How to compare tires for replacement and needed pressure

[jadatis]

Because I hyjack a lot of topics with my presssue calculation and tire-replacement advices, I made my own topic, so you can react and discuss about it here. First a part for comparing tires when wanting to chanche , wich is also the title. But later I can add the pressure calculation part.

How to compare tires for aftermarket replacement and needed pressure.

First is why we want to chanche the tires to our Travel-trailer of 5th wheeler.
1. More savety , so blowouts chanche gets practically zero.
2. Longer live time or more milage of the tires.

There are more reasons , like for good looks , better riding quality etc but thats personal.
Most travel-trailers and 5th wheelers are supplied with tires that can barely carry the Gross axle weight ratings , and because overloading , and unequall loading makes the loads on the separate wheels go over that , the tires must be save for those larger loads then the vehicle maker gives as maximum allowed. We can discuss about this but in practice it happens so you have to be ware of it.
But when you want to chanche tires to get more load to carry , you have to take 2 things into account.

1. To laws of nature the new tires must be able to bare substantially more then the old ones.
2. The new tires have to fit in the same space as the old, so larger diameter and widt is only allowed up to a sertain limit.
For part 2 there are several programmes on sites to calculate the sidewall , so I concentrate on part 1.

The tire maker gives mostly on sidewall of tire, the maximum load and pressure needed for that , also called maxloadpressure or referenc e-pressure and speedcode of tire in shape of a letter . Will shorten it further by
Lmax(load max)
Pr( Pressure reference) This is not the maximum pressure of the tire ( Pmax)
Vr ( velocity reference).
Especially for truck tires different Lmax are given for different Vr for the same sises of tires at same Pr. But also for LT(licht truck) tires and ST( special trailer) tires, this system is used.

The tire maker determines a sertain deflection of the tire for the Vr at wich the rubber does not bend that much so it gets hardened by the temperature at specific places of the tire so it gets damaged. For higher speed less bending of the rubber so less deflection is allowed wich gives lesser Pmax for the same Pr. But for lower speed more deflection allowed so more maximum load.

This is the reason you cant compare tires only by the Lmax and Pr. But also have to bring that back to the same Vr.

I will first give the most simple system , by things I noticed at different given L max at different Vr.

In Europe where I live LT tires are called C-tyres for commercial tyres and also “for trailer use only tires “ are in the market wich are speedrated N=maximum speed and so Vr of 140km/87m/h and almost always have a Loadindex of 2 higher then the same sises and Pr in C-tyre wich are Q speedrated ( 160km/99m/h) wich is then Vr ,or if higher speedrated Vr is also 160km/h. Also for truck tires a second loadindex is given for lower speed on sidewall, and then for every 20km/12,5miles/h lower loadindex LI gets 2 higher.

This loadindex system is so made that 2 higher gives a sertain percentage higher maximum load and then rounded to logical steps.

So you can say that for every 20 km/12.5 m/h lower the Lmax gets a percentage higher. If thats a good system to laws of nature is questionable but the tire makers use it and so we can use it to calculate the Pmax back to the same speed so we can compare tires if the difference is substantial to laws of nature or that its only a few percent so its not the trouble of doing.

ST ( special trailer ) tires are speedrated to 65m/ 104km/h and for that reason have a higher Lmax then the same sises and Pr tire in LT, and that is purely for reasons of more deflection allowed.
You can compare a ST tire with a J-speedrated tire wich is for 100km/62m/h and I suspect it is originally calculated in its Pmax for 100km/h and the 62 is rounded to 65 for logical step reasons.
If not so we can use it still to compare the tires in the 2 LI step /20km/12.5m/h system.

So to compare we first calculate the Loadindex back to 160km/99m/h Vr for the old and the new tires. Then we can compare the maximum load that gives the same savety and livetime deflection , so we are then comparing apples to apples again.

In America the speedrating is not always given on sidewall so then you have to google for information of the tire to determine that.

Here a list of speedratings in wich I calculated by head the miles /h so can be off a mile or so.

J= 100km/62m/h
ST= Special Trailer = Vr = 104km/65m/h but in comparing use J speedrated.
K=110km/69m/h
L=120km/75m/h
M=130km/81m/h
N=140km/87m/h
P=150km/93m/h
Q=160km/99m/h
R=170km/105m/h Vr is exeption here also 170km/h
S=180km/112m/h From here up to V speedrated Vr is 160km/99m/h for wich Pmax is calculated.
T=190km/125m/h From S is not used for traveltrailers so I will leave it at this.

So if we have a ST tire with 113 loadindex and we want to compare it to a LT tire with speedcode N with loadindex 115, we have to handle next way.

ST calculate back to N speedrate is from J to N is 40 km so 2 steps of 20 km/h so 2 times 2 LI steps =4steps lower gives 113-4 is 109 Loadindex to use for getting same save deflection of tire.

But we would even calculate it back for 160km/h=Q speedrated so even 60km higher and we simplifie by taking 1 LI lower for every 10km higher Vr.

So 6 x10 km higher gives 6 LI steps lower so 113-6=107.

Then the LT tire with 115LI N speedrated, only 2 times 10km higher speed so 2 steps lower LI = 115-2=113.

Then the difference between the ST 113 and LT115 given is for same Vr 113-107=6 Li steps difference to compare and this is a substantial difference so gives substantial more savety and livetime for the tires.

These examples I have taken from another topic in another forunm, in wich the ST had a Pr of 65 psi ( D-load/8Plyrating further PR) and the LT a Pr of 80 psi ( E-load/10PR).

So now we get to the to use pressure for the old and new tires.
I made spreadsheet for calculating advice pressure in wich I take when weighed 10% reserve to the real load for savety reasons , this makes the percentage to use from the weight the pressure is calculated for ( L% for loadpercentage) 91 % , and I concluded that above 85% L% gives still acceptable comfort and gripp, but is discussable.

I state this for a tire with Vr of 160km/h because it has to do with the deflection. That is the reason why I want to calculate all the comparing tires back to that 160km/99m/h so this bumping-border can be compared to in the needed pressure.
Also I give a dropdownbox in it, with the loadindexes ( LI ) and the maximum load belonging to that in KG and LBS so you can select that easily. In America Lmax is still given in between of those loadindexes, so you have to round it down to the first lower step for comparing and calculating needed pressure. Wont give large differences so no problem.

This makes that if you fill in ST tires or J – speedrated the maximum load is calculated for more deflection and it gives more reserve for the bumping-border but brings the damage border get closer, wich is at over 100% for 160km/h.

To make it short a very discussable system I introduce, but to compare it gives a handle.

I also give the weight-diffence R/L at wich one tire gets over the damage border of 100% and under the bumping border of 85%.

Further I calculate with a saver formula then the tire-makers , that takes care of the same deflection over the whole range of pressure/loadcapacity, wich is also the goal of the formula of the tire-makers , but wich does not make this goal, mine is saver.

So if the Pmax-es are calcuted back for the comparing tires to that same Vr of 160km/99m/h the bumping and damage border can be compared too.
In further reactions here I can add information and explain things , this is the basic principal. Also discusion can be held here , so we dont hyjack other topics.

Then I am thinking about the Pr further explained and the different Load kinds that are used with their belonging Pr . And the more pressure over the Pr that is allowed by the tire-makers organisation TRA.

[C-Bears]

I really tired to read all of this but got a headache. I never really cared about speed ratings on my trailer tires. I just scale my trailers and know exactly how much weight my axles are carrying and get the appropriate tires.

In my current FW it was obvious I needed G rated tires instead of the OEM ones they came with. Since I rarely would have the need to exceed 65mph I don't need a chart for any of that.

[jadatis]

To get back on my research on the Loadindex system .
Made a PDF of it in wich I grooped it in lines of 24 and 3 x 8 lines
From LI 0 to over 250 LI so enaugh I think.

At the top I placed the rules of tumb so in every language it is understood.

You can also use it to see what LI to use when you know GAWR.
then yust look up the GAWR and go 1 line to the left to get single axle LI with belonging KG and LBS. For Dual load go 2 lines to the left.

Placed the PDF in next map , take the Loadindex24+ version ,in wich every 2 lines blu so easyer to compare for 8 LI steps.
Extrasave pressure/loadcapacity lists with in head map the Loadindex-chard

but here the picture I made of it, all in black ,was to lazy to make a picture of the 24+ version

[jadatis]

but for those who want to know, its not more trouble then searching the internet for lists.
Weighing per wheel(pair) is a thing you always have to do.
Also determining maximum load and pressure needed for that is always a thing you have to do together with speedcode of tire, and all can be found on sidewall.

Only difference is instead of searching the internet for a wrong made list, fill the gathered data into my spreadsheet and its filled in for you.

My way makes clear what reserves you have , so dont fill yust enaug for the tire with highest load on it, why balancing on the edges of what is still save if you can have a reserve.

And if you cant handle it , give the needed data ( of yellow and orange cells) and let me do it for you.

[Allworth]

I know you mean well, but if I have to go to that much trouble I'll just sell the Titanium and stay home!

[jadatis]

Made a version 2 of the spreadsheet in wich better explanation how to fill in .
Also a small list,taken over from Goodyear list with ST sises and maximum loads and AT-pressure, so you can copy that to the right place.

Here and example picture of what you see when you open the spreadsheet and filled in with first on list Goodyear.
See here the page from Goodyear and compare the loadcapacity's
Goodyear list

And here the picture of filled in spreadsheet

[Mile_High]

My chart:

LT? Yes/No If no, move on to next tire

Made in USA? Yes/No If no, move on to next tire

Load capacity > axle rating? Yes/No If no, move on to next tire


When all = YES, buy the tire.

[Allworth]

Too much thinking!

Too little camping!

[JIMNLIN]

Hmmm....been working for 45 min trying to figure out how to use the charts. No luck.

IMO using the tire makers minimum pressure charts for each size are a whole lot easier...... for those that want to use them .

[jadatis]

laknox wrote:
Too much for me. My rules of thumb are:

1. Step up one load rating over the placard, either at delivery of a new trailer, or when you swap out the first set of tires.
2. Stay away from STs whenever possible, no matter the tire size.
3. Always run at max sidewall pressure unless running extremely light (like completely unloaded), then drop 10 lbs.

Works for me.

Lyle

But thats yust what this topic is all about.
What you do is using general rules, wich sometimes are exagerated for the situation , but sometimes even aint enaugh.
Meaning of all this is that you can make those choises 1 to 3 more secure.

Only handed as a tool , you still must keep using your common sense.

I realise the last spreadsheet looks complicated, but the list that now are given by tire-makers are made that simple that they can give dangerous answers for most situations.
If you yust open the spreadsheet and begin at the top and work yourself to the bottom, the list is automatically made and filled in .
Then you see as in the example that there is not one ideal pressure but a range in the pressure in wich comfort and gripp is acceptable and no damage to the tires done.
The higher your maximum speed and the more comfort you want, the narrower that range in pressure will be.

There is nothing wrong with ST tires, its the wrong use of them that gives the problems.

Had myself told that ST tires have lesser profile dept so lesser isolation of the rings that produce the most heat by driving.
And ST tires dont make that many miles mostly, so the profile is enaugh for the miles those tires make.

If you use the system described in this topic to determine new ST tires for your trailer or 5th-wheeler , so more reserve, they can last almost as long as when you put LT tires on them.

[Mile_High]

Wow, I would have to hire somebody to understand it. I just replace the factory G614 with G614. Kind of simple really.

[laknox]

Too much for me. My rules of thumb are:

1. Step up one load rating over the placard, either at delivery of a new trailer, or when you swap out the first set of tires.
2. Stay away from STs whenever possible, no matter the tire size.
3. Always run at max sidewall pressure unless running extremely light (like completely unloaded), then drop 10 lbs.

Works for me.

Lyle

[jadatis]

I made a spreadsheet wich makes a pressure/loadcapacity list, meanth for trailers and 5th-wheelers.

Fist wanted to make PDF for every tire sise , but would give a page for every sise and even one more for dual load.
The dual load then would be included in the list so that problem was solved, but meant more loadcapacity's per pressure.


So chanched my plan and made this spreadsheet in wich you can give the name of the tire and sises and maximum load, AT-pressure ( Pr) speedcode and it gives a list with at every pressure even 10 loadcapacity's for different speeds , with system used described as in this topic and minimum load for reserve.
Swiched the koloms and rows as is given in normal P/LC lists of tire -makers so you have to get used to it.
Explain what you have to do , but you can even give seperate tire loads maximum and minimum, and also can click the link to another sheet in the document where you can give seperate weighed tire/axle loads and it calculates for you minimum and maximum load per tire . For that sheet yust fill in what you weigh, so dont be clever to devide by 2 if dual for instance, it is all conversed to load/tire for you. At the bottom click the link to go back.

So if you fill in all the P/LC list is marked for you automatically.

For the time being, only use it for trailers and 5th-wheelers , but you can also fill in LT tire with its speedcode and it gives what this sise and AT-pressure would be if it was a ST tire.

Here an example , but if you want to know it for a tire and you dont have Excell or Open office CALC, give the needed data and I will do it for you .

Here a picture of an example , see I put the wrong sise to it , and fictiv weights max and min given so its filled in
See that the red( highest load on a tire) and green( lowest) are in the black printed area between 150 psi and 130 psi.
so this would mean that you have to fill to 150 psi even and if in time it drops to 130 ( cold) the tires are still save.

For the Canadians here, you can also use KG and at the back of the lines the pressure is given in kPa.



The spreadsheet and PDF made of it and this picture, are in next map. but normaly you would have to print it out and use a marker.
The pressure loadcapacity map with the spreadsheet, PDF and this picture

Have not tested it thourohly so can be some bugs in it.

Always keep yourselfes to the rules and regulations of tire-makers and trafic laws. So in this example if you think 150 psi would not be allowed by the tire-maker use 140 wich is mostly always allowed for a Pr of 130 psi. Also counts for the speeds to drive, but be onest , if the police dont get you driving 75m/h where 65 is allowed, Nature will punisch you if you use pressure that is yust enaugh for 65 with no reserve.

[jadatis]

You always must stay within the rules and regulations of the TRA or law.
But doing that for for instance ST tires , gives the experiënce that still often blowing tires and damage that goes with it.

So What is on sidewall counts for staying within the rules and laws .
But my conversing system takes care that your tires last longer and chanche on damage to tires by driving alone is practically zero.

So everybody happy, you stay within the rules of TRA etc and within the rules of Nature.

[FastEagle]

jadatis wrote:
Because we are only conversing to higher speed it can even be simpler.

Your posts are always interesting reading. But, not much we can take to the bank because of regulatory differences. For instance, load indexes, - even though displayed on some ST and LT tires - are not the official loads here in the USA. For us it’s the load range letters for those particular tires. The European truck tires with speed ratings set below 65 MPH by their manufacturers are also a problem for the USA market because the TRA does not allow tires with speed ratings lower than 65 MPH to be manipulated with air pressure and weight reduction to be operated above the manufacturers speed restrictions.

FastEagle