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Old 11-28-2012, 11:31 AM   #1
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Tire Safety Factors

I have read many of the tire threads in their entirety and my eyeballs are spinning like the wheels on a slot machine.

One of the major points made was safety which would include the weight safety factor which was mentioned as being 15% of the GVW.

The much maligned GYM ST225/75R15D has a max weight rating of 2,540 pounds at 65 psi. Four times that equals 10,160 pounds of load capacity. The 31 foot Airstream models have a GVW of 10,000 pounds and come from the factory with this tire. Hmm, a 160 pound margin of the GVW?

The highly touted Michelin LT225/75R16E in the threads has a load capacity of 2,680 pounds at 80 psi. Four times that equals 10, 720 pounds of load capacity. Is that a 720 pound margin?

Where is the much mentioned 10 to 15% weight safety factor?

Just looking at the GVW on the data plate on the trailer here, not tongue weight.

If one counts the number of discreet posters to the tire threads, the total number of posters is a very small percentage (under 1%) of the total user population of the GYM tire just in the Airstream world. There were no verifiable statistics on the GYM tire failures presented, just personal experiences. I believe that if there really was a 50% failure rate in the general GYM tire population, these tires would not be on the market.

I also read where folks are running in excess of 65 mph which is the recommended maximum speed for thee GYM per some of the posts. Higher speeds build more heat which in turn leads to tire failure.

In the posts was a comment about replacing tires at the three or four year mark and at as low as 5,000 miles of use. Replacing all four tires is about a $600 dollar expense. We also have storage expense, both at home (if one cannot store the unit beside their home) and on the road. We have to carry liability and comprehensive insurance. Their is an electric bill if the unit is plugged in at home, etc.

If one is concerned about expense, then perhaps this is not the best hobby. Maintenance costs money, just ask the airlines. They change those tires regularly after so many landings and cost a lot more than a GYM. Tires are a maintenance item, period.

Many folks in the forums appear to disagree with the factory recommendations. Yet I did not read about anyone wanting to exceed the posted GVW for their trailer. Why not? It is just a recommendation like tire pressure or tire size.

My concept of the trailer is that it for leisure use. Thus a self imposed 55 mph speed is easier both on me, the TV and the tires on the trailer and I use less fuel.

I have used Michelin tires on all of my 4 wheeled vehicles for years and change them out long before getting to the wear bars or after several years have passed. I think they are a great tire. I am sure they would work fine on a trailer if there was a US version LT 225/75R15D tire for a 15" wheel. The P (as in passenger) 235/75R XL is still available in Canada with a max load of 1,985 pounds at 50 psi (or perhaps a Load Range C rating). Four of them are rated as 7,940 pounds. That would carry the weight of a 25FB with a GVW of 7,300 pounds with a 640 pound weight safety margin.

That is NOT the recommended minimum tire specification. I wonder if the insurance company would walk away from a claim because of intentionally using too small a rated tire? Would they walk away from a claim with a LT tire as it is not the specified type tire? I would really get it in writing from the insurance carrier that my coverage continues if I use an LT or P instead of a ST tire on my trailer.

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
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Old 11-28-2012, 12:11 PM   #2
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I have to agree with you! It is a thing called Fear Factor, if you take care of the items that need to be taken care of, SLOW DOWN when towing and not jumping at every post as if it was the Word Of God, your life would be a lot better. We have put over 60,000 miles on GYM tires and had only one flat from a nail pick up less than 5 miles for our home.
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Old 11-28-2012, 12:13 PM   #3
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Quote:
Originally Posted by switz View Post
One of the major points made was safety which would include the weight safety factor which was mentioned as being 15% of the GVW.

Where is the much mentioned 10 to 15% weight safety factor?
This safety factor is just a rule of thumb. There is no regulation at the State or Federal level that says you can't load the tires to 100% of the rated capacity. Or even more. BUT if you do, it's at your own risk.

Tires are designed by engineers, and ALREADY have a factor of safety built into the maximum load rating. No engineer would EVER put a 2680-pound capacity mark on the tire if it would fail at 2681 pounds of load. But while I'm an engineer and know how they think, I'm not an automotive engineer and don't design tires for a living, so I can't tell you exactly what factor of safety is already built in.

The reason why YOU should leave a margin of error of 10~15% in addition to the built-in factor of safety designed into the tire is that the trailer weight may not be evenly distributed, and the load on one tire may be higher than on others. You can't just say, total weight ÷ 4 = weight on each tire. If you want to be sure, go to the public scales. As long as the weight on EACH tire is less than that tire's rated capacity, you should be good. Even if it's only 1 pound less.
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Old 11-28-2012, 12:23 PM   #4
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There is enough tire information to keep you reading for a long time.

One thing that you need to figure into your calculations for tires is that 10% or more of the weight of the trailer will be carried by the TV and not the trailer tires. This will give you more flexibility when looking at the ratings of the tires.
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Old 11-28-2012, 12:37 PM   #5
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* http://www.airforums.com/forums/f438...oll-76867.html

* TireRack.com Tire Reviews

* goodyear marathon tire failure - Google Search
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Old 11-28-2012, 01:18 PM   #6
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Quote:
Originally Posted by switz View Post
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
The event called replacing tires may be similar to replacing a light bulb. In it has to be done on a regular basis. I would suggest that if one takes on the light bulb mentality as in replace it and forget it then that person is up for unexpected surprises.

Regular tire maintence should provide a better tire experience.
Check tire pressure daily before moving the trailer
Speed does kill, so slow it down
Avoid road hazards. And if one slows down one will have more time to avoid hazards
Balance and rotate tires frequently

In my experience any brand that is rated for the job will work by maintaining the tires. More expensive tires will allow me to neglect the tires to some degree. Just my experience.

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Old 11-28-2012, 01:53 PM   #7
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Some good stuff on RV Tires. A tire expert goes over causes of failures and analyzes the blowout chronicled on LongLongHoneymoon.com

RV Tire Safety: A Tire Autopsy "Root Cause" Part 1
RV Tire Safety: A Tire Autopsy "Root Cause" Part 2
RV Tire Safety: "Blowout" A Real Life Experience
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Old 11-28-2012, 01:59 PM   #8
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The much mentioned safety factor is often for using P or LT tires in place of ST tires. Have not seen a reccomendation for 10 or 15% for ST tires. The implication is that the ST tire design is better for handling the short interval overloads that come from the poor trailer suspensions better than the P metric or LT tires.

Me, I am in the alternative tire camp. I would rather run LT or P-metric tires than 15" ST tires of any brand or size. Specifications is not the problem. Failure due to poor construction is. My information comes from going on caravans and talking to people who have had tire failures. It does happen. Not a huge percentage, but enough to make me edgy.
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Old 11-28-2012, 02:26 PM   #9
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Quote:
Originally Posted by switz View Post
The much maligned GYM ST225/75R15D has a max weight rating of 2,540 pounds at 65 psi. Four times that equals 10,160 pounds of load capacity. The 31 foot Airstream models have a GVW of 10,000 pounds and come from the factory with this tire. Hmm, a 160 pound margin of the GVW?
With the WD dialed in properly, typically around 5-10% of the weight is carried by the tow vehicle axles, so there's more margin than that.

Besides, 30/31' classics don't weigh that much even with a whole bunch of stuff in them and full tanks. More like 8,500, max.

Quote:
The highly touted Michelin LT225/75R16E in the threads has a load capacity of 2,680 pounds at 80 psi. Four times that equals 10, 720 pounds of load capacity. Is that a 720 pound margin?
There is no actual physical or measurable or testable difference in load-carrying capacity between ST and LT tires. ST tires just have higher weight ratings because the use case is purportedly different. The difference in weight ratings between LT and ST tires of the same size and construction (load range or ply rating) is around 10-15%. That's your safety factor, and that is why you can run LT tires right up to the rated maximum without the problems that people get when they run ST tires right up to the rated maximum.

Quote:
If one counts the number of discreet posters to the tire threads, the total number of posters is a very small percentage (under 1%) of the total user population of the GYM tire just in the Airstream world. There were no verifiable statistics on the GYM tire failures presented, just personal experiences. I believe that if there really was a 50% failure rate in the general GYM tire population, these tires would not be on the market.

I also read where folks are running in excess of 65 mph which is the recommended maximum speed for thee GYM per some of the posts. Higher speeds build more heat which in turn leads to tire failure.
The 65 MPH limit is part of the "use case" for ST tires. If you dig deeper, you'll find that most people who have had problems with GYMs live in hot climates or travel extensively in hot climates.

Most GYMs are not on campers, they're on utility trailers and boat trailers that are not run as far, as fast, or as heavy. There is also a sizable population of GYMs on campers that, for various reasons, move rarely or not at all.

Quote:
Many folks in the forums appear to disagree with the factory recommendations. Yet I did not read about anyone wanting to exceed the posted GVW for their trailer. Why not? It is just a recommendation like tire pressure or tire size.
Exceeding the GAWR damages the axles and bearings leading to unsafe conditions. The relationship is more clear than it is with tires, because there are fewer confounding factors like temperature and speed.

Quote:
My concept of the trailer is that it for leisure use. Thus a self imposed 55 mph speed is easier both on me, the TV and the tires on the trailer and I use less fuel.
Good for you. My situation is different, and I drive faster.

Quote:
I have used Michelin tires on all of my 4 wheeled vehicles for years and change them out long before getting to the wear bars or after several years have passed. I think they are a great tire. I am sure they would work fine on a trailer if there was a US version LT 225/75R15D tire for a 15" wheel. The P (as in passenger) 235/75R XL is still available in Canada with a max load of 1,985 pounds at 50 psi (or perhaps a Load Range C rating). Four of them are rated as 7,940 pounds. That would carry the weight of a 25FB with a GVW of 7,300 pounds with a 640 pound weight safety margin.
Michelin makes great products. I have LTX ATs on my Suburban and X-ice3 on my daily driver. They do not, however, make a suitable tire for my trailer.

Quote:
That is NOT the recommended minimum tire specification. I wonder if the insurance company would walk away from a claim because of intentionally using too small a rated tire? Would they walk away from a claim with a LT tire as it is not the specified type tire?
It is my experience that insurers don't "walk away" from claims. What they do is cancel policies with clients who they have come to consider poor risks. In my experience, the timing and severity of crashes weighs much more heavily on their thinking than the specific chain of events that led to any one crash.

Quote:
I would really get it in writing from the insurance carrier that my coverage continues if I use an LT or P instead of a ST tire on my trailer.
Good luck with that.

Quote:
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.
Well, it will be lower, that's for sure. I don't know if it's practical for me. Maybe it is for you.

Sometimes there's a bad lot of tires due to manufacturing problems and it doesn't matter much what you do.

I think that keeping the load below 85-90% of the rated max for ST tires would also help considerably with failures.
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Old 11-28-2012, 03:28 PM   #10
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+1 to the original post!
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Old 11-28-2012, 06:58 PM   #11
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I went through all the threads and all the numbers when picking new tires for my 30' Safari (8400 GVWR).

There were a couple of compelling arguments in selecting the Michelin LTX235/75R15 tires.

1. People with exactly the same trailer had had issues with the Marathons, but no issues with the Michelins,
2. They were recommended by Andy Thompson of Can-Am RV who sees more trailer tires than most everybody else posting to these sites, and

3. The economics of the RV industry. The margins in the RV industry are paper thin. When selecting OEM tires, the manufacturers are not going to want a premium tire - they want something that is cheap - that is good enough for most of their customers who will use the trailer infrequently. Call it less than 1000miles a year. As such, regardless of how well the tire might have been designed, the entire supply chain will be geared to lowest cost. The plants will not have the latest technology, perhaps less than premium materials, etc.

Switching over to a passenger car or truck tire line - here there is much more selection. A premium tire will come from a factory with all the latest technology. It will be based on a very solid design - and these tires will be heavily used. Very few folks purchase tires for their auto and then park the vehicle for 90% of the time. Trends in defects will get flagged and addressed very quickly - or the tires will be blasted in the market place.

In the end, I'm much more comfortable with the LTX rather than ST, largely because of the many many more miles of real world experience that these tires experience.
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Old 11-28-2012, 09:16 PM   #12
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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. [quote]

Of course there is. It’s molded right into each DOT certified tire. Example: The LT235/85R16E has a maximum load capacity of 3042# at 80 PSI… The ST235/85R16E has a maximum load capacity of 3640# at 80 psi.

All LT tires fitted to vehicles in the automotive industry - 10,000# or less - are required by federal regulations to provide at least 6% in load capacity reserves above the loaded vehicle’s GW. Because those vehicle’s are seldom being operated at their full load potential the OE tires are normally providing about 12% in load capacity reserves. That requirement does not exist when fitting tires - any tires - to the RV trailer axles.

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Old 11-29-2012, 11:17 AM   #13
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[QUOTE=BlackAces;1232349][quote jammer] There is no actual physical or measurable or testable difference in load-carrying capacity between ST and LT tires.
Quote:

Of course there is. It’s molded right into each DOT certified tire. Example: The LT235/85R16E has a maximum load capacity of 3042# at 80 PSI… The ST235/85R16E has a maximum load capacity of 3640# at 80 psi.
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.
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Old 11-29-2012, 11:59 AM   #14
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[QUOTE=Jammer;1232483]
Quote:
Originally Posted by BlackAces View Post
[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.
They are tested to different standards, however. Passenger-car and light truck tires are tested to Federal Motor Vehicle Safety Standard (FMVSS) 139. Trailer tires are tested to FMVSS 109 and 119.

As an aside, retreads are tested to FMVSS 117, and those dinky donut spares are tested to FMVSS 129. Deep-tread tires such as snow tires are also tested to FMVSS 139, just like p-metric and LT tires.

These standards apply to a whole lot more than just load range and speed rating, including such factors as, who much sidewall flex is required to unseat the bead of the tire when inflated to a certain pressure?

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?). They are not legally identical as a result of having to meet different safety standards.
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