[jadatis]

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 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 witdt 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 reference-pressure and speedcode of tire in shape of a letter . Will shorten it further by
Lmax(load max)
Pr( Pressure reference)
Vr ( velocity reference).

This Pr is not the maximum pressure of the tire ( Pmax)

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 spaces 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 everyt 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 and see 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, or will simplifie by 1 LI step lower per 10km/6,3m higher Vr.
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.
Normally you would say that its only 2 LI difference so whats the use.

ST calculate back to N speedrate is from J to N is 40 km so 4 steps of 10 km/h so 4 times 1 LI steps =4steps lower gives 113-4 is 109 Loadindex to use for getting same save deflection of tire.
But we calculate it back to 160km/99m/h Vr for reason I will explane later.
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 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% givestill acceptable comfort and gripp, but is discussable. I state that 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 weightdiffence 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.

Then I am thinking about the Pr further explained and the different Load kinds that are used with their belonging Pr .