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Old 07-23-2013, 09:35 PM   #29
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Originally Posted by switz View Post
Siler Hawk:
Snip...
S4.2.2.3
(a) For vehicles, except trailers with no designated seating positions, equipped with passenger car tires, the vehicle normal load on the tire shall be no greater than 94 percent of the derated load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
(b) For vehicles, except trailers with no designated seating positions, equipped with LT tires, the vehicle normal load on the tire shall be no greater than 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
I am sure I'm missing something. Both these paragraphs seem not to apply to travel trailers (for vehicles, EXCEPT trailers with no designated seating positions...)....

If that's so, what % of vehicle normal load applies? 94%? More? Less?

4 15's at 1985/ea (derated) is 7940. My 27FB Flying Cloud max is 7600. 94% of 7940 is 7463. So my 7600 gvw would exceed the 94% requirement if applicable.

Just trying to understand...thanks for going slow with me on this :-)
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Old 07-23-2013, 10:06 PM   #30
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The Discount Tire that we go to has a manager whose father owned a couple of big travel trailers. And, the manager was the one that told me about the all steel BFG Commercial T/As and Michelin XPS Ribs being used on SOBs, because his dad had switched to them after having numerous blowouts. So, I had no problems with Discount Tire regarding switching to 16-inch wheels and LT tires.
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Old 07-24-2013, 05:46 AM   #31
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Originally Posted by SteveSueMac View Post
I am sure I'm missing something. Both these paragraphs seem not to apply to travel trailers (for vehicles, EXCEPT trailers with no designated seating positions...)....

If that's so, what % of vehicle normal load applies? 94%? More? Less?

4 15's at 1985/ea (derated) is 7940. My 27FB Flying Cloud max is 7600. 94% of 7940 is 7463. So my 7600 gvw would exceed the 94% requirement if applicable.

Just trying to understand...thanks for going slow with me on this :-)
If I am reading this correctly, they are saying up to 100%.

Is this a good idea? No, it's a terrible Idea, but it is not illegal.

I am of the opinion that tires (any tire regardless of type) should not be loaded more than 85% of its rated capacity.
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Old 07-24-2013, 08:27 AM   #32
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Please note that when you cross the CAT scales with your trailer and tow vehicle, you normally receive the front and rear axle weights of the tow vehicle and the total weight on the trailer axle(s) (single or tandem or triple). Then you also get the total combination weight of the rig. By careful positioning of the trailer on the scales, it is possible to break out the weights for the individual axles which would show if the "stuff" was properly placed inside the trailer and not overloading one of the axles with more weight than the other axle.

The trailer tires are only supporting the axle weight shown on the weigh ticket. So our last trip, fully loaded with water, food, generators, etc, our trailer axles weight was 5,880 pounds. Note that the 5,880 pounds included the portion of the tongue weight transferred back to the trailer axles by the Hensley weight distribution hitch. That weight was supported by four Michelin LTX (P) 235/75R15 XL tires rated together at 7,940 pounds.

The tongue weight of our trailer loaded is 1,175 pounds, which means that the payload on our trailer axles could be 6,125 pounds at GVW which is still much less than the four tire rating of 7,940.

Our acquiring a 9,000 pound 27FB Classic eliminated the opportunity to use 15" tires as the projected tongue weight might be 1,100 pounds and thus the trailer axles would be supporting 7,900 pounds or nearly the maximum payload of the 15" Michelin tires. Thus we will install the 16" Michelin LT 225/75R16E tires that are NOT derated and have a rating of 2,680 pounds per tire or 10,720 pounds.

A prior poster was amazed at the 9,000 pound GVW of the 27FB Classic. The factory literature payload is 2,328 pounds and an empty weight of 6,672 pounds. Along with the solid wood cabinets there is a 54 gallon water tank, two 40 pound propane tanks and factory awnings installed all the way around.
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Old 07-24-2013, 09:29 AM   #33
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the payload on our trailer axles could be 6,125 pounds at GVW which is still much less than the four tire rating of 7,940.

Still need to know axle-by-axle and down to wheel-by-wheel with WD applied. It is unlikely that the load is evenly spread. 23% looks good in aggregate, but the scale tells the story.

Familes need to know those same weight values as when parked with 4-6 people aboard that the tire weight ratings are not exceeded. IOW, how often can I host an open house on the given ability of the tires to withstand being overloaded?

.
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Old 07-24-2013, 12:28 PM   #34
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O'K, So with the lower rating for trailers I'm still good to go with 15" with a 120% safety factor.

What I don't understand, and someone hopefully could enlighten, Why the lower rating for trailers? Can't come up with any rational. With the turning of the front car wheels - scrubbing and car lean into the turn (weight transfer) it seems the opposite.
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Old 07-24-2013, 02:33 PM   #35
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Perhaps some funds exchanged hands to make one type of tire have a handicap over another type of tire?
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Old 07-24-2013, 05:31 PM   #36
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O'K, So with the lower rating for trailers I'm still good to go with 15" with a 120% safety factor.

What I don't understand, and someone hopefully could enlighten, Why the lower rating for trailers? Can't come up with any rational. With the turning of the front car wheels - scrubbing and car lean into the turn (weight transfer) it seems the opposite.
My guesses;

That trailers are often loaded to near the max weight.

The much higher center of gravity (at least on travel trailers).

Another possibility is that cars normally have tires that are rated with enough extra capacity that they recommend less then 75% of the max PSI (normally max weight is only at max PSI or less PSI=less weight capacity).
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Old 07-24-2013, 05:37 PM   #37
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Trailers live their lives at the end of a pole, a lever. The forces enacted are not the same as on a car, for instance, which can settle out. Haven't either of you put a trailer into a spot from an angle or three and noticed that the tires are pushed AND pulled on either side, or on the same side, even, with a tandem or tridem axle trailer?

If I pushed on your tow vehicle from the b-pillar and caused it to pivot around that point then that set of tires would be seeing the same kind of forces in action.

Trailer tires are ALWAYS inflated to maximum sidewall pressure for this reason. Some do not do this, but the day of failure has come closer than age or miles would predict.
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Old 07-24-2013, 05:57 PM   #38
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ST (special trailer) tires have stronger sidewalls because of the side forces on them, especially when backing. This leads to the recommendation that ST tires must be inflated to their maximum rating since more pressure means stiffer sidewalls.

LT (light truck) tires have varying strengths in their sidewalls depending on the LT tire. Using a Load Range E tire on a trailer when LR D tires were the OEM tire means they may be inflated more than the OEM tire (80 psi vs. 65 psi). How much strength a specific LT tire sidewall depends on which LT tire you buy. For. ex., a Michelin Rib tire has steel belts in the sidewall and may be as strong or stronger than a LR D ST tire.

I have not seen any actual study that shows what lateral force modern LT tires can withstand compared to ST tires. Nor is there anything that shows whether the LT tires have to be inflated to maximum pressure or something more than 65 psi and less than 80 psi. It would be nice if such information existed.

ST tires came out when radial tires became common and bias ply tires were going out of style. The idea was to produce a radial tire with strong sidewalls for trailer use, comparable to the sidewalls of bias ply tires. All tires are a lot better now and it may be that contemporary LT tires are better than any ST ever made, even for backing with a trailer. I await the study that shows the actual facts. But, for the time being, we are guessing.

Our Michelin LTX tires have performed very well, even with my amateurish attempts at backing into spaces. Today I backed into a national forest site that was put in the wrong way—not a 70˚ turn or a 90˚ turn, but about 120˚. Did it in one shot and the tires are still on the wheels.

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Old 07-24-2013, 06:31 PM   #39
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The type of tire doesn't matter, friend. Tearing one off of the rim is a possibility to be avoided. As a general principle.

Granted that finding the proper load reserve in a non-ST tire is not as simple as any of us would like. Thus the best place to start is with BARRYS TIRE TECH website on ST Tires as to cross-referencing.
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Old 07-25-2013, 04:10 AM   #40
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I will say it again.

There are front to rear and side to side variations in tire loading. If you do not weigh your trailer such that you can determine what those individual weights are, then your guessing.

I think a good estimate for twin axles on a trailer are:

1) If you don't have individual axle measurements, then you need to have a 15% over capacity in your tires.

2) If you have the individual axle measurements, then a 10% over capacity is needed.

3) If you have only left vs right and not individual positions, a 10% over capacity is needed.

PLEASE NOTE: This is on top of my recommendation of only using 85% of the tires rated capacity.

Oh, and if you want to travel more than 65 mph on ST tires, I'd recommend 10 psi more for 75 mph, and another 10% reduction for 85 mph.

So just because you have 20% doesn't mean you're good to go. Be very careful. These reductions can add up quickly.
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Old 07-26-2013, 07:28 AM   #41
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All these tire reservations sound great on the the small to midsize trailers. But the larger units only have two axles today instead of three. Those new models are the 31' Flying Cloud and International with a 8,800 pound GVW and all the Classic models which have a higher GVW starting at 9,000 to 10,000 pounds.

So doing the math recommended by CapriRacer with Michelin LT 225/75R16E tires that are rated 2,680 pounds at 80psi, we find the initial weight capacity reduced to 2,278 pounds. Then, after only getting the twin axle weights together, we have to deduct another 15% to 1,936 pounds or 7,744 pounds for four tires.

Using this analysis, four Load E tires at maximum inflation would not have the load capacity for any of the trailers mentioned above with a load or the longer Classics empty. They would also lack the load capacity for the 19' Bambi with a single axle and a 4,400 pound GVW. Certainly, the factory supplied GYM ST225/75R15D tires would be overloaded even before loading the trailer since their capacity per this formula is only 1,835 pounds or 7,340 pounds for four tires. Per these calculations, the GYM tires should only be used on the tandem axle trailers with 7,300 pound GVW or less.

At this point, I do not see 17" Load E tires fitting in any existing Airstream wheel well. The Michelin LT245/75R17E is rated 3,195 pounds and is 31.5" in diameter or 3.2" larger diameter than the GYM ST tires. However, their derated capacity per the above suggestions would be 2,308 pounds or 9,233 pounds for four which would carry the two longer Classics axles as well as the 19' Bambi single axle.

These number follow the suggested formula. Not everyone has access to a CAT scale or a grain elevator scale. Of course one could spend close to $5,000 and have their own set of five wheel pad scales to verify the weights before each trip.

Federal law requires each new vehicle be weighed as it comes off the line. Adding a few words to the law could make sense. "the weight per wheel and it's location on the unit" must be disclosed on the door label.

Airstream, for whatever reason, supplies four GYM ST225/75R15D tires on the tandem axle trailers (or two on the single axle trailers) plus one as a spare. They have to be responsible that the tire will do the job or there would be a huge liability exposure.

My purchase of four Michelin LT 225/75R16E tires exceeds the weight capacity of the GYM tires and has no maximum speed restrictions and they fit on the trailer. I will have a current tire design with a greater safety margin in terms of load capacity and there seems to be NO chronic reporting of tire failures for Michelin.

Regardless of tire choice, I still must use common sense when loading the trailer, it is not a moving van.
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Old 07-26-2013, 10:15 AM   #42
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[QUOTE=switz;1331836]
Airstream, for whatever reason, supplies four GYM ST225/75R15D tires on the tandem axle trailers (or two on the single axle trailers) plus one as a spare. They have to be responsible that the tire will do the job or there would be a huge liability exposure.

/QUOTE]

That's assuming Airstream knows what it is doing. They have a tendency to do what they've always done and find out later they screwed up. I doubt they go through the painstaking analysis that goes on here.

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