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Old 10-27-2015, 01:48 PM   #1
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Tire overloading on curves....

We had a tire failure recently that has prompted me to take a close look at the factory tires on my 2015 Rockwood 2604WS.

The factory tires have a load rating of 1760lbs each, times four, or 7040lbs total load.

The loaded trailer GVWR is 7715lbs, so the tires aren't rated to carry the full weight of the trailer.

The dry tongue weight of my trailer is 715lbs, so the weight actually supported by the tires is 7000lbs, plus or minus.

Comparing tire load to the total load rating, there's only 10lbs of margin between the load rating of each tire and the load applied to them when my loaded trailer is sitting still on a flat surface with a perfectly balanced load.

I started wondering what will cause additional loading on my tires and maybe create an overload condition. There are lots of things that come to mind...differences between axles, left/right weight differences, lumps and bumps on the road, and so on. However, one source of additional load stands out: weight transfer to the outside tires on curves.

So, I decided to calculate the weight transfer to the outside tires when I take a turn. This involves using the 'advisory speed' for a curve shown on a yellow sign and defined in the MUTCD (federal law) to calculate the lateral g forces, a guestimate of the height of the center of mass of my trailer, and the side to side distance between my wheels. I'll show the math if anyone asks. I *think* it's correct.

BTW, if anything, using the advisory speed is conservative since many people seem to ignore advisory limits.

To make a long story short, I came up with the following table for my trailer:

Advisory Speed / Added Load Per Tire
35MPH / 427lbs
30MPH / 503lbs
25MPH / 503lbs
20MPH / 580lbs

Since there is almost no margin between static load and load rating of my tires, the added load per tire is entirely an overload of the outside tires.

So, I figure if I take a curve with my loaded trailer at the posted 30mph advisory speed limit, I'm overloading my two outside tires by 503lbs each. If I'm driving Monarch Pass in Colorado, as I was when my failure occurred, I'm not doing this just once, but repeatedly while also braking heavily at times.

If these numbers are valid and apply to other trailers as well, no wonder there are so many complaints of tire problems on travel trailers. The tires aren't rated for the load that they see on the road.

What am I missing?
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Old 10-27-2015, 02:18 PM   #2
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The manufacturers put the cheapest tires on that can slip by the GVW of the trailer. IMHO, I suggest you go up a load rating over what it came with. I went down this road to the tune of 4 blowouts, three on one 350 mile trip.
Make sure your spare is upgraded also. I carry two spares.
Also, a TPMS is a must have.
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Old 10-27-2015, 02:23 PM   #3
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Yup. Fool me once, shame on you, fool me twice, shame on me.


I went up to Kumho 857's. The load rating went from 1760 to 2270 per tire.
I also got myself a TPMS, an infrared thermometer, and a digital pressure gauge.


I'm not going to repeat this bad experience if it can be avoided.
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Old 10-27-2015, 02:39 PM   #4
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You can do a search and see hundreds in the same situation as yourself.

The tires are rated to carry the full TT because you subtract TW since that part is carried by the TV. This makes them "within" limits. This is why most people negotiate a deal for better tires or take the TT straight to the tire shop after purchase and slap a better rating of tires on.

The load range on the sidewall is for static measurement and not meant to be calculated for dynamic applications. To correctly calculate the dynamic factor (and point of purposed failure) you would have to know the speed traveled, ambient temp, temp of the tire as well as the current force that the pothole, bump or turn generated at the time of impact (or failure). This is the exact reason these numbers aren't posted on the side wall (because there are too many unknown variables and 1000s of situations that the manufacturer cant determine). Because of this the manufacturer posts the max speed rating as well as max load with an associated air pressure.

I agree for piece of mind you should go with a "buffer" and get higher load range tires (it only makes sense). I am not saying what the trailer manufacturers are doing is right Im just offering an explanation as to why they do it and how they legally do it.
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Old 10-27-2015, 02:54 PM   #5
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I am glad I came across this thread, my DH and I have been discussing new tires for our Windjammer 3025 which we just bought about a month ago. We like tires made here in the USA, and Michelins are our favorites.

Our choice of tires are 16" Michelin XPS tires, but the smallest they come in are 16", and we haven't checked to see if our wheel wells have enough clearance in them for the 16" wheels. Also, our present wheels have 5 lugs and we haven't checked yet to see if we can find any trailer wheels in 16" which have five lugs.

On our Airstream 25', we put on new axles to upgrade to 3500#/axle (two axles) which also increased the size of our brakes and put on 16" wheels and the Michelin XPS tires, we did this in the Spring of 2011. So far, we have not had one issue with the axles; wheels; or tires -- and we've, thankfully been able to travel stress-free without worries of tire blowouts all this time, and we've been out West, numerous trips to Florida, down to the Carolina's, etc.

Those of you who have posted on this thread seem to be very aware and concerned about tires, etc, and we would like to know if any of you have any thoughts regarding our situation? We'd love to hear from anyone who does.

Thanks in advance.

Deb
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Old 10-27-2015, 02:54 PM   #6
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They are on the edge it's a really poor design but will obviously only be divulged when there is a few deaths caused by this, when profit is at stake only government regulation works and these days governments are reactionary rather than proactive.
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Old 10-27-2015, 03:02 PM   #7
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Yup. The problem isn't with one vendor, but with the entire industry.

A lot of the weight transfer problem on turns will be specific to the location center of gravity of the trailer. This, and other dynamic effects, will be different with each trailer model. Yet the method used to size tires to trailers doesn't take this into account at all. By raising the center of gravity, I could design a trailer that would blow it's tires coming out of the parking lot, and yet would still meet the industry standards.

That many people immediately swap out their brand new tires is a pretty clear statement that there is a problem here.


I'm just not inclined to accept that it's ok.
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Old 10-27-2015, 03:08 PM   #8
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Windjam2, I talked to a Dexter engineer about tires and axles. He warned me not to significantly increase the diameter of tires because the axle spindle is designed for a specific diameter and may fail with a larger tire. He seemed credible, but I don't completely understand why the axle would be so sensitive to tire diameter. However, unless I have reason to know better, he's the expert. So, if it were mine, I'd try to go to a higher load rating in the same size as the factory tires.
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Old 10-27-2015, 03:19 PM   #9
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More leverage on the spindle maybe?


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Old 10-27-2015, 03:34 PM   #10
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Quote:
Originally Posted by Iwannacamp View Post
More leverage on the spindle maybe?


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Yeah. That was my thought too.
But we're not talking about a lot.
An inch or two out of maybe 26"?

I should call the Dexter engineer back and ask him......
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