How to compare tires for replacement and needed pressure

[jadatis]

This topic is split of from this one
http://www.keystonerv.org/forums/showthread.php?t=15172
for reasons that I was hyjacking that topic to much, and I promesed to make my own topic for how to compare tires for replacement and how to handle the tire-pressure advice.
Made ducument about it in word first and placed it as reaction here , so you have to scroll reaction #12 to find it.
under here the first split off reaction .

Because the maximum load was not clear of both old and final new tires I did some googling.

The old Maxis are STR ( think Special Trailer RADIAL)
found them on amazon with speedcode R ( up to 170km/94m/h) so made a remark to them because this does not stroke with ST wich is always max speed normal of 65m/104km/h, but allowed 75m/120km/h when 75psi used . for that 65m/h the maximum load is calculated.
of loadindex 113 wich stands for 1150kg/2535lbs.
Must be on the sidewall somewhere as loadindex or "maximum load 2535lbs AT 65 PSI ( cold)" also see if you can find a speedcode letter like that R they mention on the page of amazon.

The final new Bigfoot LT 225/75R16 LRE ( loadrange E= maxloadpressure 80 psi. found them at bigotires here
http://www.bigotires.com/Tire-Detail...FOOT-H-T/12602
with Loadindex 115 = maximum load 2680lbs/1215kg AT 80 psi( cold) with speedcode N = max speed of tire 140km/87m/h .

This difference in speed for wich the maximum load is calculated makes the difference small between the tires , tough the bigfoot is larger and has higher maxloadpressure.

You did not have troubles with the maxis and probably used 65 psi for them.I think 2 years is a short livetime so am not surprised that you did not have any troubles with them. Better would have been if you had treated the Maxis as an LT tire and then even for dual load for tandem-axle.
this gives 81 psi as advice wich is not allowed by TRA ( 10 psi extra is for LT and ST tires) so if you would have put them to 75 psi from the beginning , the tipical wear probably would not have happened and you probably made a longer livetime with them or more miles.

The bigfoot with 115 li are for 140km/87m/h calculated in their maximum load.
In Europe this is the maximum speed for "for trailer use only"tyres, and mostly have 2 loadindex higher then an C(omercial) tire ( Eur LT) with max speed of 160km/99m/h So the lower the speed the more maximum load .
And this is because at lower speed more deflection is allowed .
When I fill this in in my Traveltrailer calculator and use the dual load maximum load wich is 7.5% lower then single so mostly 2 LI lower + 2 LI lower for 160km/h it comes to a LI( loadindex) of 111 wich stands for 1090kg/2403lbs, it gives as answer 84 psi . So that is even as high as is allowed, with still acceptable comfort and gripp. Then my spreadsheet even gives warning of poor reserve of the tires and because of that maximum speed of 60m/h.
But with this speed all the reserves count of 1000lbs weightdifference R/L before discomfort or tire damage begins at high speed of 160km/99m/h.
Seems a bit high but used this for the bumping border . For lower speed damage begins later, so this weight difference gets more before damage begins.

End conclusion, this long story was needed to advice you a pressure that gives your new tires a longer life more miles . Probably even 6 years and 40000 miles or more.

Mind thoug that the valves must stand the 84 psi too. This is sertainly with metal valves but the HP TR 600 series snap in rubber valves are allowed a cold pressure of 6.3 bar/95psi cold.

Here a picture for the bigfoot with the reduction for speed and dual load used , so 4 LI lower then given on the page.
Mayby the difference between the maximum loads is courced by the other speeds that are used by the tire-specialist, mayby they have a list for that how far they can go. ST has 15% more maximum load then LT , I once calculated of 2 same sises ST and LT. Mayby they used that system to calculate more maximum load for the bigfoot for speed of 65m/h instead of 87m/h.
In Holland where I live cars with trailers behind them are allowed 90km/56m/h and it has chanched recently, before that is was the same as for Trucks 50m/h. So the 60m/H I gave as max adviced speed is enaugh, you probably dont drive faster.

Here the picture, and sorry it has gotten a too long story .

[tileman]

[Festus2]

jadatis -
I am not sure whether or not you saw and read JRTJH's response to both your and CWtheMan's posts but I think the message was that Poppy's 5th Wheel, the member who was the OP, never has asked for any advice or explanation about tire pressure.

He simply went to Big O and bought a set of tires. Period.

If and when he asks for advice about tire pressure, I am sure he will but I would guess that he now has far more than enough technical data to think about. I believe it is called "information overload". Can we give it a rest?

Please, give this member some credit - he can figure it out for himself.

Thank you.

[Ken / Claudia]

I guess he was writting english? But, I could not understand it. I can read the side wall of a tire ok.

[CWtheMan]

I think Jadaist feels he has something to say. Rather than say it under someone else’s name/post he has opened the topic under his name. As a member in good standing the rules should allow him to do so. If the team of moderators feel he needs some guidance in the form of a reprimand they should do so in private or just delete the topic as being unsatisfactory/unsuitable for forum debate.

CW

[JRTJH]

Originally Posted by CWtheMan

I think Jadaist feels he has something to say. Rather than say it under someone else’s name/post he has opened the topic under his name. As a member in good standing the rules should allow him to do so. If the team of moderators feel he needs some guidance in the form of a reprimand they should do so in private or just delete the topic as being unsatisfactory/unsuitable for forum debate.

CW

You are correct, CWtheMan. The rules do allow any member in good standing to post when they have something to say. Unfortunately, some members tend to pirate other threads and then get off topic and long winded with their thoughts. This is what happened here. The initial post in this thread was posted elsewhere in the forum. I moved Jadaist's post out of the original thread in which he posted it and started this thread to allow him (and anyone else who wishes) the opportunity to "talk technical" for as long as they want without having to pirate a thread to do so.

Feel free to join the thread and post as much technical data as you feel is appropriate. The purpose of this thread is to hopefully keep this kind of data out of threads where it is not related to the topic being discussed.

[CWtheMan]

Actually I’m a tire regulations & tire specifications researcher, specializing in RV trailer tire Original Equipment (OE) fitment and replacement tire selections.

My experience in the tire industry could be best described as hands-on or technical. I’ve also done some technical training as an instructor for military aviation tires. However, The rules I worked under there were almost entirely from the Naval Aviation Maintenance Program.

My forum posts are safety oriented. Our National Traffic Safety Administration (NHTSA) bends over backwards to present the safest procedures to follow. From tire selection by the vehicle manufacturer to replacement tire selection by the individual vehicle owner.

In the motorized vehicle community the overall selection process is almost entirely in the hands of the retailer. When it comes to replacement tires, the regulations for RV trailer tire fitment are not the same. Many RV trailer owners are not familiar with the differences and become very argumentative when confronted with regulations they are not familiar with.

The most common mistake consumers make when searching for replacement tires is to disregard safety warnings/regulations.

When NHTSA safety warnings and tire industry standards are 100% under the same blanket, consumers must consider that decisions to step out from under that blanket as being unsafe.

CW

[Javi]

One of the most difficult things I've had to learn in this life is....

That most of the time when someone asks for the score in the last Super bowl, they really are just wanting to know the score and not how to play the game.

[jsmith948]

Originally Posted by Javi

One of the most difficult things I've had to learn in this life is....

That most of the time when someone asks for the score in the last Super bowl, they really are just wanting to know the score and not how to play the game.

VERY well stated!!!

[jadatis]

OK I realise that I was hyjacking this topic a bit, and a special topic about it I will open when I have all together. But what CWtheMan and I wanted to make clear that topicstarter had to use higher pressure, what he probably would not have done when we did not wrote it here.

So I begin with a new sheet here and react as if it was my first reaction here.

A good idea to chanche to those Bigfoot LT tires at the end from the maxis ST tires.
But be ware that the Bigfoots E-loads need higer pressure for the same loads.
If you keep to 65 psi the bigfoots probably have blowouts in a short while.
Calculated , taking into account the speedcode that you can even put them on 84 psi , wich is higher then the 80 psi given for E-load ,But allowed by TRA up to 10 psi above that 80 psi. This would give the tire maximum savety and still screws dont come loose from the woodwork by shuttering. With that mayby even more then 6 years livetime /40000miles

Even the Maxis ST D-load would have been better off when you had filled them to 75 psi from the beginning, I think 2 years is short livetime even for an ST tire.
If you want to know How I came to this conclusions read back my too long posts

Better this way

[CWtheMan]

JADATIS

Within the document referenced below you will find the industry standards and procedures used to plus size tires in the USA market.

If you have a TRA manual you can compare it with the referenced procedures.

http://www.tiresafety.com/images/Tir...t%20Manual.pdf

CW

[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 .

[Ken / Claudia]

The date code is as inportant as everything else that is printed on the sidewall.

[CWtheMan]

Since this thread is a platform for technical stuff about the tires used on RV trailers, I’m going to use it to see if I can explain some of the underlying myths. And, just maybe something about regulations that muddy the waters surrounding the myths.

The paragraph quoted below can be found in your Keystone owner’s manual. Why? Because it’s mandated to be there by the DOT. It’s a NHTSA safety standard. It supports tire fitment and vehicle certification regulations that empower vehicle manufacturers to set recommended (correct) tire pressures for the tires on your trailer. The recommended tire pressures insure your trailer’s tires are providing enough load capacity to satisfy the needs of the trailer’s certified GAWR.

“To maintain tire safety, purchase new tires that are the same size as the vehicle’s original tires or another size recommended by the manufacturer. Look at the Tire and Loading Information label, or the sidewall of the tire you are replacing to find this information. If you have any doubt about the correct size to choose, consult with the tire dealer.”

Here is a copy of one of the NHTSA documents.

http://www.safercar.gov/Vehicle+Shop...ting/Tire+Size

When reading these posts it’s best to remember that I’m often writing from my point of view. Dissemination of government regulations and documents is not an easy task and many - me included - will sometimes draw conclusions that are not 100% truthful or factual. Whenever I’m in doubt or feel more information is necessary to the reader I’ll post the most current reference I can find or have in my files.

When large volumes of information became available and released to the WWW, government agencies jumped right on the bandwagon and downloaded almost everything they confirmed to be unclassified. But, would they make timely updates? It’s a toss-up. But, NHTSA has been very good so far as I can see.

I’m sure if you’ve read this far you know that there are DOT regulations for tires that are often talked about and quoted in all of the internet forum tire threads.

Have you ever asked yourself anything about such information and how it’s applied? Who is supposed to take action to insure information in such regulations is properly applied? Are we as end users supposed to take information from such regulations and use it to our advantage or disadvantage? For instance, this is a quote often used by tire thread participants referencing a DOT regulation. It is taken out of context from a large paragraph on the same subject.

“Except in the case of a vehicle which has a speed attainable in 2 miles of 50 mph or less, the sum of the maximum load ratings of the tires fitted to an axle shall be not less than the gross axle weight rating (GAWR) of the axle system as specified on the vehicle's certification label required by 49 CFR part 567.”

People will use that statement to justify their recommendation that a lesser load capacity tire can be used as long as it meets the requirement of the trailer’s GAWR. But wait, lets hash that over a bit. The DOT has presented the trailer manufacturer with a list of regulations they must abide by. That quote is from one of those regulations. If that regulation is directed at the vehicle manufacturer how come we, the private citizen have a right to apply it? Well, we don’t. No matter what is perceived by trailer owners about tire selection and fitment it still is in the hands of the trailer’s manufacturer. During the vehicle certification process the trailer builder is going to have to make a final selection on a number of items that are going to be put on the vehicle’s federal certification label and tires is one of them. Those certified tires are the minimum standard for that trailer. When it comes replacement time you can put on whatever your old buddy down at the local tire shop recommends or do some researching and use tires equal to the task certified on the vehicle’s certification label. With RV trailer tire replacements bigger - more load capacity - is always going to be better. Regardless of what you decide to use don’t pass-up an recommendation from the vehicle manufacturer. It’s always the first recommended place to ask questions. You may be surprised and find they have added an upgrade to your model line-up that supports a recommendation to another tire line.

Here is the list of regulations trailer manufacturers must apply when building your trailer.


https://docs.google.com/viewer?a=v&q...IyEkdLI94e_VGA


CW

[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 10km/6.3m/h lower speed .but in spreadsheets we 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( instead of 4 times 3% =12%) 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 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

[jadatis]

@ CWtheMan

The system of the tire makers is based on laws of nature.
But it is set up long time ago, when the knowledge about those laws of nature for tires was still incomplete.

You can compare it to company's that already early began with computer systems, and now have a system with many band-aids and suplements to keep it working properly.
Some want to set it up from the beginning, but are risk loosing all the old data.

For instance the system and formula to calculate maximum load for a tire and to calculate needed pressure for lower loads then that, is introduced in 1928.

In time it has been adjusted for instance for radial tires and when problems occured of still blowing tires when the calculations seem to be right, a band aid was put on it by for instance substracting 10% of the maximum load of a P-tire before doing the calculation.
Also In America and Europe different calculations where made , and still are made for calculating that pressure for lower loads.
Or better the other way around is more used, and that is to calculate loadcapacity for a sertain pressure, as the tire makers give for their tires.

As late as 2005 the American TRA swiched over to the calculation of European ETRTO for reasons given of globally using the same system, but the old formula used yust came to to low pressure.
this universal used formula has a power in it and the closer that power to 1 the lower the maximum load for a sertain pressure in the lower pressures.
Europe uses Power 0.8 for all kind of radial tires since decades ( about from 1970) and America used different powers From 0.5( root) for P-tires with Aspect ratio of above 50% and 0.65 for lower then 50% AR.
For LT tires and truck-tires America used and stil uses 0.7 power wich leads to to much deflection in the lower pressures, because higher loadcapacity is given.

also the maximum load of a tire is calculated with a formula in wich also Speed is a factor, as we learned already.
But this formula is generally given to be based on the internal vollume of a tire.
My opinion is , that thats a misunderstanding in the beginning, that is taken over as faxt in time. To my opinion The allowed deflection and the surface on the ground it gives is the thing it has to be based on to laws of nature.
Both for calculating internal vollume and surface for allowed deflection, you need the same data like Overall radius of tire and rimm radius , width and highth of tire.

In the document you gave first, a comparison is made between P tires and Metric tires, and its written that you cant use a list for metric tires for P tire.
But you can even better use the metric list for P tires, it gives lower so saver maximum load.

But that will be another contribution I will make.
The different used system in tires in America and Europe and an overview of the different Pr for the load-kind of the tire.

[jsmith948]

Huh???

[Ken / Claudia]

I will read the trailer tire information on the trailer and match it with what a tire sidewall says at the tire store. Maybe go to a heavier ply with a higher max. rating. Really what else should we know?.

[CWtheMan]

Originally Posted by Ken / Claudia

I will read the trailer tire information on the trailer and match it with what a tire sidewall says at the tire store. Maybe go to a heavier ply with a higher max. rating. Really what else should we know?.

The only thing I would add to that is to first seek information from the vehicle manufacturer about larger tires. Then I would read the warranty for the replacement tire to insure I'm not making a "misapplication" of their tire. Actually I'd ask them to explain the misapplication statement and if it applies to fitment on my vehicle.

CW