Max tire pressure or trailer recommended pressure?

[iamrwnut]

Sorry for another tire pressure question. Searched for 1/2 hour and did not come across an answer that I am sure is somewhere in the 12000 posts on tires.

My tires have a pressure rating of 80 psi cold. My trailer shows 65 psi cold.
Which one should I use? I have been setting them at 65 psi.

[SAD]

Sorry for another tire pressure question. Searched for 1/2 hour and did not come across an answer that I am sure is somewhere in the 12000 posts on tires.

My tires have a pressure rating of 80 psi cold. My trailer shows 65 psi cold.
Which one should I use? I have been setting them at 65 psi.

The 65 psi recommendation ~may~ be imposed by a limitation of your wheels mounted on the trailer.

Try and determine the rating for the wheels....

If you find that the wheels are rated for up to 80 psi as well, the recommendation may be simply the result of the fact of the load that is imposed upon the tires - i.e. your RV is not heavy enough to warrant an 80 psi recommendation.

All tire manufacturers have a "recommended pressure" based upon the specific weight they carry - not just the maximum rated weight.

Simply because a tire is rated for up to 80 psi, does not mean that it is "under inflated" at 65 psi.

[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)

[iamrwnut]

Looks like I am going to have to hit the scales to find out my true weights, then adjust my tire pressure accordingly.

[hankpage]

Rob, According to Keystones webpage, your trailer came from the factory with 15 in. load rated D tires. (65psi max.) When the tires were upgraded to E rating (80psi) the rims should have been upgraded also. Before you increase tire pressure above 65psi be sure to check the rating of your rims. Have they been changed to 16in ??????

[Bugle Boy]

Looks like I am going to have to hit the scales to find out my true weights, then adjust my tire pressure accordingly.

A safe RV trip depends on knowing the weight and weight rating of your trailer, tongue weight, weight on the tires, weight ratings of your TV, weight rating of your tires, weight rating of your hitch etc.

Without knowing the true weights and these weight ratings you have no idea if anything is overloaded.

I think it very important to get hitched up, loaded for a typical trip, and head straight to a set of scales. We will do that Tuesday right after we pick up our new Cougar.

I don't mean to sound preachy here, but having spent 50 some years in the nuclear industry I have been conditioned to "what if" to a fault sometimes. "Questioning attitude" and fault/event review are burned into my old brain.

[gepaine]

My Keystone Cougar came with 15" tires rated at LRD. I was able to upgrade to 15" LRE's on the original rims because the rims were rated at 2830 pounds which was the max load of the new LRE's. (The LRE's have a max psi of 80.) However, I will inflate them based upon my actual load plus a 10-15 psi safety margin. That will be about 65 psi which happens to be the recommendation on the trailer label.

I'm just mentioning this so people won't assume they have to upgrade their rims if they upgrade their tires. It all depends on the actual ratings of their equipment.

[iamrwnut]

My tires are Carlisle ST 225/75R15s. The rims show 2600 lb limit.

[SAD]

My tires are Carlisle ST 225/75R15s. The rims show 2600 lb limit.

Hence the 65 psi recommendation.

[diugo]

So now we have two posters in this thread who have to run an LRE at 65psi instead of 80---instead of an LRD at the 65psi full sidewall pressure.

Am I the only one here who thinks this is bad?

[SAD]

It is not bad as long as the pressure chosen is appropriate for the weight placed upon the tire.

[Festus2]

So now we have two posters in this thread who have to run an LRE at 65psi instead of 80---instead of an LRD at the 65psi full sidewall pressure.

Am I the only one here who thinks this is bad?

diugo -

I may be wrong here but in another and earlier thread you indicated that you were going to run your newly-acquired Maxxis D's at 55psi rather than the stated sidewall pressure of 65 psi - a reduction of 10 psi - based on the weight on the tires.

In this thread, someone will be running their E rated tires at 65 psi instead of 80 psi - a reduction of 15 psi.

Perhaps I am not following, but why do you think it is "bad" for this person to reduce the pressure but it is ok for you? I realize there is a slight difference of 5 psi but the end result is the same - a reduction in pressure other than what is stamped on the tire. Is it an issue of wheel/rim size?

Puzzled. :confused:

[diugo]

Festus2,

It's not a question of "just 5 psi", but of proper load range. In my case, 55psi is simply too high for LRC---yet 65psi results in "gross overinflation", according to RVSEF.

If my trailer were just a little lighter---such that 50psi was indicated---I would have seriously considered LRCs when I bought my Maxxis. Because I contend that an LRC at 50psi is superior to an LRD at 50psi. Simply put, the LRC is designed for 50psi. The LRD is not. It is underinflated at 50psi---it no longer has the rigidity necessary for trailer use, and the extra plies only make it less pliable and run hotter. If you don't believe me, Google is your friend.

[gepaine]

Diugo,

I can find nothing on RVSEF to support your position. (If support is there, please point it out to me.) If a Google search will return support for your position, why not save us the time and provide us with a URL?

Thanks,
Gene

[iamrwnut]

Just an FYI. I did not purchase the tires. They came on the "new to us" trailer when we purchased it last year. I had been following alot of the posts about tires and was still confused. The trailer was 5 years old when we purchased it. It is my un-educated guess that the original owner replaced the tires one or two years ago. I would like to neither over or under inflate my tires. Just looking to do "the right thing".

[iamrwnut]

Another FYI. We purchased from a dealer not a private owner so no way to know when they were replaced.

[diugo]

Gene, the RVSEF document citing my "gross overinflation" reference can be found at http://rvsafety.com/2011/09/tire-load-and-inflation-ratings . There you'll find their general recommendation that towable tires be inflated to the sidewall pressure. If you only inflate an LRE to 65psi, then you're not doing that. There are posts on numerous other forums debating the equivalence of LRD/E at 65psi, all easily googleable.

OP, I share your concern about doing the right thing. Many RVers---myself included---find ourselves in a similar dilemma: we all seem to be a load range too high with our tires, and there's just no good solution. My advice to every RVer casually considering a higher load range: look before you leap!

[gepaine]

Diugo,

Your RVSEF reference says:

"Note: Towable – Travel Trailer/ 5th Wheel owners Due to the severe use conditions experienced by tires when axles are very close together – tire industry experts recommend maximum (sidewall) inflation pressure for towable tires unless this causes a severe over-inflation situation (20psi+), often referred to as the ‘basketball effect’. If this is your situation allow a 10 – 15psi safety margin above the minimum required inflation pressure."

I am doing exactly what this paragraph says when I inflate my LRE's to 65 psi. That is, I am avoiding a severe over-inflation situation by allowing a 10 – 15psi safety margin above the minimum required inflation pressure.

[diugo]

Gene, yes, and I am doing the same thing with my Maxxis LRD's at 55psi. However, that doesn't make it right/ideal. Neither of us is running a trailer tire at the designed sidewall pressure---our load range is too high, and as a result, extra heat is being generated in our tires.

In your case, you could run LRDs and still have your 10-15psi margin, right? In my case, I would have to run an LRC at 5psi over the sidewall, which is something I will personally discuss with RVSEF when I get each wheel individually weighed, a month from now.

There's a good discussion here: http://forums.woodalls.com/index.cfm/fuseaction/thread/tid/23003859/print/true.cfm

[gepaine]

Diugo and all,

I read the discussion you pointed me to on Woodall's forum. Even though it was all opinion with no verifiable authority to support the opinions (as opposed to what we see on RVSEF), it has convinced me that I need to do more research. The good thing in my case is that my rims can handle 80 psi should I decide to go that way.

Thanks for your input. Good discussion.

Gene

[CWtheMan]

Looks like I am going to have to hit the scales to find out my true weights, then adjust my tire pressure accordingly.

Here is a “what if” for you. Lets say your trailer came with OEM 15” tires. The 65 psi on the tire placard indicates they were LRD. If your current tires are in fact 16” and the same design as the OEM tires than the 65 psi would still be valid. If the new tires are 16” LT tires the correct tire pressure for them is the amount of pressure it takes to equal the load capacity of the OEM tires at 65 psi.

CW

p.s. Remember, tire manufacturers DO NOT set vehicle tire pressures. Vehicle manufacturers do.

[CWtheMan]

Another FYI. We purchased from a dealer not a private owner so no way to know when they were replaced.

Maxxis is the only ST tire manufacturer that I have found that will allow their tires to be manipulated with air pressure below the maximum sidewall pressure without invalidating their warranty.

Maxxis canned answer is it’s OK with them as long as it’s approved by the vehicle manufacturer.

You can call Mr. Shawn Brennan - east coast Maxxis rep - at 687-404-6772 for conformation on that.

CW

[CWtheMan]

For reasons unknown to me, tire manufacturers are often reluctant to print industry standards. Industry standards are normally a combination of government regulations and inputs from contributing agencies such as the RMA or T&RA.

By DOT regulation the manufacturer of your RV trailer is tasked with determining and setting the correct tire pressure for your Original Equipment (OE) tires. The correct tire pressure is depicted on the trailer’s federal certification label and is depicted as the recommended cold tire pressure.

Industry standards will not violate those recommended pressures for your OE tires or replacements of the same size and load capacity. When replacements are different in size and/or design new recommended tire pressures will be determined by the installer. The information for determining the new recommended pressures will be determined using the amount of load capacity the OE tires provide by their recommended pressure.

In the reference provided below you will see the emphasis placed on the words; “NEVER use inflation pressures lower than those printed on the vehicle placard.” That’s a reinforcement of an industry standard.

http://www.trucktires.com/bridgestone/us_eng/press/zip/WeighForm.pdf

CW

[chuck&gail]

This can be debated forever. I will say that in over 200,000 towing miles with ST tires the only tire issues have been a few repairable nail punctures.

AND

I have not yet inflated a TT tire to the max on the sidewall. I use the designers tables, plus 5 psi for luck. Note I DO know the weights of each tire.

Must admit I have never used Chinese tires. Some say that is not meaningful, and MAYBE they are right, but it works for me. I also do not run tires over 7 years old per DOT code.

[CWtheMan]

This can be debated forever. I will say that in over 200,000 towing miles with ST tires the only tire issues have been a few repairable nail punctures.

AND

I have not yet inflated a TT tire to the max on the sidewall. I use the designers tables, plus 5 psi for luck. Note I DO know the weights of each tire.

Must admit I have never used Chinese tires. Some say that is not meaningful, and MAYBE they are right, but it works for me. I also do not run tires over 7 years old per DOT code.

I’ve seen a number of your posts and I’m pretty sure you’re using Maxxis tires.

When the tire industry talks about the correct or recommended (cold) tire pressure they are referring to the recommendations found on your trailers certification label or tire placard.

Some people are very successful operating outside of the established standards. You are obviously one of them and I’ve left your posts alone in the past because you’re so adamant about it.

Here is a link to the Maxxis trailer tire warranty. It does not cover tires under or over inflated or overloaded.

http://www.maxxis.com/Industrial/Trailer/Trailer-Warranty.aspx

I’ve talked with Mr. Sawn Brennan (Maxxis tire rep) about tire pressures for the Maxxis trailer tires. Maxxis supports the trailer manufacturers tire pressure recommendations 100%.

CW

[diugo]

When the tire industry talks about the correct or recommended (cold) tire pressure they are referring to the recommendations found on your trailers certification label or tire placard.

My truck has a genuine placard, showing four different pressures for the two axles, loaded and empty. The figures were calculated by a bona fide automotive engineer and reflect actual loads.

My trailer, on the other hand, has a sticker that simply indicates the OEM tire's sidewall pressure for all four tires. While I applaud the trailer industry for conveying the fact that the sidewall pressure is usually the correct pressure, absolutely zero engineering went into this labeling.

While trailer tire pressure is not as load sensitive as LT tire pressure, there still must be some adjustment based on actual load---which could vary by 1000 pounds per tire. RVSEF clearly recognizes this.

So I find the statement that "Maxxis supports the trailer manufacturers tire pressure recommendations 100%" extremely disappointing.

[CWtheMan]

The certification label/tire placard regulations are basically the same. However, the method used is also based on the regulations and they differ dramatically for the automotive industry as compared to the RV trailer industry. Reserve tire load capacity is not required - by regulations - for the RV trailer industry. It is required - in two parts - for the automotive industry.

That often confuses many RV trailer owners that have never had to deal with those tire pressure differences.

The Special Trailer tire (ST) is universally manufactured to be operated at their maximum allowed tire pressure. Because their manufacturer’s build the tires for the RV trailer industry there is an unwritten understanding that the RV trailer manufacturer will use the maximum sidewall pressures on the trailer’s certification label and tire placard. It’s a two fold safety valve for the trailer builder. One; they are taking advantage of the maximum load capacity to set GAWR and GVWR. Two; They are insuring the owners are going to have the necessary load capacity up to and including the full GAWR. Beyond that is not their responsibility.

CW

p.s. Remember, the vehicle manufacturer - by DOT regulations - has the sole responsibility for setting the recommended cold tire pressures found on the vehicle’s placarding.

[gepaine]

Regarding previous statements in this thread that tire manufacturers universally recommend inflating to the maximum psi molded into the sidewall, I found the following statement on the Bridgestone website referenced by CWtheMan:

"Always determine correct tire inflation pressure based on actual loads on the tires. Use the tiremakers’ recommendations (which you will find in load and inflation tables)."

Clearly, if Bridgestone was recommending inflating to maximum psi, this is where they would say that rather than saying to find your psi in the load and inflation tables.

Furthermore, Maxxis on their website does not say to always inflate to max psi. Instead they also provide a load and inflation table to use to determine proper psi.

Regarding, the previous post that points to the Maxxis warranty excluding under-inflated tires, I believe Maxxis would consider a tire to be under-inflated if the psi was less than the recommended value listed in their load and inflation table. I do not think Maxxis would consider a tire to be under-inflated just because it was inflated to less than the max psi.

[Festus2]

Regarding previous statements in this thread that tire manufacturers universally recommend inflating to the maximum psi molded into the sidewall, I found the following statement on the Bridgestone website referenced by CWtheMan:

"Always determine correct tire inflation pressure based on actual loads on the tires. Use the tiremakers’ recommendations (which you will find in load and inflation tables)."

Clearly, if Bridgestone was recommending inflating to maximum psi, this is where they would say that rather than saying to find your psi in the load and inflation tables.

Furthermore, Maxxis on their website does not say to always inflate to max psi. Instead they also provide a load and inflation table to use to determine proper psi.

Regarding, the previous post that points to the Maxxis warranty excluding under-inflated tires, I believe Maxxis would consider a tire to be under-inflated if the psi was less than the recommended value listed in their load and inflation table. I do not think Maxxis would consider a tire to be under-inflated just because it was inflated to less than the max psi.

Consider this scenario for a moment. Let's say I was heading out for 10 days of dry camping and for this particular trip I would need to load up the 5th wheel more than usual. For example, the FW tank might be filled rather than empty, propane tanks topped up and extra food and clothing thrown in as well. In the truck, I've added a generator, wood and a few other items which would normally not be taken along.

All of this extra stuff in the RV and truck adds to and obviously changes the load on the tires. For argument's sake, let's say there was an additional 500 pounds added - 350 to the RV and 150 to the truck.

Prior to this last increased load, I am running the tires at 10 psi lower than the recommended psi based on the data in the tables - not the recommended psi stamped on the tire.

Question - Because of the new increased weight of the RV and the truck, should I head to the scales, reweigh everything, refer to the tables and adjust the pressure in both the TV and the 5th wheel tires before heading out?

[jsmith948]

Festus2,
That seems to be the general consensus of this thread. Adding to your scenario, after having used the firewood and the water and the food and the propane; does that mean you should re-weigh your rig and reduce your air pressure before heading home?:confused:

[Bob Landry]

Just a question, not a hijack.. Someone posted a link on one of the tire threads to the manufacture that provides aluminum wheels for Keystone. Would someone please repost that link. I can't seem to find it.

Thanks

Edit

A call to Keystone answered my question. The aluminum wheels used by Keystone are manufactured by Tredit. They have a page that shows all of the wheels they sell. Click on one and it will give you the load rating for that wheel. it does not give the PSI rating, but a call to them gave me an answer of 80PSI for my particular wheel. A check on a couple of their steel, spoke type wheels shows PSI ratings that are significantly lower than the aluminum wheels until you get into the 16" wheels, so I think that if anyone is planning to upgrade to a higher load range tire, a call to them might be the prudent thing to do.

http://www.tredittire.com/Aluminum-Wheels-C10.aspx

[gepaine]

Of course, you don't do all those trips to the scale! (Unless you are super anal!) I think you weigh once while loaded for your typical trip and determine the proper psi from the load and inflation table provided by the tire manufacturer. Then add 10-15 psi to that value (but not to exceed max psi), air your tires to that psi, and forget about it. The extra 10-15 psi should handle variations in your load.

[Festus2]

Thanks. Not being anal, I won't be making multiple trips to the scales. I did, however, follow up on Bob's post and also phoned Tredit - the folks that made the wheels on the 5th. I gave them the info stamped on the wheel and asked them if it was good for 80 psi. He also asked if it was steel or aluminum and when I told him it was a steel wheel, he said it would be fine to run the tire at 80 psi. From that, I take it that aluminum wheels may not have as high a load rating or psi capability as a steel one ???

But Tredit's saying that the wheel is capable of safely handling 80 psi answers my concern about running the Maxxis at 80 psi - the recommended pressure.

[Javi]

Me thinks some folks are being a bit anal....:D

In the majority of cases the manufacturer isn't putting tires that exceed the necessary load by any appreciable margin, therefore running them at the maximum pressure isn't a bad thing... I'd way more worried about overloading the whole thing including the axles than running the tires at max pressure...

[Bob Landry]

Have to agree. I think too many people have too much time on their hands. I think just about everyone would agree that running tire pressure at max gets the greatest longevity on drivable vehicles. Why would the same not hold true for travel trailer tires. More inflation, less rubber hitting the ground, less heat generated, longer tire life.. What am I missing?

[gepaine]

Bob and Festus2,

You have convinced me that it is time for me to stop participating in this thread for fear of being accused of having too much time on my hands. (I wonder, does that apply to everyone that participates on this forum or only to those you disagree with?)

Just to be clear, I have no problem with people who choose to run at max psi. I just can't find anything on the web (such as at RVSEF, the Maxxis website, the Bridgestone website, etc.) that says it's bad to run at the psi that is recommended in the tire manufacturers load and inflation tables.

Bye

[Festus2]

The teacher made me write this on the blackboard ....

"I promise not to submit any more posts in this thread."
"I promise not to submit any more posts in this thread."
"I promise ......."

Only 98 more to go.:D
But when I'm finished and have more time on my hands, then what will I do?

[davidjsimons]

The teacher made me write this on the blackboard ....

"I promise not to submit any more posts in this thread."
"I promise not to submit any more posts in this thread."
"I promise ......."

Only 98 more to go.:D
But when I'm finished and have more time on my hands, then what will I do?

Keep posting. I appreciate your knowledge and insight. Much of the input from the administrators of this forum is great information. John, Hank, Steve and yourself, Festus, do a great job. I feel fortunate to have found this group, I don't have to re-invent the wheel. I may not always like some of the answers, but at least they are rooted in experience.
As for being anal about certain things, I'm as guilty as they come, especially if I think I am right based on my own personal experience. Obstinate and down right thick headed may be a more accurate description.
Keep doing what you do.

Sent from my iPad using Tapatalk

[Bob Landry]

Now Festus, you're just being anal. :jump:

[hankpage]

Now Festus, you're just being anal. :jump:

http://www.keystonerv.org/forums/picture.php?albumid=251&pictureid=1177

[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/tire_care_manual/Chapters/complete%20manual.pdf

CW

[diugo]

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]

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=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21904
for the loadcapacity.

and for the calculator
https://skydrive.live.com/?cid=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%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]

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]

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]

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]

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]

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]

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=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%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=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21601

Dont worry , I will stop now
Greatings from Holland
Peter

[CWtheMan]

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=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%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=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%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]

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 .
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.