Another tire debate

[Tireman9]

Originally Posted by CapriRacer

First, it is true.

Why? I traced this back to the introduction of P metric tires in the 1960's. The very first appearance of this note coincides with the first appearance of the P metric load table in the Tire and Rim Association yearbook.

An educated guess on my part is that "trucks" are different than "cars" by the fact that the suspensions on "cars" are much softer than they are on "trucks". This means the tires on "cars" don't have to absorb as much road input as the tires on "trucks".

A principle of tires is that load carrying capacity is a function of the size of the tire (its physical dimensions), the inflation pressure, the operating speed AND the roughness of the road surface. While that last one only seems to apply to specialty tires, I think that was the rationale behind the derating.

I put the words "cars" and "trucks" in quoted because it is getting harder and harder to distinguish the boundary between the 2. Is an SUV a "truck". How about a van? And remember, the note was first applied in the 1960's where the difference is much clearer.

I don't think I have to explain why a trailer is considered more like a "truck" than a "car".

The rule is applicable to "multi-purpose passenger vehicle, truck, bus, or trailer". I think that first entry means vans.

As an official "old fart" I can add a couple details.
In the beginning there were passenger cars and there were trucks. The expected service (including % time at or near max load) was tougher with trucks so they had different standards.
The introduction of "multipurpose passenger vehicles or "Station Wagone was the first de-rating of passenger tires since the % load was higher.
All was good till SUV's were introduced which were more like derated trucks than uprated cars. These also required derating of passenger tires.
As Capri says today things are a mess as we have "crossover" vehicles etc. but no station wagons as in the good old days.

The 1.1 de-rating factor is in my 1975 TRA yearbook.

Interesting side note the "strongest" ST type tire back then was an 8.85-15ST that was rated for a Max of 1920# at 50 psi

[CapriRacer]

Originally Posted by jcanavera

You threw me for a loop on your comment on E rated tires. So are we saying that a 16" LT Michelin has a higher fatigue level than say an E rated ST? But in either case, you have concerns whether an E rated tire, no matter ST or LT is lacking?

Again with my D rated ST failing at the end of year three and then 2 E's failing at the beginning of season 4, does the answer lie in fatigue? It would seem that the weight many of us carry seems to be a major contributing factor in accelerating fatigue.

Jack

You have to be careful here. Because size plays a huge role in load carrying capacity, and because LT's and ST's are rated for load carrying capacity in different ways, it becomes confusing.

So you have to be very specific when you're talking about loads on tires.

I'm assuming your vehicle has ST225/75R15 Load Range E's on it. That means the load carrying capacity is 2540# at 65 psi and 2830# at 80 psi.

If I have calculated correctly, your vehicle has a maximum estimated tire load of 2444#. That means the Load Range D's were barely adequate, and the Load Range E are better - BUT - you are completely restricted to 65 mph max. If I factor in 85 mph, the tire has to have a minimum of 2957# - and that is what I was referring to.

[gmw photos]

Originally Posted by CapriRacer

You have to be careful here. Because size plays a huge role in load carrying capacity, and because LT's and ST's are rated for load carrying capacity in different ways, it becomes confusing.

So you have to be very specific when you're talking about loads on tires.

I'm assuming your vehicle has ST225/75R15 Load Range E's on it. That means the load carrying capacity is 2540# at 65 psi and 2830# at 80 psi.

If I have calculated correctly, your vehicle has a maximum estimated tire load of 2444#. That means the Load Range D's were barely adequate, and the Load Range E are better - BUT - you are completely restricted to 65 mph max. If I factor in 85 mph, the tire has to have a minimum of 2957# - and that is what I was referring to.

Noe I'm getting confused by your followup Capri. I can only assume you are referring to the fact that Jack "used to run" LR D and then LR E ST tires. In his post 87 he describes the issues he "had" with failed ST tires. Capri, you may in fact be correct that part of his issue regarding failing load range E ST tires was that he was still close to an overload situation for the tire, but the bottom line is that his vehicle ( trailer ) "no longer has" LR E ST tires on it. He has moved on to LT tires on his trailer, and at least so far has had no LT tire failures.
Jack, correct me if I am wrong on any of this. Just trying to keep this clear for the benefit of some folks that might be new to the discussion who may not have followed the entire thread.

Good discussion. Glad to hear some well thought out responses, ideas and real world experiences.

george

[switz]

The ordered five 16" SenDel T03-66655T wheels arrived at the local Discount Tire yesterday. The Centramatics are here as well for both the Dodge pickup and the Airstream and the McGard lug nuts are here.

I received the trailer build and completion dates (9 January to 15 January 2014) yesterday along with the VIN number for our 2014 27FB Classic with twin beds. I will be at Jackson Center to photograph the entire build process of our trailer.

I plan to order the five Michelin LT 225/75R16E LTX M/S2 to arrive the week before the ETA of the trailer to have the freshest ones possible to start out with on the trailer.

All these forum tire thread discussions have reinforced my decision to retire the GYM ST tires (and thus the smaller wheels) before use and go with the Michelins.

YMMV

[jcanavera]

Originally Posted by gmw photos

Noe I'm getting confused by your followup Capri. I can only assume you are referring to the fact that Jack "used to run" LR D and then LR E ST tires. In his post 87 he describes the issues he "had" with failed ST tires. Capri, you may in fact be correct that part of his issue regarding failing load range E ST tires was that he was still close to an overload situation for the tire, but the bottom line is that his vehicle ( trailer ) "no longer has" LR E ST tires on it. He has moved on to LT tires on his trailer, and at least so far has had no LT tire failures.
Jack, correct me if I am wrong on any of this. Just trying to keep this clear for the benefit of some folks that might be new to the discussion who may not have followed the entire thread.

Good discussion. Glad to hear some well thought out responses, ideas and real world experiences.

george

Correct my original failure was the ST D rated 225 75R 15 Marathon, inflation 65 psi. The two tires that failed next were ST E rated 225 75R 15 Maxxis inflation 80 psi.

Currently I using the LT E ratedMichelin 225 75R 16 inflation 80 psi. No failures as of now but I just am on one year of use. The Goodyear failed on my last tow of its 3rd season. The 2 Maxxis tires failed on their first tow of season 4.

Jack

[jcanavera]

Originally Posted by CapriRacer

You have to be careful here. Because size plays a huge role in load carrying capacity, and because LT's and ST's are rated for load carrying capacity in different ways, it becomes confusing.

So you have to be very specific when you're talking about loads on tires.

I'm assuming your vehicle has ST225/75R15 Load Range E's on it. That means the load carrying capacity is 2540# at 65 psi and 2830# at 80 psi.

If I have calculated correctly, your vehicle has a maximum estimated tire load of 2444#. That means the Load Range D's were barely adequate, and the Load Range E are better - BUT - you are completely restricted to 65 mph max. If I factor in 85 mph, the tire has to have a minimum of 2957# - and that is what I was referring to.

I tow around 60 with some peaks of 65. So 85 mph isn't anywhere in my plans. My tow vehicle eats enough gas as it is. Its sweet spot for fuel economy is between 55 and 60 when towing.

Jack

[Tireman9]

Lots of posts here but one thing seems constant. Incomplete information as to the tire being discussed.
The complete tire size you are talking about would help clarify issues and avoid many questions.

Example
Michelin LTX is a "P-Metric" tire that has an "XL" Load Range
Shown on Michelin web as P235/75R15/XL 108 T with a max single load of 1985 at 50 psi.
If this tire is applied to a Pick-up or Multi-purpose vehicle (SUV) the load is to be de-rated by 1.1 or 1805# max load at 50 psi. This de-rating is an industry standard as published by Tire & Rim Association.

Michelin does not currently list an LT235/75R15

A quick look at Tire Rack and I see a Firestone Transforce
LT235/75R15 104/101R LR-C with a max load of 1985 @ 50 psi. This tire does not have to be de-rated as it is an LT type tire.

Tire Rack does not show an ST235/75R15

Maxxis does not have a ST235/75R15 but does have a smaller M8008 ST225/75R15 rated at 2150# @ 50 psi with a 65 MPH MAX.

A google search for an ST235/75R15 did find a number of companies advertising "Cheap" tires.


If you are going to change tires you need to first confirm your individual tire load. There are a couple of worksheets with instructions on how to learn and calculate your real loading
One is aimed at 5th wheel trailers but the math is the same.
This site has a worksheet you can download and covers any type RV.

Once you know your loads you need to confirm the tire clearance before selecting a size.

If you know the size then you can select the type a De-aRated P-Metric or LT or ST.

Finally whatever tire you select I strongly suggest you run the inflation on the tire associated with its max load but never exceed the max load or inflation rating of your wheel when setting the cold inflation.

[BlackAces]

Originally Posted by Tireman9

Lots of posts here but one thing seems constant. Incomplete information as to the tire being discussed.
The complete tire size you are talking about would help clarify issues and avoid many questions.

Example
Michelin LTX is a "P-Metric" tire that has an "XL" Load Range
Shown on Michelin web as P235/75R15/XL 108 T with a max single load of 1985 at 50 psi.
If this tire is applied to a Pick-up or Multi-purpose vehicle (SUV) the load is to be de-rated by 1.1 or 1805# max load at 50 psi. This de-rating is an industry standard as published by Tire & Rim Association.

I'm a little confused about your figures. The load index chart for 108 shows a maximum single load to be 2205#. That would put the deraterd value at 1985.

BA

[eheffa]

Originally Posted by Tireman9

Lots of posts here but one thing seems constant. Incomplete information as to the tire being discussed.
The complete tire size you are talking about would help clarify issues and avoid many questions.

Example
Michelin LTX is a "P-Metric" tire that has an "XL" Load Range
Shown on Michelin web as P235/75R15/XL 108 T with a max single load of 1985 at 50 psi.
If this tire is applied to a Pick-up or Multi-purpose vehicle (SUV) the load is to be de-rated by 1.1 or 1805# max load at 50 psi. This de-rating is an industry standard as published by Tire & Rim Association.

Michelin does not currently list an LT235/75R15

A quick look at Tire Rack and I see a Firestone Transforce
LT235/75R15 104/101R LR-C with a max load of 1985 @ 50 psi. This tire does not have to be de-rated as it is an LT type tire.

Tire Rack does not show an ST235/75R15

Maxxis does not have a ST235/75R15 but does have a smaller M8008 ST225/75R15 rated at 2150# @ 50 psi with a 65 MPH MAX.

A google search for an ST235/75R15 did find a number of companies advertising "Cheap" tires.

If you are going to change tires you need to first confirm your individual tire load. There are a couple of worksheets with instructions on how to learn and calculate your real loading
One is aimed at 5th wheel trailers but the math is the same.
This site has a worksheet you can download and covers any type RV.

Once you know your loads you need to confirm the tire clearance before selecting a size.

If you know the size then you can select the type a De-aRated P-Metric or LT or ST.

Finally whatever tire you select I strongly suggest you run the inflation on the tire associated with its max load but never exceed the max load or inflation rating of your wheel when setting the cold inflation.

In the end I think the math is pretty simple...

If you want to replace the ST225/75-15's with a non-ST tire that matches or exceeds the original Load Range D tires, you have to move up to 16 inch LT Tires.

- evan

[gmw photos]

Originally Posted by eheffa

In the end I think the math is pretty simple...

If you want to replace the ST225/75-15's with a non-ST tire that matches or exceeds the original Load Range D tires, you have to move up to 16 inch LT Tires.

- evan

That may in fact be a correct answer, but is it the answer to the correct question ?
To my mind, the correct question is, how much tire capacity do you need in each position on the ground ? If I am right, then the next question would be, which tires are available to meet those needs ?

Someone jump in here and correct me if I am getting my thought process out of whack.

[Tireman9]

Originally Posted by BlackAces

I'm a little confused about your figures. The load index chart for 108 shows a maximum single load to be 2205#. That would put the deraterd value at 1985.

BA

Well after re-reading the Michelin web site I too am a bit confused.

As ref in TRA we find a P235/75R15 Standard Load tire with a Load Index of 105 is rated at 2028#@ 35
An Extra Load with a Load Index of 108 is rated at 2183#@41 psi

The Michelin web site indicates they rate their P235/75R15 with a Load Index of 108 at 1985@50 psi.
They also state on the same page that " Passenger sizes used in Light Truck/SUV applications have reduced load capacity. This will differ from the maximum load branded on the tire sidewall."
I do not know what is actually on the tire. I note that 2183 de-rated by 1.1 gives 1985# so it looks like their spec page includes the de-rating but I also note a bump of 9 psi over normal XL inflation levels.

Michelin has a history of saying itas tires carry slightly different loads than are published in industry standards books. Sometimes I think they do this simply because they can.

So maybe the correct statement would be that you can use a Michelin P235/75R15XL and carry 1985 but you must inflate the tire to 50 psi.
If you choose the Michelin be sure you get something in writing from the Michelin dealer on the load capacity for non-passenger car application.

It really looks like Michelin has an P tire that they rate like an LT as 50 psi in an LT is rated at 1985.

[Ridgerunner3]

Originally Posted by Tireman9

Lots of posts here but one thing seems constant. Incomplete information as to the tire being discussed.
The complete tire size you are talking about would help clarify issues and avoid many questions.

Example
Michelin LTX is a "P-Metric" tire that has an "XL" Load Range
Shown on Michelin web as P235/75R15/XL 108 T with a max single load of 1985 at 50 psi.
If this tire is applied to a Pick-up or Multi-purpose vehicle (SUV) the load is to be de-rated by 1.1 or 1805# max load at 50 psi. This de-rating is an industry standard as published by Tire & Rim Association.

Michelin P235 75 15XL is rated at 2183 lbs with a max inflation press of 50 psi. This is per the side wall of the new tires on my trailer and the specs on Tire Rack and other web vendors. The derated capacity is 1985 lbs. I don't understand why Michelin shows them at the derated load capacity on their websites. It may be a carry over from when the tires were classified as an LT tire. I asked Michelin about the website rating and got a response that did not even attempt to answer the question.

[BlackAces]

Here is a nice new tool from Michelin.

All Tire Sizes | Michelin Tires

BA

[CapriRacer]

Originally Posted by jcanavera

Correct my original failure was the ST D rated 225 75R 15 Marathon, inflation 65 psi. The two tires that failed next were ST E rated 225 75R 15 Maxxis inflation 80 psi.

Currently I using the LT E ratedMichelin 225 75R 16 inflation 80 psi. No failures as of now but I just am on one year of use. The Goodyear failed on my last tow of its 3rd season. The 2 Maxxis tires failed on their first tow of season 4.

Jack

So to complete the picture, an LT225/75R16 Load Range E has a load rating of 300# at 80 psi - and if we want to compare to an ST tire, you have to adjust one or the other because the ST is speed limited to 65 mph and that is reflected in the load rating.

If I do the math, the LT225/75R16 Load Range E when limited to 65 mph gains about 12% more load carrying capacity = 3360# at 80 psi.

Just a word of caution: The differences between the way ST tires are rated for load carrying capacity and LT tires are rated is not well publicized. You won't find a translator anywhere. I had to do a lot of digging to sort this out.

So summarizing:

ST225/75R15 Load Range D = 2540# at 65 psi.

ST225/75R15 Load Range E = 2830# at 65 psi.

LT225/75R16 Load Range E = Calculated at ~ 3360# at 80 psi (3000# rated)

I want to point out that referring to a "Load Range E" is confusing as the 16" and one of the 15" both fit that description - and they have significantly different load carrying capacities. That was the point I was trying to make.

[CapriRacer]

OK a bit of more mucking around with load ratings:

First, tires built to the US tire standards, which are published by the Tire and Rim Association (TRA), have their load ratings in pounds (or kgs) and those are rounded to the nearest Load Index - unlike tires built to the European and Japanese tire standards - European Tyre and Rim Technical Organization (ETRTO) and Japanese Automobile Tyre Manufacturers Association (JATMA) - where the Load Indices are EXACT.

So don't be surprised that the published load rating doesn't exactly match the Load Index. This has everything to do with trying to fit something into a slot it wasn't designed to fit into.

Because there are difference in application, the way the load carrying capacities are calculated are different for P type, LT type, and ST type tires. It's the same for other types of tires, too, but we don't need to talk about those.

Some rules: P type tires have to be derated by a factor of 1.1 for use on trucks, vans and trailers.

ST tires are speed limited to 65 mph, so they get a higher load carrying capacity than LT and P type tires.

So it's difficult to do an "apples to apples" comparison.

And, frankly, if trailer manufacturers did a better job of sizing the tires, this whole area would be a mute point. (Please do not take this to mean I think the entire problem is with the trailer manufacturers. I just think they are part of the problem, and that is discouraging high quality tire manufacturers from entering the market.)

[VTSafari]

Made the switch to 16" and Michelin LT's. The Sendel TO3 looks nice on the stream.

[Roadtoaster]

Originally Posted by VTSafari

Attachment 192404

Made the switch to 16" and Michelin LT's. The Sendel TO3 looks nice on the stream.

Same wheel I have, same wheel I bought! Great looking I might add.

[Lance M]

Great information here from everyone.

So how much additional load could the Michelin P235/75/15 XL (or any other brand this size) carry if the speed was limited to 70 MPH?

[switz]

Post #134 says the Michelin LT 225/75R16E tires are rated 3,000 or more pounds at 80psi.

I found that the Michelin LT 245/75R16E tires have a rated capacity of 3.042 pounds. The challenge in installing these on an Airstream is that they are at least an inch wider, they are 1.3 inches taller than the LT 225/75R16E and 7" wheels might be harder to find designed for the Airstream bolt pattern and offset.

However, the Michelin website and their tire inflation charts show that the LT 225/75R16E LTX M/S2 tire has a load range E rating of 2,680 pounds at 80psi. I think 2,680 pounds is the correct tire rating number for this specific tire.

[Tireman9]

Originally Posted by CapriRacer

OK a bit of more mucking around with load ratings:

snip


And, frankly, if trailer manufacturers did a better job of sizing the tires, this whole area would be a mute point. (Please do not take this to mean I think the entire problem is with the trailer manufacturers. I just think they are part of the problem, and that is discouraging high quality tire manufacturers from entering the market.)

Well Capri as a fellow tire engineer, I do hold the RV mfg a lot more responsible for a good portion of the tire failures.
They are the ones that fail to allow sufficient clearance for larger tires. They are the ones totally responsible for selecting tires and setting inflations and vehicle load capacities. They do a very poor job of educating the buyer that an RV is not just a big car but a completely different vehicle.
RV dealers are so intent on making the sale they will never say anything to the buyer that implies the buyer has additional responsibilities for managing the load and speed.

If RV mfg did the same homework as "Detroit" when it comes to selecting tires we would see larger tires with significant reserve load and TPMS as part of the package. Since the majority of RVs only have a 12 month warranty, the mfg has no incentive to make a better quality, longer lasting product. If however the RV mfg had to stand behind their product for 3 to 5 years as most cars mfg do today, we would see an immediate and significant improvement in RV durability, fit and finish and quality.

Tire failures are just the most obvious failing of the RV industry as a whole to take Quality seriously. If or when a company like Toyota, Honda, Kia or Hyundai decide to enter the RV market, I predict that "Elkhart" will suffer the same fate as "Detroit" did in the 80's when US mfg were crushed by the vastly superior quality from off-shore mfg. Detroit learned their lesson but I doubt that "Elkhart" would survive direct competition from a mfg who focused on real quality and durability.

But that's just my opinion.