Max tire pressure or trailer recommended pressure?

[CWtheMan]

It takes many hours of researching the various regulations and industry standards to get on solid ground with tire pressures and where the recommendations come from.

I’ve written numerous blog and internet forum posts about recommended tire pressures. They are always challenged to some degree. It is a subject with a mind set and it’s very difficult to penetrate some mind sets.

When I post information about recommended tire pressures I do it from the industry accepted perspective. The industry must follow the written government regulations. Researching those regulations is tedious and a certain amount of experience is very beneficial to the researcher.

Because there are more than one standard to follow the RV trailer owner is often bewildered because what they have known about their automotive vehicles tires may not be acceptable for their trailer tires.

One thing that is standard across the board is where the recommended cold tire pressures come from. By DOT regulations the only authorization for tire pressure certification is the vehicle manufacturer. When they put a figure on the vehicle’s certification label it becomes the minimum standard for correct tire pressure (s) for that vehicle. Sure, there may be exceptions and deviations. They will be found in the owner’s manual.

RV trailer owners that decide to manipulate their tire pressures below the vehicle manufacturers recommendations are the people you pass parked on the side of the road waiting for roadside assistance to help change their failed tires. Here again there are exceptions. Not very many though.

There is only one good thing to do about your tire pressures on your RV trailer. Get your trailer weighed. Insure no tires are overloaded. Insure your trailer is loaded under its GVWR. And make sure your tires are aired to the recommended cold tire pressure (s) found on the tire placard, vehicle certification label or owners manual.

Here is an up-to-date reference with loads of good and useful tire information. Chapter four is all about Recreational Vehicles. It is worth noting that the RMA mimics the industry standards for NEVER using less tire pressure than what is depicted on the vehicle’s labeling.

http://www.rma.org/tire_care_info/ti...e%20manual.pdf

CW

[diugo]

Originally Posted by CWtheMan

Here is an up-to-date reference with loads of good and useful tire information. Chapter four is all about Recreational Vehicles. It is worth noting that the RMA mimics the industry standards for NEVER using less tire pressure than what is depicted on the vehicle’s labeling.

Thank you for the excellent reference. I concur that it does indeed say that under no circumstances, should a tire ever be inflated to a pressure less than that indicated on the vehicle placard/label/manual---even if a tire manufacturer's load/inflation table says otherwise.

These are mighty strong words. If I were a strictly "by the book" kind of guy, I could see why it might be frustrating to encounter impenetrable mind sets that constantly challenge That Which Is Written.

However, the one constant in this universe is change. Already, RVSEF---prominenty mentioned at the beginning of Chapter 4---is suggesting that ST tires can, in fact, be severely overinflated even at the sidewall pressure. So it is certainly possible that future versions of this reference will reflect this.

In the mean time---although DOT may well say In Keystone We Trust---I'm just not the kind of guy to blindly trust a single, questionable source. Especially where safety is concerned. Otherwise I might drive off the edge of the earth!

As I've stated many times before, I will personally discuss this issue with RVSEF when they weigh my trailer. If anyone is interested in what they have to say, please let me know.

[JRTJH]

I think some people are comparing "apples to apples", some are comparing "apples to oranges", some are comparing "oranges to oranges" and then there's the guy who's asking "where are the apricots"

In many of the situations where people are increasing their tire pressure, they have replaced the standard tires with increased ply tires or bigger tires. That makes the "apples to oranges" comparison a consideration.

Let's say that the standard tires on an RV are ST225 75R15 LRD and the owner replaces those tires with ST225 75R15 LRE. The placard on the side of the RV indicates for the "STANDARD" tire, the recommended cold inflation pressure is 65PSI. The sidewall also says 65PSI. Nobody is going to argue that fact (I hope).

Now, the increased load range tires have a sidewall recommended max inflation of 80PSI. What does the owner do? Should he run his LRE tires at 65PSI or should he increase to 80PSI or should he weigh his wheels individually and follow a "tire manufacturer's load/psi chart recommendation"

That, I think, should be the question, not whether tires are "basketball inflated" or "overinflated"

I'd think that it is safe to say that even if you weigh the wheels and find the load to be below that recommended by the RV manufacturer's placard cold pressure, you shouldn't go below that PSI in each tire. And, if the owner chooses to "just run the maximum (80PSI), he still is within the recommended "add 10-15 PSI" when he increased from 65 to 80.

Now if we're talking about upgrading to LRG tires from LRD, then the pressure difference may be "where's the apricots"

But the bottom line, I think, is don't run tires below the recommended pressure located on the "yellow placard" even if you upgrade a load range, and make sure your wheels will carry the increased pressure (there won't be an increased load unless you do something dramatically different in how you travel). So where is all this confusion coming from??????

[theeyres]

Originally Posted by SAD

Here's a load inflation chart for the Maxxis trailer tire....

http://www.maxxis.com/Repository/Files/m8008load.pdf

See that the LR E tire is rated for 3420 lbs at 80 psi...

But also:

2870 @ 60 psi
3130 @ 70 psi
(ratings are inclusive across lower load ranges)

No, the chart does not say to run the tires at a given inflation. It says, "LOAD/INFLATION INFORMATION FOR ST METRIC TRAILER TIRES
TIRE LOAD LIMITS (LBS) AT VARIOUS COLD INFLATION PRESSURES (PSI) FOR ST TRAILER TIRES USED IN NORMAL HIGHWAY SERVICE"

That quote is from the Maxxis chart. Read it carefully. That chart says Don't go over that weight if you put that much air in the tires. It does not say Put that much air in the tires if that is your weight. That is a critical difference. You can put more air in the tires than the weight and be safe. The label on your trailer will tell you how much air to put in your tires. Read it and go by it. That will prevent a lot of blowouts that people have because they misread those charts.

[jadatis]

Registered to this forum to inform you about tire-pressure calculation.
I dont have an RV at the moment, and live in Holland ( Europe) , so excuse me for some strange words once in a while. Wont introduce myself in a seperate topic. Hovered over the 5 pages of this topic so might overread some good advices.

End of 2007 got hold of a copy of page 14 of the standards manual 2007 of the ETRTO ( European Tyre & Rimm Technical Organisation = European TRA), in wich formula and extra addings to calculate a minimum pressure for a load on a tire.
This formula is used to make advice-pressures for cars with radial tires.
Declared the formula holy and learned myself Excell to make spreadsheets for it.
Translated a few from Dutch to English to go worldwide with it, and react on many fora I found by googling for tire/tyre-pressure.
Found out that this formula was basically the same in America, but with an other power in it, wich lead to lower pressure for the load or the other way around higher loadcapacity for the same pressure.
This gives more deflection at lower pressure/loadcombinations the the tire can stand.

For the tires that are mostly used on RV's ( C- to H-load) even the European Power gives somewhat to low pressure for the load.
This together with some tires that have to high maximum load given on the sidewall already gave problems in the Ford/Firestone-Affaire.
After that tire- and car-makers got nervous and adviced higher pressures then calculated up to the maximum of the tire.
What the tire-lobby tries is that they can describe the pressure, and we dumb users have to obide that, and dont use our own common sense.

I found an article of an American IR J.C.Daws with another way of calculating and comparison with the old way and different used powers in Europe and America. Endconclusion was that the European power used is adequate for Standard load tires and XL/reinforced/Extraload ( different names for the same kind of tire), but for C-load/6PR and up it even gives to low pressure for the load, or to high loadcapacity for the pressure.

So now about this topic.
I saw a link to a pressure/loadcapacitylist from Maxis here, and that is made wrong for different reasons.
1. the wrong power is used of 0.7 as is normal in American TRA system, it leads to to much deflection of the tire at lower pressure/load combinations.
2. C- to E load is placed in the same list, mind that the steps in loadcapacity is inconsequent around the reference-pressures (Pr further will come back to this what it is) of the loadratings( that C-to E-load)
For every loadrating a different list should be made because a stiffer tire ( E stiffer then C) needs a higher pressure for the same load.
They made a calculation for one of the loadratings( fi D load) and placed the maximum load ( further Lmax) of the other loadratings in that list.
You dont have to be an Ingenieur to understand that this is unlogical to laws of nature.

already saw good advices for determining the loads to search back in those lists ( weighing ) but if you then look in a wrong list , the carefully determined reserves gets lost for a part.
I even made my own univeral formula from wich the old and the new "Daws" formula can be made by filling in the right power and construction load ( my own input= part of the maximum load that is carried by the construction of the tiresidewall depending on the deflection) and use that formula with those settings as I think is right in my spreadsheets.
Also made Pressure/loadcapacitylists for the most used loadindexes.
If you look back in those lists , the calculation part of it is even saver then the European calculation.

With this all I think the determining of the pressure for a load is to laws of nature save, what laws and standards of country's and organisations say can be different, but those are yust what they decided.

for the question of topicstarter it means that if the 65 psi is the right calculated pressure for his load, the 80 psi tire needs higher pressure for that same load, because its a stiffer tire.
Sertainly the front tires dont need that high pressure mostly, the front axle is seldomly used to the GAWR ( gross axle weight rating).
That 65 and 80 psi is called the reference-pressure ( Pr ) and is not the maximum pressure of the tire( Pmax) . Even TRA allows 10 psi for LT tires extra and 20 PSI for trucktires. Mind the stemms then must be HPrubber stemms ( up to 90 psi) or metal ( more then 145psi) cold .

to make this long first post complete 2 links , one to the map with PDF's of pressure/loadcapacity lists , and the other to my map with RV-tirepressure-calculator,still under constuction.
https://skydrive.live.com/?cid=a526e...E092E6DC%21904
for the loadcapacity.

and for the calculator
https://skydrive.live.com/?cid=a526e...E092E6DC%21793

From these maps you can navigate my complete public map of skydrive that belongs to my hotmail adress with the same username as here, like in a forum.
a lot about tire-pressure.
To use a spreadsheet first download by Rightclicking , then choose download from dropdownlist, not use the first 2 items like open in Excell or web-app. After download and eventual virus-check , open in Excell or Open Office Calc to use it.
The PDF's can be leftclicked too but then opened in the browser , if you want to have it on your computer, download like described above.

Greatings from Holland, and if wanted, to be continued.

[diugo]

Hi Jadatis,

Welcome to the forum and thank you for compiling the info and taking the time to post it.

What you point out is something I've always wondered about---how a single pressure/load table can be used to describe four distinctly different tires: Load Range B (LRB), LRC, LRD, and LRE. I've always questioned the apparent equivalence, for example, of an LRD and LRE at 65 psi. Your data confirms my suspicions---published load tables are often woefully inaccurate.

You specifically questioned the Maxxis table, suggesting all the pressures were too low. However, all your data seems to apply to what we here call "LT" (Light Truck) tires---and that table describes "ST" (Special Trailer) tires. A dominant feature of the ST tire is that, due to its different construction, it can support a higher load than an LT tire, both at the same pressure.

[CWtheMan]

Originally Posted by diugo

Hi Jadatis,

Your data confirms my suspicions---published load tables are often woefully inaccurate.

Before the lists are standardized they are scrutinized and approved by such organizations as The Tire & Rim Association, The European Tyre & Rim Technical Organization, Japan Automobile Tire Manufacturers Association, Inc., British Standards Institute, The Tyre & Rim Association of Australia, just to name a few.

FastEagle

[jadatis]

What i meanth is that the lists give to high loadcapcity for the pressure, you Diugo writes that the pressures are to low, wich aproximately says the same .

The higher maximum load of an ST tire is yust because they calculate that maximum load for more deflection such a tire may have when used for lower speed ( 65 m/h instead of 99m/h ).
Truck tires in Europe and probably also in America , always have a additional loadindex with a lower allowed speedrating written on the sidewall.
Some LT tires also have it, search for it on your sidewall.
If for instance you have 107/105 Q written as loadindexes, somewhere else on the sidewall something like 109N is written that means that if you stay below 140km/85m?/h you can have the maximum load that belongs to loadindex 109 with the same pressure.

[CWtheMan]

Originally Posted by jadatis

If for instance you have 107/105 Q written as loadindexes, somewhere else on the sidewall something like 109N is written that means that if you stay below 140km/85m?/h you can have the maximum load that belongs to loadindex 109 with the same pressure.

This discussion is basically about RV trailer tires. Our DOT only recognizes load range as the official load capacity identifier for all LT & ST tires. Seldom are tires from another design used on RV trailers. Load Indexes are officially used for automotive tires other than LT.

CW

[diugo]

Originally Posted by CWtheMan

Before the lists are standardized they are scrutinized and approved

Perhaps it is this "standardization" that I most object to. All tires are not the same. There are differences between manufacturers, differences between models of the same manufacturer, differences between load ranges of the same model, and of course between individual tires.

This morning, I discovered to my horror, that the Maxxis m8008 load/pressure table is completely identical---down to the digit---to the boilerplate table in the 2001 TRA manual. I strongly suspect the Power King Towmax table is identical as well.

In other words, the heavily "scrutinized" table simply lists bare minima that only the worst possible individual tire could fail to attain. This only acts to redouble my confidence in going with 55 psi on my new Maxxis LRDs.

The other awesome standardization is that whenever any RV manufacturer puts ST225/75R15D tires on a trailer, on goes a sticker decreeing 65 psi. This, without any due diligence to the vehicle's (or even model's) actual dry weight, and the unequivocal fact that the trailer's weight could vary by thousands of pounds depending upon whatever the owner decides to put in it.

While I agree that tire underinflation is still a major problem facing RVers, the correct solution is not a brain-dead one-size-fits-all sticker. Trailer placards should bear a range of correct tire pressures tailored to whether the trailer is empty or full---just as is done with light trucks.

[diugo]

Originally Posted by jadatis

The higher maximum load of an ST tire is just because they calculate that maximum load for more deflection such a tire may have when used for lower speed ( 65 m/h instead of 99m/h ).

So what you're saying is that ST tires are not constructed any differently than LT tires---and that their supposed superiority is merely derived from their inferior speed rating?

I thought STs had thicker sidewalls and thinner treads.

[CWtheMan]

Originally Posted by diugo

Perhaps it is this "standardization" that I most object to. All tires are not the same. There are differences between manufacturers, differences between models of the same manufacturer, differences between load ranges of the same model, and of course between individual tires.

This morning, I discovered to my horror, that the Maxxis m8008 load/pressure table is completely identical---down to the digit---to the boilerplate table in the 2001 TRA manual. I strongly suspect the Power King Towmax table is identical as well.

In other words, the heavily "scrutinized" table simply lists bare minima that only the worst possible individual tire could fail to attain. This only acts to redouble my confidence in going with 55 psi on my new Maxxis LRDs.

The other awesome standardization is that whenever any RV manufacturer puts ST225/75R15D tires on a trailer, on goes a sticker decreeing 65 psi. This, without any due diligence to the vehicle's (or even model's) actual dry weight, and the unequivocal fact that the trailer's weight could vary by thousands of pounds depending upon whatever the owner decides to put in it.

While I agree that tire underinflation is still a major problem facing RVers, the correct solution is not a brain-dead one-size-fits-all sticker. Trailer placards should bear a range of correct tire pressures tailored to whether the trailer is empty or full---just as is done with light trucks.

If you also look at the Maxxis M8009 ST bias tire line-up you will also find the identical load inflation chart.

Tire manufacturers build tires to specific specifications. Standardization is a very necessary part of the industry and its ability to readily identify suitable replacements for Original Equipment tires.

CW

[jadatis]

Originally Posted by diugo

So what you're saying is that ST tires are not constructed any differently than LT tires---and that their supposed superiority is merely derived from their inferior speed rating?

I thought STs had thicker sidewalls and thinner treads.

I now realise that I am writing to a Travel-trailer forum , thougt is was for motorhomes because of the RV in it.
So I will give the direct link to the map for the caravan/trailer calculator.
Yust recently chanched it a bit , added American English in wich preset is 10% tonge-weight ( give me a better word and I will change it).
This is in Europe , so the other languages , 4% because the axles are placed more to the middle of the box here.
But you can give in the small orange cell your own lbs and it calculates the % for you.
https://skydrive.live.com/?cid=a526e...E092E6DC%21348

But if you want to read the article of J.C.Daws, about tirepressure at reduced loads, I have it on my public map too, navigate to the head map, but better get it from his own site
http://www.dawsengineering.com/desite4_003.htm take the 6th dot .
Hover to page 11 for the discussion and the conclusions, I wont ask you to understand the rest, some Equations I cant follow either.
I think the article wil acnowledge things you wonder about too.

So you see , I could write a book about the used systems and what is done wrong for tires maximum load and calculation of pressure all over the world.
The tire-makers know this, but dont want us to know.
And its the reason for regulations by law for having TPMS in the car , and the higher adviced pressures after 2000.
To my opinion an overreaction to the tire-failures courced by to low calculated pressure. In Holland institutes write that 50% uses to low pressure, but if you would look at wich pressure is save , only 10% to low pressure would be left of it. Its yust what you use as reference.
In the map "all about tire-pressure" my last spreadsheet "pressureloadcapacitylistandconversion " in wich 4 tabs at the bottom.
1st is to make graphics of different calculations.
2nd is to compare differrent calculations or tire tipes, but also little history about the used formula's in time. Made my own universal formula, from wich the old and new can be made, and give the different Power and construction-load used in time, and what I think is the X and Lc that have to be used to get the same deflection over the whole range.
3th is to converse to other market, with other Pr system and weightkind, and to make your own pressure loadcapacity list for one tire.
It gives 4 lists in every used pressure and loadkind( PSIand bar/kpa,and KG Lbs) to copy to any other file.
4th is the disclaimer of 3th.
Play with that spreadsheet and get wiser.
https://skydrive.live.com/?cid=a526e...E092E6DC%21601

Dont worry , I will stop now
Greatings from Holland
Peter

[CWtheMan]

Originally Posted by jadatis

I now realise that I am writing to a Travel-trailer forum , thougt is was for motorhomes because of the RV in it.
So I will give the direct link to the map for the caravan/trailer calculator.
Yust recently chanched it a bit , added American English in wich preset is 10% tonge-weight ( give me a better word and I will change it).
This is in Europe , so the other languages , 4% because the axles are placed more to the middle of the box here.
But you can give in the small orange cell your own lbs and it calculates the % for you.
https://skydrive.live.com/?cid=a526e...E092E6DC%21348

But if you want to read the article of J.C.Daws, about tirepressure at reduced loads, I have it on my public map too, navigate to the head map, but better get it from his own site
http://www.dawsengineering.com/desite4_003.htm take the 6th dot .
Hover to page 11 for the discussion and the conclusions, I wont ask you to understand the rest, some Equations I cant follow either.
I think the article wil acnowledge things you wonder about too.

So you see , I could write a book about the used systems and what is done wrong for tires maximum load and calculation of pressure all over the world.
The tire-makers know this, but dont want us to know.
And its the reason for regulations by law for having TPMS in the car , and the higher adviced pressures after 2000.
To my opinion an overreaction to the tire-failures courced by to low calculated pressure. In Holland institutes write that 50% uses to low pressure, but if you would look at wich pressure is save , only 10% to low pressure would be left of it. Its yust what you use as reference.
In the map "all about tire-pressure" my last spreadsheet "pressureloadcapacitylistandconversion " in wich 4 tabs at the bottom.
1st is to make graphics of different calculations.
2nd is to compare differrent calculations or tire tipes, but also little history about the used formula's in time. Made my own universal formula, from wich the old and new can be made, and give the different Power and construction-load used in time, and what I think is the X and Lc that have to be used to get the same deflection over the whole range.
3th is to converse to other market, with other Pr system and weightkind, and to make your own pressure loadcapacity list for one tire.
It gives 4 lists in every used pressure and loadkind( PSIand bar/kpa,and KG Lbs) to copy to any other file.
4th is the disclaimer of 3th.
Play with that spreadsheet and get wiser.
https://skydrive.live.com/?cid=a526e...E092E6DC%21601

Dont worry , I will stop now
Greatings from Holland
Peter

Dr. John Daws has numerous publications and reports available about tires on the internet.

I read most of them, but, to me, the most interesting are the ones about tire forensics, and more pointedly, tread separations. Here is a link to those reports.

http://www.dawsengineering.com/desite4_003.htm

CW

p.s. The last major approval of new government regulations for DOT certified tires was in early 2007. So some statistics written before that date may be invalidated by the new rules.

[jadatis]

Originally Posted by diugo

So what you're saying is that ST tires are not constructed any differently than LT tires---and that their supposed superiority is merely derived from their inferior speed rating?

I thought STs had thicker sidewalls and thinner treads.

I stumbled on my mistake in my earlier post with this quote and so forgot to answer your wonderings.

Probably there is some difference between built of ST and LT tires, but this does not influence the way they calculate the maximum load for it.
This also is pretty standardized , as CW the mann writes.
Thicker sidewalls for instance would make the tire stiffer and so gives smaller surfacelength , so surface on the ground, with the same deflection, because the overgoing curve from the outside radius of tire to flat on the ground is larger.
So this would only mean lower maximum load and not higher.
Thinner treaths I wonder if they do that to ST tires. Sooner more profile and an off-road kind of profile, with profile-blocks that have more distrance between them and cover a part of the sidewall, witch makes this sidewall less deflection allowed to prefent damage by driving.

Mayby this is the reason why, together with overloading, ST tires often fail.

[jadatis]

My reactions in blue directly after your text .

Originally Posted by diugo

Perhaps it is this "standardization" that I most object to. All tires are not the same. There are differences between manufacturers, differences between models of the same manufacturer, differences between load ranges of the same model, and of course between individual tires. Some standardisation is completely OK but there are some tires that has to have different standards.
These are low aspect ratio tires wich naturaly right maximum load is lower then written on the sidewall of it, about 10 to 20 %, this is yust marginally compensated by the larger load bare by the construction of the tire.
Other are off-road tires wich are allowed less deflection and so lower maximum load, because the profile blocks cover a part of the sidewal wich makes shorter sidewall part to deflect, I estimate again , depending of the sise of those blocks again 10 to 20 % less

This morning, I discovered to my horror, that the Maxxis m8008 load/pressure table is completely identical---down to the digit---to the boilerplate table in the 2001 TRA manual. I strongly suspect the Power King Towmax table is identical as well. Interresting , do you own that 2001 TRA manual, mostly only owned by tire-specialists. Would like copy's of some parts of it, but further contact about that in personal message then.

In other words, the heavily "scrutinized" table simply lists bare minima that only the worst possible individual tire could fail to attain. This only acts to redouble my confidence in going with 55 psi on my new Maxxis LRDs.
Dont agree with that, even a A-brand tire could fail if you use those lists. Your 55 psi could even be to low if you got this information out of this list

The other awesome standardization is that whenever any RV manufacturer puts ST225/75R15D tires on a trailer, on goes a sticker decreeing 65 psi.
And that is yust what they do after 2000, because they realized that the calculation came to to low pressures.
Example in Holland where the often used advice for campers ( motorhomes) is now 5.5 bar/80 psi all around, on D-load tires wich have 65 or 70 psi reference-pressure. Rear it is probably yust needed to cover the almost standard overloading of those 3500kg/7700lbs GVWR ( so it may be driven with B-driverslicence for persons-cars). Front axle seldomly gets overloaded, so there mayby even 3.0 bar /60psi is alright, and gives much more comfort and gripp. Especially the gripp is important for the mostly frontwheeldriven campers in that weight-standard but also for braking This, without any due diligence to the vehicle's (or even model's) actual dry weight, and the unequivocal fact that the trailer's weight could vary by thousands of pounds depending upon whatever the owner decides to put in it.

While I agree that tire underinflation is still a major problem facing RVers, the correct solution is not a brain-dead one-size-fits-all sticker. Trailer placards should bear a range of correct tire pressures tailored to whether the trailer is empty or full---just as is done with light trucks.
It is beginning to be compared to that book of George Orwell 1984. In Belgium and Germany the car maker has to homologise the tires for their car, so you have to ask them , and they look in their secret list,if they may bare that load to rules of nature. In Holland the tires must bare at least the GAWR and maximum speed must be the same or higher then the maximum technical car speed. A system that might be streched up a bit for behind , but has always worked fine, assuming that what is written on the sidewall is the maximum load to laws of nature, and that sometimes is not. Wont be long thoug before the tire-lobby gets this trough the Dutch law too, I think.