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Old 11-29-2012, 03:28 PM   #15
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Originally Posted by Protagonist View Post
Equally-sized ST and LT tires may be physically identical (but I haven't cut cross-sections out of them to confirm that the inner workings are also identical, have you?).
No, but more to the point, there was an extensive discussion on this on rv.net, with links and a summary from this forum. In that discussion, an engineer who had worked in the tire industry for many years asserted that there was no difference in construction, and in the ensuing discussion, there was no rebuttal of that assertion. I looked briefly and couldn't find the link, but it's out there somewhere.

On the testing angle, it is my conjecture that either an LT or an ST tire would pass the tests required for tires of the opposite type. The tests are minimum standards that modern tires exceed by wide margins.

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They are not legally identical as a result of having to meet different safety standards.
That's right.
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Old 11-29-2012, 04:09 PM   #16
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That article is 12 years old.
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Old 11-29-2012, 04:11 PM   #17
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…there was an extensive discussion on this on rv.net, with links and a summary from this forum. In that discussion, an engineer who had worked in the tire industry for many years asserted that there was no difference in construction, and in the ensuing discussion, there was no rebuttal of that assertion.
I'd believe that engineer when he says they're the same. What I wouldn't trust is the quality control on the assembly line. I read in the Federal Register, when the subject of adopting FMVSS 139 was first brought up, that the market for ST tires was so small that there was no benefit to holding them to the higher standard, which is the only reason why the less-stringent FMVSS 109 is even still on the books.

Our tax dollars at work.
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Old 11-29-2012, 04:12 PM   #18
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Yeah, I missed the date at first. Gone now.

Thanks. Do not want bad Karma. Can not afford bad Karma. I actually have a set of GYM's on my heavier trailer that does not move much.
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Old 11-29-2012, 05:31 PM   #19
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I'd believe that engineer when he says they're the same. What I wouldn't trust is the quality control on the assembly line. I read in the Federal Register, when the subject of adopting FMVSS 139 was first brought up, that the market for ST tires was so small that there was no benefit to holding them to the higher standard, which is the only reason why the less-stringent FMVSS 109 is even still on the books.

Our tax dollars at work.
Well, to paraphrase the RV.NET discussion on this point, ST tires do tend to be made in lower volume plants, which does tend to mean older plants. Nonetheless, these same plants also, in general, produce passenger car and LT tires (I presume in unusual sizes or specialty treads like studded snows or something). The overall plant approach to quality isn't any different between runs. True, the occasional tire that gets sacrificed to the test machine will be run at somewhat lower speed and higher load, but that doesn't mean much in and of itself and those changes tend to balance out anyway. The tire guy wasn't of the opinion that there was a material difference in overall quality as a result.
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Old 11-29-2012, 05:37 PM   #20
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Well, to paraphrase the RV.NET discussion on this point, ST tires do tend to be made in lower volume plants, which does tend to mean older plants. Nonetheless, these same plants also, in general, produce passenger car and LT tires (I presume in unusual sizes or specialty treads like studded snows or something). The overall plant approach to quality isn't any different between runs. True, the occasional tire that gets sacrificed to the test machine will be run at somewhat lower speed and higher load, but that doesn't mean much in and of itself and those changes tend to balance out anyway. The tire guy wasn't of the opinion that there was a material difference in overall quality as a result.
Okay. Thanks for the info.
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Old 11-29-2012, 06:21 PM   #21
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Here is a picture of the tires I use. I have never had a flat.
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Old 11-29-2012, 08:18 PM   #22
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Tire testing: Why? To achieve a goal. What is the goal? The tire must be able to operate safely within the envelope of it’s design purpose.

ST tires are designed for a single purpose. The mundane everyday task of operating safely on free rolling trailer axles. They are tested for that purpose.

LT tires, by DOT definition, are designed for the rigors of powered/unpowered - steer, drive, and sometimes tag - automotive axles and are designated by their manufacturer as primarily intended for use on lightweight trucks or multipurpose passenger vehicles. (FMCSA-109). They are tested for that purpose.

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Old 11-29-2012, 08:44 PM   #23
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[QUOTE=Jammer;1232483]
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[quote jammer] There is no actual physical or measurable or testable difference in load-carrying capacity between ST and LT tires.

Sure, the sidewall brand is different. So what -- that's not a physical difference.

It's still the same tire carcass, same number of plys, same arrangement of belts, etc.
The (hypothetical) tire builder gets specifications to build a tire with carcass cording of .032 diameter for the polyester cord and .025 diameter for the steel cord. In his next build specifications for the same tire the polyester cording is .040 and the steel is .033. You cannot see any physical difference but the larger cording will increase the tires strength. A prime example would be the ST225/75R15. LRD & LRE look identical.

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Old 11-30-2012, 04:05 AM   #24
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ST and LT tires are definitely built differently. In comparing an unmounted Goodyear Marathon to a Michelin LTX MS, the GYM sidewalls were floppy, like rubber coated tent fabric; and the LTX sidewalls were relatively rigid. Also, there was a vast difference between the GYM and a Michelin XPS Rib with steel sidewalls.

I don't see how someone could say that ST and LT tires are identical in construction. If that were the case, why aren't ST tires used on pickup trucks.
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Old 11-30-2012, 05:53 AM   #25
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I don't see how someone could say that ST and LT tires are identical in construction. If that were the case, why aren't ST tires used on pickup trucks.
Partly because, by Federal regulation 49 CFR 571, they can't be.

Refer to my previous posts about which Federal Motor Vehicle Safety Standards (FMVSS) each type of tire must meet. Tires mounted on any motor vehicle that carries passengers must meet FMVSS 139. ST tires are not tested to FMVSS 139 standards. They're tested to FMVSS 109 or 119 standards.
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Old 11-30-2012, 06:23 AM   #26
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I have read many of the tire threads in their entirety and my eyeballs are spinning like the wheels on a slot machine.


Replacing tires is like replacing light bulbs and any other consumable item in the trailering experience. I would not be surprised that if the tire pressures are kept where recommended, speeds are kept down and the tires were replaced on a low mileage or elapsed time period, that one would have a low probability of a tire failure.

YMMV
Good post. I'd just add another point on maintenance. While in WA state for Thanksgiving (on a 3300 mile R/T from CO), I took our 2011 FC23FB into Schwab Tire to have the original Goodyear Marathons inspected, rotated and balanced at just under 20,000 miles. I had rotated them at 10,000 miles. Schwab is a large and well regarded tire chain in the NW and there seem to be more RV's per capita in the SeaTac area than I've seen anywhere else. Amazingly, when I said that I wanted my trailer tires balanced, the manager responded "Are you sure? That costs $$ - hardly anybody bothers to balance trailer tires unless they have a problem." This reinforced my opinion/observation that trailer tire maintenance is often neglected, rated speeds exceeded, and tires run well past their recommended life expectancy.

When we bought a Casita a few years ago, I caught the GYM fear factor from postings on that forum, and immediately replaced them with MAXXIS Load D's. OK, I also wanted to upsize to 15" tires on nicer wheels. With the new Airstream, I decided get my use out of the GYM's. No failures so far at 20,000 miles. We shall see....
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Old 11-30-2012, 09:10 AM   #27
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We had Maxxis tires on our 1985 Excella 30'er and got good service over the time we had it traveling around 30k miles over 4 years. Last summer when we picked up our
NEWer 2001 AS 30'er I noted that it had GYM's on it. They were only 1 year old so I decided to leave them alone. Last summer we towed it 7k miles with temps over 100 deg. at highway speeds in Missouri & Ill. to cooler temps at Rocky Mtn. altitudes with no problems. I was somewhat disappointed when seeing the trailer shoed with GMS's but I must say that so far they have served me well. We'll see how they hold up when we head south in Feb but so far ( where's that wood ) they are serving me nicely.
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Old 11-30-2012, 09:37 AM   #28
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Any tire that has a DOT certification for highway service can carry passengers. Here is a reference that has each state’s restrictions.

http://www.towingworld.com/articles/TowingLaws.htm

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