-speed of 160km/99m/h for up to V tires, At higher speed the deflection may be less and there is a system for highening up the referencepressure for that.

I made a map on my skydrive "all about tire-pressure" wich I used to support my discussion with an American J.C.Daws, who described an other way of calculation wich gives saver answers, and compares it to the old power formula and its different used powers. You can navigate to it from the link I gave , like in a forum. There also spreadsheet "myownformula, where my universal formula is explained and the different formulas are mentioned.

You can also use that , to see what differences it will give with the formula you describe in your beginning post. Then you have to fill in for X=1 and Lc=0. That spreadsheet can be used in the browser, but in Excell you can see the remarks.

 

Enaugh for now, but if you want, to be continued.

And again, if you cant work it out, ask me here or on my hotmail adress with the same username as here ( spam robots cant get my adress like this, people can).

[Thunder240]

Hey Peter, I ran through the calculations using the following inputs:

 

GAWR - 2360 lbs (front), 2340 lbs (rear)

GVW - 4435 lbs

Tire maximum load rating - 1521 lbs (ie 95 load index)

Speed rating - Y

Tire type - XL

Maximum speed - 150 mph (may be a little optimistic)

 

The results are 31 psi front, 30 psi rear (advice pressure - heavy). These pressures are ~ 3 psi less than the recommended pressures with OEM tires. Using a reference pressure of 47 psi instead of 44 psi, the results are 35 psi front, 35 psi rear.

 

Out of curiousity then plugged in the values corresponding to the OEM tire (91 load index, V speed rating, SL tire type), and got 30 front 30 rear. Adding 3 psi to the reference tire pressure, these results increased to 33 front, 32 rear. This is 1 psi less than the actual advice pressure printed on the label on the car.

 

Do you believe that the reason the actual recommended pressures on OEM tires are substantially (ie 4 psi) above the pressures that your spreadsheet computes is simply an extra safety margin that Subaru is building in? Or are they taking a different approach to computing the recommended tire pressure? Given the new tires I bought, would you suggest using the 35 psi / 35 psi pressures computed using the safety cushion as a starting point, and then adjust from there to suit my taste?

[jadatis]

I think your car is from after 2000, and after that the advice-pressures in Europe and probably in Japan where held higher then the calculation gave.

To my opinion this was an over-reaction to the Ford/Firestone Affaire.

Search for it with google, but in short, my conclusion is that in America they calculated with with a lower power in the universal formula, wich gave to low advice pressures , wich gave tire damage, and so roll-over-accidents .

More then 100 people died because of that, so no joke.

 

So you see that there is much more to tell about tire-pressure.

The European calculation prooved to be right for SL and XL tires, but in America they still calculate with a lower power for C-load and up, wich can give trouble in the future , whenn going to the edges of what is possible for a tire.

 

If you are able to determine the loads on the tire accurately and you can measure the pressure accurately , and the maximum load of the tire is give accurately , the calculation of Europe ( and since 2006 also America for SL and XL) gives a save deflection of the tire, an so no damage to it.

Point is that these items can never be determined accurately , even the maximum load of the tire is calculated with a formula wich is to my opinion not sufficiënt for low aspect ratio tires.

That is why you better take some reserve. But if you take to much , things go bouncing and you have less gripp. Concluded by reactions, that if the real load on the tire is under 85% of the weight you calculate the pressure for, then bouncing begins. Can be that if you calculate it to be 85% with the given load , in practice the Load% ( as I babtised it) is 95%.

But the advice pressures are given for GAWR and in real mostly the weights on the axles are lower ( only driver and a little load). so even that advice pressure can give bouncing.

 

enaugh for now,but.... to be continued.

[jadatis]

now you know how it is calculated by the tire-makers, you can see where the formula you give in your starting post can go wrong.

 

First the relation between the maximum pressures can be different from the relation between the reference pressures.

Here maximum pressures probably, but look on the side-wall, max press 44 to 51 for the Standard load , and about 55psi for the XL.

But in America it is still often given like this "maximum load xxxx lbs AT yyy psi ( cold) " and then this is always the reference pressure.

But if you have an European tyre, only the maximum pressure is given on the sidewall, Michelin always 50 psi and others 44 psi , while the referencepressure is always 35 or 36 psi.

 

But then an example with the tires you have and a relatively low GAWR.

If the advice-pressure is almost the referencepressure for the speed it is calculated for, the differences will be rounded zero.

So thats why I use a lower load.

 

GAWR 2000 lbs, OEM tire LI 91= 615* 2,2045=1355lbs maximum load at 35 psi.

then it would give (1000/1355)^1,25 X 35 psi= 23,94 psi =rounded up 24 psi.

With the new tires LI95/max load 1521lbs AT say 42 psi

then it would give (1000/1521)^1,25 X 42 psi=24,86 psi=rounded up 25 psi.

 

"your " formula would give next.

24psi X ( 1355/35)X ( 42/1521)= 25,65 so higher then needed because rounded up to 26 psi. This would never be bad. So this calculation can do

But if you do it the other way around so from XL to SL it can give to low answers.

 

More important is to determine if the old advices where calculated with the wrong American power in the calculation of 0,5 to calculate load capacaty for a sertain pressure is power 1/0,5= ^2 to calculate pressure for a sertain load. Then the differences can get more, because you then calculate for a lower load then it is meanth for.