Safe Tire Pressure

[A W Warn]

Yesterday I sent off another inquiry to Michelin, trying to get a more specific answer to my questions, re: the light truck tires, how/if they warranty them when installed on a RV trailer. I specifically asked about the LT tires being installed by manufacturers on some of their new trailers. If they respond I'll post their response here.

[Ag&Au]

Originally Posted by A W Warn

Yesterday I sent off another inquiry to Michelin, trying to get a more specific answer to my questions, re: the light truck tires, how/if they warranty them when installed on a RV trailer. I specifically asked about the LT tires being installed by manufacturers on some of their new trailers. If they respond I'll post their response here.

It would be also interesting to know how the tire warranty is handled on the Eddie Bauer trailers. Does Michelin provide the warranty or does Airstream?

Ken

[richinny]

how would michelin reply to replacing LT tires on a trailer that came with LT tires? (such as older Airstreams)

[Moflash]

All tires are designed for different uses Lt (light truck) for example were designed tested and built for light trucks.The have not been built or tested for travel trailers (or wheel barrows) so if you ask the manufacturer how will they would perform on your wheel barrow(or travel trailer) due to litigation issues they will not and cannot answer the question.
But light trucks go down the road with these on.They turn corners,they also have weight added to them in the form of payload.So make your own decision whether or not they will work on a travel trailer(tough question).
Oh also I might add they do actually stamped on the side of every tire the maximum weight each tire is rated to be safely used for .And the maximum tire pressure is also stamped on every tire.If you inflate to maximum air pressure listed on the tire you can safely carry the maximum weight that is also stamped on the tire.
Now here is where you have to pay close attention if you inflate the tire to a lower pressure than what is stamped on the tire it will still haul weight but that weight will be less than that is spelled out by the stamped amount on the tire.
These lesser air pressure weight ratings are usually listed on mfg's site.
No you do not have to run the max rated air pressure in your tires.Only if you need the max weight rating for the tire would you be required to do this.
Nobody explains this to their tire reps anymore.That is why the confusion.

[jcanavera]

Originally Posted by w7ts

It would be also interesting to know how the tire warranty is handled on the Eddie Bauer trailers. Does Michelin provide the warranty or does Airstream?

Ken

Typically the tire manufacturers provide the warranty. If I remember correctly when I bought my Classic new, the documentation provided by Airstream included a packet from Goodyear which included their warranty terms. Same goes when I bought my GMC van new. There was separate documentation from Uniroyal regarding warranty.

This is an interesting question as to how Airstream and Michelin are dealing with this. You would think that if Michelin is providing warranty information which is passed though with the trailer documents, then obviously Michelin is approving the use of that specific tire with the Eddie Bauer model. Quite honestly if I took a tire into a Michelin dealer, he probably would have no idea as to the vehicle it was mounted on. In my case National tire mounted my tires on my supplied wheels. They have no idea as to what type of vehicle the tires are on. Technically I could say my van since it uses the same exact size tire.

Jack

[Ag&Au]

Originally Posted by jcanavera

Typically the tire manufacturers provide the warranty. If I remember correctly when I bought my Classic new, the documentation provided by Airstream included a packet from Goodyear which included their warranty terms. Same goes when I bought my GMC van new. There was separate documentation from Uniroyal regarding warranty.

This is an interesting question as to how Airstream and Michelin are dealing with this. You would think that if Michelin is providing warranty information which is passed though with the trailer documents, then obviously Michelin is approving the use of that specific tire with the Eddie Bauer model. Quite honestly if I took a tire into a Michelin dealer, he probably would have no idea as to the vehicle it was mounted on. In my case National tire mounted my tires on my supplied wheels. They have no idea as to what type of vehicle the tires are on. Technically I could say my van since it uses the same exact size tire.

Jack

Once Discount Tire decided that I had researched what I was doing, and was either going to buy from them or someone else, they only required me to sign a statement saying that I understood that these wheels and tires where not the same as the ones that were supplied with the vehicle (trailer in this case). They also did some research on load ratings. After that they gave me their standard warranty and free services. No mention was made of any reduction in Michelin's warranty. They also offered me their extended warranty, which I declined. Not that any of this proves anything.

Ken

[jcanavera]

Originally Posted by w7ts

Once Discount Tire decided that I had researched what I was doing, and was either going to buy from them or someone else, they only required me to sign a statement saying that I understood that these wheels and tires where not the same as the ones that were supplied with the vehicle (trailer in this case). They also did some research on load ratings. After that they gave me their standard warranty and free services. No mention was made of any reduction in Michelin's warranty. They also offered me their extended warranty, which I declined. Not that any of this proves anything.

Ken

I bought mine from the Discount Tire Direct Internet site. No disclaimer to sign since they were shipping the tires with no idea of the vehicle I was putting them on. I also declined the extended warranty offer.

Jack

[Ag&Au]

Originally Posted by jcanavera

I bought mine from the Discount Tire Direct Internet site. No disclaimer to sign since they were shipping the tires with no idea of the vehicle I was putting them on. I also declined the extended warranty offer.

Jack

My wife convinced me to have them mount the tires. Sometimes she has a good idea. It went great and I was able to stand right there and supervise/observe. Supervision was limited to where they placed the jacks. The shop supervisor took a lot of interest in the project. As far as pressure is concerned, he told me that they would only inflate to the max PSI (80) and recommended that I continue to use that pressure.

Ken

[CapriRacer]

Originally Posted by w7ts

After reading about this on dealer, manufacturer and government web sites, I am left with the following impressions. (I cannot find this issue addressed directly, so this is conjecture.)

It appears that the term passenger tire refers to tires whose label starts with P such as P225/15. However, it seems the term passenger rated tire is not an official one, but refers to any tire rated to carry people including drivers. This seems to, in conversation, include LT tires. It also only seems to come up when trailers are mentioned.

I may have jumped tor a wrong conclusion, but I cannot find these terms definitively defined anywhere. It anyone knows where these terms are discussed in more detail, please post a link.

This is one possible explanation for why a question about LT tires evokes a response about passenger rated tires. Here is another reason I believe this. The dealer I bought my LT tires from called them passenger rated tires, when he was telling me they did not recommend them on trailers. He looked at me strangely when I said "They're not passenger tires, they're truck tires", so I dropped it.

Ken

Perhaps I can help here.

The tire industry uses the term "passenger car tires' to mean both tires that have the letter "P" in front of the tire size and their metric equivalents that don't have a "P" in front of the tire size. These tires are designed for uses on passenger cars. These tires CAN be used on trailers with a 10% reduction in load carrying capacity (The reduction also applioes to usage on light trucks - say pickups.)

I come back to that point in a minute.

Side note: The term "passenger rated" does NOT have a meaning within the industry, but you will find folks using the term and similar sounding phrases, when they really mean "P type tires".

OK, back on topic:

LT tires are a group of tires that are designed for usage on light trucks (again, pickups), and, in general, have higher loads that "P" type tires. The letters "LT" are used in front of the tire size and the Europeans don't use the letters, but bnteresting, the Japanese put the letters behind the tire size.

I'll come back to that as well.

ST tires are a group of tires that are similar to LT type tires. They can carry higher load than "P" type tires - albeit at higher pressures, but at pressures similar to LT type tires.

A practical example:

P245/75R16 Standard Load: Max load 2271 # @ 35 psi (please note, the tire's sidewall COULD say max pressure 44 psi or 51 psi, but the relationship delineated earlier in this sentence still applies)

P245/75R16 Standard load - used on a light truck or trailer: 2065# @ 35 psi (again, the same comment about the max pressure listed on the sidewall)

LT245/75R16 Load Range C: 2205# @ 50 psi

LT245/75R16 Load Range D: 2623# @ 65 psi

LT245/75R16 Load Range E: 3042# @ 80 psi

LT245/75R16 Load Range F: 3415# @ 95 psi

ST245/75R16 Load Range B: 2090# @ 35 psi

ST245/75R16 Load Range C: 2600# @ 50 psi

ST245/75R16 Load Range D: 3000# @ 65 psi

ST245/75R16 Load Range E: 3420 @ 80 psi

Notes: First, recognize that there are different tire standards throughout the world. The US standards have letters in front of (and sometimes behind) the tire size to designate the type of service the tire is designed for. The Europeans don't use letters at all, and the Japanese only sometimes use the letters.

- and for those who will point this out, both European and Japanese tire manufacturers will build tires to the US standards and use the letters.

Second, I chose this "size" (meaning dimensions) because it is pretty much the only "size" common to all 3 types. I recognize that the ST version isn't made by anyone, but it does appear in the ST standards and could be made if someone had a mind to. Nevertheless, what I am about to write would apply regardless.

P type tires have a lower load carrying capacity than LT type tires. This is why tire manufacturers do NOT condone the use of P type tires on trailers. - however, if done carefully, P type tires can be successfully used on trailers. Just don't expect a tire manufacturer to say so.

LT type tires have a lower load carrying capacity than ST type tires. This is why tire manufacturers do not condone the use of LT type tires in place of ST type tires. - however, if done carefully, LT type tires can be used successfully in place of ST type tires. Just don't expect a tire manufacturer to say so.

Any questions?

[Ag&Au]

Originally Posted by CapriRacer

.................................................. ....

Any questions?

First of all thank you very much for this explanation. It was very helpful.

What seems to be the biggest bone of contention here right now is how to determine what pressure is appropriate when using P or LT tires on trailers. Because of the reasons you stated manufacturers don't seem to want to discuss this.

The two schools of thought here are:

1. Use the manufacturer's' PSI vs Load charts.

2. always use the maximum PSI on the tire sidewall.

Is one or the other the best practice, or is there some other method we haven't discussed?

Ken

[Phoenix]

Two schools of thought (restated):

1. Always use the maximum pressure printed on the tire sidewall like many sources recommend; e.g., numerous trailer manufacturers (including Airstream) and tire stores (Discount Tire). Also, see repeat of Goodyear quote, below:

"It's a common practice for RV owners to lower tire pressure in their search for a smoother ride. This is not only dangerous, it's relatively ineffective, as the difference in ride quality is not significant."

Or,

2. Take a chance and gamble against the experts that lower tire pressure will provide a smoother, softer ride.


Personally, I'll risk replacing a couple of rivets, rather than pay to repair/replace wheel wells, trim, and belly pan and body sheet metal (and possibly much more extensive damage).

If we took a poll, I would guess that the people who are seeking a smoother ride are those who have not yet had a tire failure that damaged their Airstream. And, those running the maximum (or near maximum) tire pressure have probably had numerous blowouts, tread separation and other failures, and had tire-related Airstream damage repaired.

[CapriRacer]

Originally Posted by w7ts

First of all thank you very much for this explanation. It was very helpful.

What seems to be the biggest bone of contention here right now is how to determine what pressure is appropriate when using P or LT tires on trailers. Because of the reasons you stated manufacturers don't seem to want to discuss this.

The two schools of thought here are:

1. Use the manufacturer's' PSI vs Load charts.

2. always use the maximum PSI on the tire sidewall.

Is one or the other the best practice, or is there some other method we haven't discussed?

Ken

First, let me tell you why tire manufacturers are reluctant to discuss "alternative" inflation pressures.

Every "Light" vehicle sold in the US has a vehicle tire placard that lists the original tire size and the proper inflation pressure for that size. This is required by law. This placard is specific to the vehicle manufacturer, so the inflation pressure listed there is the vehicle manufacturer's specification.

The term "Light" refers to cars, trucks under 10,000# GVW, trailers, etc.

Also, tire sizing is standardized so ALL tires of a given "size" have the same load vs inflation pressure relationship. That means it doesn't matter who manufactures the tire, the pressure listed on the vehicle tire placard is appropriate.

Some other quirks: I put the word "size" in quotes to indicate that the letters in front or behind the tire size that indicate the kind of service the tire was designed for are part of the "size". A P245/75R16 is in my world is a different "size" than an LT245/75R16.

A tire manufacturer would be foolish to specify something other than what is listed on the vehicle tire placard. There would be a whole bunch of legal liability that would open up. Besides, tire manufacturers do not conduct vehicle specific tests, so they CAN'T know how a vehicle handles with inflation pressures other than what the vehicle manufacturer specifies. The tire manufacturer CAN be sure the vehicle manufacturer tested the vehicle at the specified inflation pressure.

Also, if you calculate the load carrying capacity of the tires as specified on the vehicle tire placard, you will find them larger than the GAWR's - and if you think about it, this would make sense. The GAWR's are the maximum load the axle is designed to carry, and there is always some side to side difference, plus there should always be some reserve (unused) capacity. Nowadays you'll find that the tires have about 15% more capacity than the GAWR's.

Now we come to RV trailers. Trailer manufacturers have not done a good job of 1) Estimating the amount of stuff folks put in their trailers. 2) Folks tend to ignore the loading limits, and 3) The tires put on trailers usually do not have any reserve capacity (not a good engineering practice).

The net effect is that manufacturers of ST type tires are in a very uncomfortable position. They have the trailer manufacturers specifying a tire size that may or may not have enough load carrying capacity - and they have consumers who may or may not be careful about tire loads. The only suitable recommendation a tire manufacturer can make is to point to the vehicle tire placard for the pressure recommendation - and in EVERY case that I know of, this is the same as what is written on the sidewall of the tire - that is, the maximum.

There may be folks who will point to the tire load tables - and I would point out that it is good engineering practice to size the tire/inflation pressure to be more than the minimum needed - and MY recommendation is 15% MORE.

Now to P type tires: There is a 10% derating for use on trailers - and I would add that the 15% over-capacity in tires applies here as well.

But LT tires gets a bit more complex - and I am not going to go into this because if the the load tables are used directly (that is, without a 15% over-capacity), then this has a built in over-capacity for trailer usage.

PLEASE NOTE: The above discussion is about TRAILERS!! There are things that would apply to cars and trucks, but not everything. Be careful parsing this.

And a couple of other thoughts:

Many folks do NOT take into account that when they weigh a vehicle that the loads are NOT evenly distributed among the tires - that there is both side to side variation and a front to rear variation. A tire doesn't know what the other tires are experiencing. It only knows what it can see - and if that tire is overloaded, it doesn't matter if its companions are not.

[dznf0g]

Excellent writeup Capri.

[Bill M.]

Many thanks to CapriRacer for taking the time to do this write up.
I have run P Michlens (LTX) on my 25' for the last 4.5 years. Looking at replacing them this spring so I have been reading all the tire stuff again. Probably just go back with the same tires. Trailer came with load range C tires from the factory (at least that is what the tire tag says) and that seems to be a good match. I have 5900 on the wheels when hitched and the tires carry 7940 or so at max so I have the 10% covered. I was thinking about 16" wheels, but it just does not make sense to me to put 65 psi tires and wheels on and then run them at a reduced pressure.

[Ag&Au]

Originally Posted by CapriRacer

..................................
But LT tires gets a bit more complex - and I am not going to go into this because if the the load tables are used directly (that is, without a 15% over-capacity), then this has a built in over-capacity for trailer usage.
.................................................. ............................................
.

I understand everything, except this paragraph. Does it say to or not to add 15%? If you add 15% is that to the load prior to entering the table, or to the pressure in the table, or does it matter?

Ken

[A W Warn]

CapriRacer,
I also would like some additional clarification on the "built in over-capacity" of LT tires. Are you speaking of added material strength for the different stresses anticipated in normal use of the LT tires? (like turning and acceleration)
What kind of tires do you run on your RV trailers, or other types of trailers?
And, thank you for your informative responses!

[dznf0g]

Alan, I suspect that this question is exactly like the, "how much safety margin is in the GVW, GAWR, GCWR.... numbers". I do know the answer to that one direct from my employer's engineers, but I cannot responsibly discuss it, as human nature......someone will exceed that number thinking there's more wiggle room. Not a good recipe for safety.

[A W Warn]

Originally Posted by dznf0g

Alan, I suspect that this question is exactly like the, "how much safety margin is in the GVW, GAWR, GCWR.... numbers". I do know the answer to that one direct from my employer's engineers, but I cannot responsibly discuss it, as human nature......someone will exceed that number thinking there's more wiggle room. Not a good recipe for safety.

I was actually asking CapriRacer a "general" question about his statement, not anything specific with regard to LT design or capacity.

However, I am curious what he uses on his trailers, since tire design is his profession.

[CapriRacer]

I was hoping to avoid this discussion because lots of folks don't buy into this - but here goes:

If I design a generic tire, I can describe how it can be used based on the type of service it will be experiencing - and I can use load carrying capacity as a way to describe the differences.

Higher speeds = Lower load carrying capacity
Torque = Lower load carrying capacity
Rough roads = Lower load carrying capacity
Higher ambient temperatures = Lower load carrying capacity

If you'll notice, the difference between ST type tires and LT type tires is described in the first 3 items. That's why I say there is a built in over capacity for LT tires used in trailer service.

- BUT -

I am somewhat uncomfortable recommending that folks use LT tires in place of ST tires. Aside from the issue that LT tires don't come in sizes where you can make a direct swap (13", 14" and 15" LT tires are really scarce.), I also know I'd recommend that even LT tires have a 15% over capacity when used in LT service. Also, my experince with LT tires in trailer service indicates that there are issues there as well.

Then there's the issue of the stated load carrying capacity. It is very understandable that a tire dealer might refuse to mount tires with a lower stated load carrying capacity. I don't want folks thinking they can make this type of exchange without resistance.

I am uncomfortable because of a lack of experience and the complex nature of these kind of swaps. One example is that it is common for folks to swap ST235/80R16's for LT235/85R16's. Notice the difference in size (and I'm not talking about the letters!) That difference results in an increased load carrying capacity that I struggle to account for in my evaluations.

And lastly: What tires do I have on my trailer?

First, I am not an RV'er. I have a trailer that I used to haul my racecar around. It has P metric tires that I applied the 15% rule and the 10% derating and they are over sized at that. I checked my tires and the pressures before every tow. That means twice every race weekend. At worst I was towing 200 miles (I can only think of 2 exceptions) and the total mileage per year was only a couple thousand. I don't think folks should point to my experience as an example.

[rp709]

Every pneumatic tire equipped conveyance that I have owned or operated has a manufacture specified tire inflation. The door jam of all cars requires a sticker showing the inflation pressure appropriate for the load in the vehicle. My BMW motorcycle shows 36 psi front and rear normally and 36/42 with a passenger. Neither tire has a maximum pressure on the sidewall matching these recommendations. This is the case with my Pontiac G8 and Mercedes C-300, and every recent car I can remember.
Racing bicycles use very high pressure tires. Michelin has recommendations for inflation pressure reflecting rider weight. Their 700/23c race tires list a maximum pressure of 135 psi and a recommended inflation of 110-115 depending on rider weight. Rolling resistance tests have shown inflating to maximum rated pressure increases it. This is because the tire will 'bounce' over road surface irregularities rather than flex to follow the contour of the pavement. This 'bounce' causes greater heat build up.
How does this relate to trailer tires? If you inflate the tire to the maximum when the load does no require it, you may be generating more heat within the tire than at a lower pressure. This may seem counter intuitive but the data back this up in other tire testing. Maxxis provides a load chart. Why doesn't Airstream? Who knows. My guess is that the lawyers have become involved so much in liability, that we have been dumbed down as vehicle owners. If you recommend your trailer tires be inflated to the maximum, then the owner can't overload them if staying within the trailer's load limits. Why confuse the driver? Whenever I have had tires serviced or replaced on my cars, the dealer always inflates to the maximum sidewall number--never what the manufacturer of the car specifies. That doesn't make that the best, but covers them from possible under inflation if the vehicle is fully loaded. The DOT requires a load/tire inflation table be highly visible. Why would that be required by law if it isn't to be used?
Others have posted articles explaining how ST trailer tires have stiffer sidewalls to withstand high loads and pivoting issues with tandem axle trailers. Why the 65 mph speed rating? I would like to know. It may have been an industry agreement for a testing standard. But, given the construction of ST tires, it clearly is a heat issue when traveling over this speed. This may be the primary cause of tire failure more than inflation pressure variation. ( I have also observed that my trailer tires get far hotter from a long mountain descent than at any other time. This comes from braking action that heats the wheel which in turn heats the tire. Since I switched to disc brakes, my wheels don't get hot anymore on a long descent). Using an LT tire may be the solution for travel in excess of 65 mph. Maybe this is why Airstream is experimenting with it on the Eddie Bauer. They had to switch to 16" wheels in order to have enough load capacity. I'm assuming that the tandem axle 'skidding' isn't a problem with an LT tire (maybe for a P rated passenger tire it is) or Airstream wouldn't be using it. ST tires are built with greater UV resistance and sidewall strength to resist deformation when stored for long periods of time. If you don't treat your tires to those conditions, maybe LT tires are the way to go. But, because of the smaller diameter of the 225/70 LT tires, I'm staying with a Maxxis ST, E rated tire this time around. I'm also not ready to spend money on 16" wheels, and to research whether or not they pose clearance issues. Wish me luck.