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.

[jadatis]

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

So converse every tire you find and want to compare first to 160km/99m/h.

J =100km/62m/h| -6 LI-steps|-15.0%
ST=104km/65m/h| -6 Li-steps|-15.0%
K =110km/69m/h| -5 LI-steps|-12.5%
L =120km/75m/h| -4 LI-steps|-10,0%
M =130km/81m/h| -3 LI-steps| -7.5%
N =140km/87m/h| -2 LI-steps| -5.0%
P =150km/93m/h| -1 LI-step | -2.5%
Q =160km/99m/h and higher up to V no reduction.

Then now what maximum load do you need for that 160 km/99m/h.
Law is that your tires must bare at least the GAWR ( gross axle weight rating) so for 2 tires on one axle half of that and 4 on the axle 1/4th of that.
This would mean that an ST tire to law can support a higher GAWR.

But to laws of nature its better to go from the conversed to 160km/99m/h maximum load and the tires together must bare at least 110% of the GAWR.
If you ever weigh per wheel(pair) then you are sertain of the weights, and might prove that my reserve of 10% is absolutely needed to cover unequall loading R/L.

Mind also that if you have Dual load ( 4 tires on one axle) that the maximum load is 7.5% less or 2 LI steps (5%??) lower in European system, and American system even often uses 9% lower or 4LI steps lower ( 10%??) .
This strange difference between percentage and LI-steps is because of rounding down from the calculated maximum load to the first lower LI step. And the difference between EUR and USA is a matter of different opinion about what is needed for it in the system.
so if you want to be on the save side substract for every tire 9% fromm single maximum load, regardless of what is printed on the sidewall.

[jadatis]

Deb and Ed M wrote:
Wow!!! Thank you for this! I will print it, and save it for future tire-shopping use.

Then print this reaction , where I have put it all in rules of tumb.

First the loadindex system
Fist rule is the translation for Dutch expression for rule of tumb.
Every LI step higher add 3% to the maximum load of tire.
Every LI step lower substract 2,5% of the maximum load of tire.
8 steps higher gives 25% more maximum load.
8 steps lower gives 20% less maximum load.

Then the speed thing.

Conversing from lower to higher speed for lesser deflection.
every 10km higher 2,5% lesser maximum load or 1 LI step lower
The other way around for every 10 km lower 3% more maximum load or 1 li step higher.

But you want it in miles/hour.
conversing from lower to higher speed for lesser deflection.
Every 5 miles higher, 2% lesser maximum load.
The other way around, so conversing from higher to lower speed.
For every 2 miles lower 1% more maximum load, 10 miles is 5%.


This will work for between 100km/62m and 160km/99m, for larger steps these rules of tumb will give to much foult.
And always calculate every tire back to 160km/99m/h because for that speed you have best milage or livetime with the calculations for pressure I give in my spreadsheets.

[jadatis]

Lowsuv wrote:
In simple terms :
Does your comparison boil down to the calculation that an ST tire with a load index of 113 on the sidewall really only has the capacity of 91 % of an LT tire with a load index of 115 on the sidewall ?
Calculated (113-6)divided by ( 115-2 ) = 94.6 %

No you cant devide the LI's , they stand for a sertain maximum load, as I explain in the reaction between ours.
But will make the right calculation here to se whats the difference.
113-6=107LI stand for 975 kg and 115-2=113 LI stands for 1150 kg Lmax.
then same division 975/1150= 84.8%

further it boils down to that , with adding that specific ST-tire has 84.8% of the maximum load of the that specific LT tire to give it the same livetime and savety.

Edit: but you can use the system I described so the LT has 6 LI steps higher so rougly 6*3% more maximum load= LT113=1.18* 107ST.
Then ST is 1/1.18= 0.8474=84.74 % maxload of the LT wich is pretty accurate
someone once called this on the road methode, the back of the sigar-box methode.

[jadatis]

The loadindex system research

I have been studying the loadindex system and found out next.
Already clear is that it is originally made for KG wich is international standard, and conversed to LBS.

For every 24 LI steps higher the Lmax gets 2 times more, and the the other way around , for every 24LI steps lower the Lmax gets half of it

Example LI 108 Lmax 1000KG/2205 LBS
So 108-24=84 LI Lmax 500kg/1102LBS
And 108+24=132 Lmax 2000kg/4409LBS

But you can compare this with every LI step of 24.
I calculated this back to 1 step and came to times 1.029302 for every LI-step higher. For on the road use thats 3% higher for every Li step so in the LI/speedstep-sysem here given 10km/6.3m/h lower speed .but in spreadsheets we can calculate as accurate as possible.

This system is somewhat distorded because the steps are grooped in logical steps like 25 kg /20kg/15kg to prevent that it gives for instance 802kg as Lmax. So sometimes it gives 3 li steps more for 20 km/12.5m/h lower.

So instead of comparing with Li steps for speed you can also say that for every 10 km lower the L max gets times 1.029302 more. But 2 steps gives squaire 1.029302=1.059463 and so on.
So you cant say that for 24 steps it would give 24 times 3 % more maxload = 72% more , we already came to the conclusion that its 2 times as much so 100% more. It the same as combined interest on you loan.

This would mean that if you compare a J Vr tire to a N Vr tire thats means 4 times 10 km lower speed means 1.029302 to the power 4 = 1.122462 times Lmax so 12.25 % more maximum load then the N speedrated tire because of the lower speed Lmax is calculated for.

This strokes with what I once calculate for ST that it had about 15% higher maximum load then an LT tire. Only compared one tire for wich I did not notice the speedcode of LT, so probably Q speedrated LT.

So the loadindex-step /speed-step system I first gave can be refined by this combined multiplication factor system.

In my spreadsheets I will implement this system to give more realistic damage and bumping border.
But this means that you have to give yet another extra input , think I will pre fill the fields with same speeds and optionally you can chanche it.
Here a list I made with Excel use it like this. If J speedrated and you want to compare with N , look behind N and see 1.122462 , then divide given Lmax by 1.122462 to get the Lmax for if it was calculated for N speedrated.


K 1,029302
L 1,059463
M 1,090508
N 1,122462
P 1,155353
Q 1,189207
R 1,224054

[Lowsuv]

In simple terms :
Does your comparison boil down to the calculation that an ST tire with a load index of 113 on the sidewall really only has the capacity of 91 % of an LT tire with a load index of 115 on the sidewall ?
Calculated (113-6)divided by ( 115-2 ) = 94.6 %

[azdryheat]

Too technical for me. I'll just stay far away from ST tires to be safe.

[Deb_and_Ed_M]

Wow!!! Thank you for this! I will print it, and save it for future tire-shopping use.