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Old 08-03-2013, 11:16 AM   #127
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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.
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Old 08-03-2013, 01:47 PM   #128
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Quote:
Originally Posted by Tireman9 View Post
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
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Old 08-03-2013, 04:00 PM   #129
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Quote:
Originally Posted by Tireman9 View Post
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
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Old 08-03-2013, 04:14 PM   #130
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Originally Posted by eheffa View Post
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.
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Old 08-03-2013, 04:53 PM   #131
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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.
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Old 08-03-2013, 04:57 PM   #132
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Quote:
Originally Posted by Tireman9 View Post
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.
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Old 08-04-2013, 12:59 AM   #133
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Here is a nice new tool from Michelin.

All Tire Sizes | Michelin Tires

BA
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Old 08-04-2013, 06:31 AM   #134
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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.
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Old 08-04-2013, 06:57 AM   #135
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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.)
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Old 08-04-2013, 07:11 AM   #136
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Made the switch to 16" and Michelin LT's. The Sendel TO3 looks nice on the stream.
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Old 08-04-2013, 07:39 AM   #137
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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.
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Old 08-04-2013, 07:59 AM   #138
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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?
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Old 08-04-2013, 10:09 AM   #139
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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.
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Old 08-04-2013, 10:31 AM   #140
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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.
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