Calculating the correct tire pressure to use

[SteveSueMac]

Originally Posted by CapriRacer

Third, vehicle inflation pressure specifications - the ones on the vehicle tire placard - are chosen by engineers. The good ones, of course, have listened to what the lawyers have said and applied those words to the values. I have publicly taken issue with those values - and I do so when I don't think the engineers have consulted with their attorneys. The values specified are based on the load tables and the weights of the trailers, but the engineers don't always do a good job. But they are always based on the published load tables. In the case of trailers, it would be wise to weigh the trailer and do your own calculations. In the case of cars, vans and pickup trucks, I think you can trust recently produced vehicle placards (say 2008 and newer.)

Thanks, CapriRacer - In another thread, there was a specific question about this. I assumed the placard is based on original equipment. For example, my 27FB FC had GYMs and the placard says 65 lbs. But in switching to the 15" Michelins (whose sidewall says 50 lbs max) I am assuming the tire's sidewall takes precedence. And - that go get the full load bearing capacity (2183 derated by 10% for 1984/tire for trailer application) they are to be loaded at the 50 lb max - but definitely not overloaded to the 65 lbs on the placard (which assumes GYMs). That is also why I assume I should update the placard.

Are those assumptions accurate, faulty, or something in between?

[Tireman9]

HowieE
In your post # 75 you said "all tires of a given size are required by Federal law to meet the same load standards." This is partially correct but incomplete statement. DOT regulations require all tires of each type (P or LT or ST) must meet the same regulations but the regulations identify the test loads based on industry published tables or tables published by the tire MFG if different. While it is true that well over 90% of all tires made in a given type & size follow published industry standards, there are a number of exceptions. Some of these exceptions show up in the tables you can find on the tire company web site or possibly in literature at a store but there are a small number of tires made where the importer has issued an exception letter directly to DOT identifying a specific tire as having a non-standard load. So I stand by my statement that you should consult the information found for RV application of your tires on the web site for the manufacturer of your tires. I assume you already know that some have different specs for RV application than when the same tires are used in non-RV application but not everyone realizes this.

RE "What option would you suggest to determine the proper inflation?" I have covered that extensively in my blog. I can't publish links here but the summary is that the multi axle design of trailers places higher structural forces on tires than tires experience, even with identical inflation and normal load, when placed on the "corners" of a motorhome. Therefore, to decrease these destructive forces you should always run the max inflation pressure as shown on your tire sidewall. This inflation is also probably the inflation shown on your RV tire placard.

RE your +10 over the chart. Are you running Passenger type tires that have a 35 psi pressure on the sidewall? Since you didn't provide the size tire info or your individual tire loads I can only speculate.


RE your post #77
While lawyers may be involved it is the tire company engineers that do the load calculations and it is engineer representatives to the US Tire & Rim Association that approve the numbers in the standards book.

RE the RV MFG selection of tires. I agree that they many times seem to choose the lowest cost option that allows them to meet the regulatory requirements.

[Tireman9]

Originally Posted by SteveSueMac

Thanks, CapriRacer - In another thread, there was a specific question about this. I assumed the placard is based on original equipment. For example, my 27FB FC had GYMs and the placard says 65 lbs. But in switching to the 15" Michelins (whose sidewall says 50 lbs max) I am assuming the tire's sidewall takes precedence. And - that go get the full load bearing capacity (2183 derated by 10% for 1984/tire for trailer application) they are to be loaded at the 50 lb max - but definitely not overloaded to the 65 lbs on the placard (which assumes GYMs). That is also why I assume I should update the placard.

Are those assumptions accurate, faulty, or something in between?

Your switch from tires that said 65psi on the sidewall to tires that say 50psi sounds like you went down in Load Range and you also lost load carry capacity and the level of reserve load. NOT a good move when contemplating tire failure.

Can you provide the full size information shown on your placard and as shown on the sidewall of your current tires? Also what are your individual tire loads when your TT is fully loaded? Lacking that what are the individual axle loads?

[switz]

When I walked past the Eddie Bauer trailers on the line at the factory with the 16" SenDel wheel and 16" Michelin LT225/75R16/E LTX M/S2 tire option, the trailer tire pressure placard said inflate to 80 psi, the same maximum number seen on the tire sidewall. Those models with the GYM tires has a maximum pressure of 65 psi, the same as on the sidewall.

When I bring the trailer back from the solar and disc brake conversion at the end of next week, we will put the twin bed mattresses back into the coach, load all of our "stuff" and put the rig up on my two sets of four wheel scales. These wheel scales are rated 5,000 pound per scale and are claimed to be accurate to five pounds.

The visual design of the Classic model 30 makes one think the unit would be heavier street side since the bathroom and refrigerator and cabinetry appear more massive on that side. The street side has the storage area for kitchen pots/pans food as well as an additional external access area. The scales will tell the tale.

[nickmeloy]

Originally Posted by CapriRacer

...
Second, Load tables. While there are some exceptions, hardly ever are the load tables specific to a given tire manufacturer. Yes, you have to be careful to get the right table. The tables for ST tires is different than it is for LT tires, and those are different than for P type tires. But it just isn't the case where EACH tire manufacturer has its own load table, but you have to be careful to get the right one.

...

This post was very helpful. Can you point me to a source for load tables for ST tires that will contain information for ST225/75R15 LRE tires?

[nickmeloy]

Originally Posted by switz

...

The visual design of the Classic model 30 makes one think the unit would be heavier street side since the bathroom and refrigerator and cabinetry appear more massive on that side. The street side has the storage area for kitchen pots/pans food as well as an additional external access area. The scales will tell the tale.

This is an interesting observation. My previous post of individual wheel loads seem to support your observation. Please post your individual wheel weights when you obtain them.

[dkottum]

I would think a few hundred pounds difference in load on one side to the other matters little on a 7000# dual axle trailer. I can't imagine most folks including us weighing individual wheels as we move about the country with changing loads as we go.

There is also the reality of frequent changes in altitude, temperatures, front to rear and side to side shifts in load caused by uneven roads, and the sunny side of the trailer that affect tire pressures.

I do think we need to keep weight and balance within the trailer in mind as we go, making an effort to concentrate loads near the axles rather than at the ends.

I'm looking for a tire pressure that can accommodate the changes of day-to-day travel, not the ideal for a particular moment. My 16" Michelins indicate 80 psi maximum pressure. The placard on Eddie Bauer Airstreams (which are virtually the same design as my own) which come standard with these tires indicates 80 psi as correct. Some tell us that is too much, others tell us their suggestion is too little, charts for other tire brands, engineers and second-guessers.

Weighing individual wheels to find a good tire pressure for a particular travel moment? I'm getting burned out.

[SteveSueMac]

Originally Posted by Tireman9

Can you provide the full size information shown on your placard and as shown on the sidewall of your current tires? Also what are your individual tire loads when your TT is fully loaded? Lacking that what are the individual axle loads?

Sure. Attached is a pic of the placard.





GVWR 7600
Mfg 5821
Tire pressure for GYMs 65

Those are 2450/tire I believe @65psi or 9800# total capacity @65 PSI.

Attached is a recent scale ticket fully loaded for camping:





Hitched and fully loaded (empty black and grey which is how I travel, 50% fresh, full propane, clothes, dishes, bedding, accessories, loaded truck with full fuel tank, etc) the trailer platform came in at 5880.

I did do my truck alone in a separate scale ticket and that was 7900. So before hitching, the trailer loaded must come in at 6840 (+7900=14740 combined weight).

The Michelins are (if I recall correctly) P23575R15/XL (the LTX M/S2). These have 2183 load capacity at 50 PSI. They have to be derated by 10% when used on a trailer (I forget the DOT standard that's been posted in the forums...).

So that means 1984 lbs load capacity per tire at 50 PSI. Applied to the trailer when hitched and ready for camping, that's a total capacity of 7936# and 26% headroom against the de-rated capacity (5880/7936). Just the trailer alone loaded for camping but not hitched leaves 14% headroom (6840/7936).

That's what I understood about capacity and there wasn't a need to upgrade to 16s (and new axles too). I talked with Airstream about this before making the change and I specifically asked if that would violate any warranty and they said it wouldn't.

I'm wide open to thoughts if I'm missing something.

[SteveSueMac]

And you asked for a pic of the sidewall which I don't have but here's a pic of the specs from Tire Rack (you'll have to zoom a bit - or visit the website:
http://www.tirerack.com/tires/tires....TR5LTXMS2OWLXL ).

[Tireman9]

Originally Posted by nickmeloy

This post was very helpful. Can you point me to a source for load tables for ST tires that will contain information for ST225/75R15 LRE tires?

What brand ST tire are you using? I want to be sure I point you to the correct table.
What is the pressure shown on your tire placard?
Most LR-E tires have 80 psi molded on their sidewall and as I have previously posted there are technical reasons (Finite Element Analysis) for always running the tire max pressure.

[Arcticfox]

This is getting interesting.....
Tagging into this thread so I can find it again.

[Tireman9]

Originally Posted by dkottum

I would think a few hundred pounds difference in load on one side to the other matters little on a 7000# dual axle trailer. I can't imagine most folks including us weighing individual wheels as we move about the country with changing loads as we go.

There is also the reality of frequent changes in altitude, temperatures, front to rear and side to side shifts in load caused by uneven roads, and the sunny side of the trailer that affect tire pressures.

I do think we need to keep weight and balance within the trailer in mind as we go, making an effort to concentrate loads near the axles rather than at the ends.

I'm looking for a tire pressure that can accommodate the changes of day-to-day travel, not the ideal for a particular moment. My 16" Michelins indicate 80 psi maximum pressure. The placard on Eddie Bauer Airstreams (which are virtually the same design as my own) which come standard with these tires indicates 80 psi as correct. Some tell us that is too much, others tell us their suggestion is too little, charts for other tire brands, engineers and second-guessers.

Weighing individual wheels to find a good tire pressure for a particular travel moment? I'm getting burned out.

Understand the "opinion overload". My background is 40 years as a tire design & quality engineer. I base my posts on facts.

A "few hundred pounds" can be significant if one tire is already overloaded due to the build of the RV. I suggest you look at the unbalance in post #67 of THIS thread.

1. You only need to try and get individual tire loads once, to discover how much unbalance you have. Some may be built into the RV and some may be in what you pack where. I gave an example Mar 29 2014

2. Until you get the individual tire loads you should get individual axle loads. We see that some times there is significant unbalance axle to axle

3. Cold pressure is the only pressure need to be concerned with. Tires are designed to tolerate the pressure increase from normal load (not overload) and normal speed (less than 65 all the time)

[dkottum]

Our four Michelin LT245/75R16 are rated 3042# maximum load each at 80psi maximum, which I have been using based on Airstream's trailer placard for these tires. Our Airstream is 5600# empty with a maximum load to 7700#. Heavy appliances are concentrated over the two axles.

We load with concern for weight and balance. Our towing speed is 60-65 mph. Do we really need to know individual tire or axle loads?

I'm not saying this is a waste of time, but suggesting many, perhaps most of us are not weighing trailers for this purpose.

[CapriRacer]

Originally Posted by SteveSueMac

Thanks, CapriRacer - In another thread, there was a specific question about this. I assumed the placard is based on original equipment. For example, my 27FB FC had GYMs and the placard says 65 lbs. But in switching to the 15" Michelins (whose sidewall says 50 lbs max) I am assuming the tire's sidewall takes precedence. And - that go get the full load bearing capacity (2183 derated by 10% for 1984/tire for trailer application) they are to be loaded at the 50 lb max - but definitely not overloaded to the 65 lbs on the placard (which assumes GYMs). That is also why I assume I should update the placard.

Are those assumptions accurate, faulty, or something in between?

You've got to be very careful here. I'm assuming you're also putting on a different type of tire, so that complicates things.

So let's start with the vehicle tire placard. What did it say for tires size and inflation pressure? (OK, I see in a later post you've got a photo of the placard)

Then you switch to 15" Michelin's - what size? (I'll take a stab that this is P235/75R15 Extra Load, given the info on the load carrying capacity. (It turns out that was correct as well.

So while I am having problems with the tire math - replacing a tire with a load carrying capacity of 2540# with one rated to 1985# - I also see a weight scale printout.

If the tires are seeing 5880# and there are 4 of them, the average load is 1470# - and to account for side to side and front to rear variation, I'm adding 15% to 1691#. Then adding another 15% because I think tires shouldn't be loaded to more than 85% of their rated capacity = 1944#. That compares favorably to the rated load of 1985#.

So while your assumption that the tire sidewall takes precedence is incorrect, it turns out that is indeed the correct answer (If that seemed confusing, think of it this way: You just stumbled onto the right answer, but you could have easily gotten a different and incorrect answer using your methodology.)

[SteveSueMac]

Originally Posted by CapriRacer

So while your assumption that the tire sidewall takes precedence is incorrect, it turns out that is indeed the correct answer (If that seemed confusing, think of it this way: You just stumbled onto the right answer, but you could have easily gotten a different and incorrect answer using your methodology.)

It is a little confusing so let me try to say what I think I'm hearing from you.

If I still had the GYM ST tires - it would be wrong to inflate to less than 65 PSI even if the trailer sees only 5880#. The placard takes precedence.

And I didn't see it as "stumbling", personally. :-) Knowing the trailer was seeing 5880, I sorted through everything I had read here and in the DOT standard that was posted about derating a P tire for trailer application and felt confident in making the switch because of the load capacity and 26% headroom on the derated figure. Different tire entirely, plenty of capacity, but 50 PSI is the max for that tire and that takes precedence over the placard as far as I could tell.

Blowing these up to 65 PSI would be a mistake, in my (admittedly unprofessional but hopefully forum-informed and self-researched) opinion.

[nickmeloy]

Originally Posted by Tireman9

What brand ST tire are you using? I want to be sure I point you to the correct table.
What is the pressure shown on your tire placard?
Most LR-E tires have 80 psi molded on their sidewall and as I have previously posted there are technical reasons (Finite Element Analysis) for always running the tire max pressure.

I am currently using a tire brand called Gladiator. This is an import from China. The tire sidewall information is DOT J2 TD GTS and 2830 pounds at 80 psig.


The previous tires were GYM ST225/75R15 LRD.


The placard on the side of the trailer is for the GYM classification and lists the tire pressure at 65 psig.


Many thanks for this insight.

[Tireman9]

Originally Posted by SteveSueMac

It is a little confusing so let me try to say what I think I'm hearing from you.

If I still had the GYM ST tires - it would be wrong to inflate to less than 65 PSI even if the trailer sees only 5880#. The placard takes precedence.

And I didn't see it as "stumbling", personally. :-) Knowing the trailer was seeing 5880, I sorted through everything I had read here and in the DOT standard that was posted about derating a P tire for trailer application and felt confident in making the switch because of the load capacity and 26% headroom on the derated figure. Different tire entirely, plenty of capacity, but 50 PSI is the max for that tire and that takes precedence over the placard as far as I could tell.

Blowing these up to 65 PSI would be a mistake, in my (admittedly unprofessional but hopefully forum-informed and self-researched) opinion.

Is there a "maximum" inflation on the Michelin tires or does it say "Max load xxx at 36psi"
Lacking a clear statement of "maximum inflation xxx psi" you would use the inflation associated with the "maximum load" as the max cold set inflation.


De-rating by 10% is correct when using Passenger tires on other than passenger cars.

With the lower inflation you will see a LOT more side deflection with the P type tires than an ST with 65. This will result in a significant increase in belt interply shear (see my blog for more on Interply shear)

[Tireman9]

Originally Posted by dkottum

Our four Michelin LT245/75R16 are rated 3042# maximum load each at 80psi maximum, which I have been using based on Airstream's trailer placard for these tires. Our Airstream is 5600# empty with a maximum load to 7700#. Heavy appliances are concentrated over the two axles.

We load with concern for weight and balance. Our towing speed is 60-65 mph. Do we really need to know individual tire or axle loads?

I'm not saying this is a waste of time, but suggesting many, perhaps most of us are not weighing trailers for this purpose.

Trailers are different than motorhomes. The close spacing of multi-axle trailers places about 24% more force into the belts trying to tear them apart. You can lower, but not eliminate, this increase with increased inflation, that is why I advocate trailers run the tire sidewall inflation. Most tire placards also spec the tire sidewall inflation.

Getting individual weights at least once will tell you how unbalanced your TT is both side to side and axle to axle.
Lacking actual numbers I suggest you use 45/55 when dividing the load on the two axles and further assume 45/55 side to side. This means you should assume 31% of the trailer load is on one tire (but you don't know which one) so you would need a MINIMUM inflation rated for that load.

If your tire sidewall pressure would not result in matching the 31% load figure you have a problem that must be fixed.

Once you know you are not in the 57% of RV owners with an overloaded tire you can keep an eye on the load by a simple CAT scale check and comparison to the numbers of the individual calculations.

[Tireman9]

Nickmeloy

RE Load & Infl tables.
I left a message with the importer of Gladiator tires. Will let you know what I learn.

[dkottum]

Originally Posted by Tireman9

Trailers are different than motorhomes. The close spacing of multi-axle trailers places about 24% more force into the belts trying to tear them apart. You can lower, but not eliminate, this increase with increased inflation, that is why I advocate trailers run the tire sidewall inflation. Most tire placards also spec the tire sidewall inflation.

I think this is new information, at least to me. The importance of this recommendation is that it eliminates the use of inflation charts to determine which tire pressure to use on our Airstreams. Just air them to maximum sidewall pressure. And maybe that's why Airstream also recommends doing so on the trailer placards and at their factory service center in Jackson Center.