Tire overloading on curves....

[Gyrogearloose]

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?

[Blackhat6mike]

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.

[Gyrogearloose]

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.

[SuicideSaints]

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.

[Windjam2]

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

[DDC]

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.

[Gyrogearloose]

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.

[Gyrogearloose]

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.

[Iwannacamp]

More leverage on the spindle maybe?


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[Gyrogearloose]

Originally Posted by Iwannacamp

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......

[Windjam2]

Gyrogearloose, I had a feeling you would be on top of this, thanks for sharing. That was something I had not considered. If you hear anything further, we'd really like to hear it.

Well, Michelin does make a 15" LT tire, but I'll have to check to see what the load rating is.

Thanks again.

Deb

[Gyrogearloose]

Originally Posted by SuicideSaints

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.
......
The load range on the sidewall is for static measurement and not meant to be calculated for dynamic applications.

I'm not so sure the tire manufacturers agree with this.

Carlisle Tire has a document titled "Trailer Tires: Tips & Best Practices". In that document they give the following advice:
"The combined capacity of the tires should exceed the loaded trailer weight by 20 percent."
There is no mention of subtracting tongue weight, which adds another 10%.

The net result of this recommendation is that tires should have about 30% more capacity than the ones put on my trailer by FR.

The reason for this excess capacity would be because the static load rating of tires doesn't have margin for lumps, bumps, and weight transfer on curves. The overloads that I calculate on turns just about go away if the advice from Carlisle Tire is followed.

I'm trying to contact an engineer at Carlisle to probe this further....

[youroo]

Gyro,I/We knew that our O/E Tires were Questionable so we Called our Tire Dealer and placed a order for Maxxis 8008 15" Tires! They were installed and down the road we go! Good Luck on your Quest for Tires! Youroo!!

[Gyrogearloose]

Thanks, youroo!

I've changed my tires to Kumho 857 trailer tires and raised the load rating to where it ought to be. In the process I made a new mistake. The Kumho's are size 205R14. I asked for 205/75R14 as recommended for my trailer. The new tires are 0.9" larger in diameter than I expected, which was not my intent. I'm now trying to figure out if this did any harm. The gap between the two tires was already small, and got smaller. This is my new concern.


Otherwise, I'm trying to raise awareness of the tire problem on trailers. It doesn't make sense that we all should buy a new trailer and immediately go out and buy new tires. And it doesn't make sense that the large number of reports of tire failures in RVs should be written off as pilot error by owners. This wouldn't be tolerated in cars owned by the same people...or by the NHTSA.


A friend of mine would call what I'm doing 'Tilting at Windmills'. Maybe this is futile; maybe not. :-)

[Gyrogearloose]

Well, I heard back from Dexter on two questions:


I asked whether the 0.9" larger diameter of my Kumho 205R14s would cause any problem with the axle or spindle. The answer was no. This makes me suspicious that the earlier warning I received from a Dexter engineer concerning increasing the tire diameter may have been a little overly conservative. I suggest going to the Dexter website and use their tool to ask similar questions.


I also asked whether overloading of the axles due to weight shift on curves might be a problem. Technically, this doesn't exceed the axle rating because as weight gets added to one end of the axle, it comes off the other end. The answer to this was also 'no'. I was told that the axles have extra capacity to handle this kind of overload. This begs the question 'how much extra capacity?'


So at the moment, my slightly bigger tires aren't a problem for the axles, and I can take curves without worrying about axle failure.


I'm still trying to get a response from credible tire manufacturers about whether a 580lb overload on curves is ok.

[Windjam2]

Thanks so much for sharing this info. Getting all of this "right" is so crucial, and when it's done, makes traveling and camping so much more enjoyable!

Deb

[mnoland30]

To Replace my 205/75 R14 tires with something reasonable, I found Taskmaster tires at etrailer.com that are rated for 81 mph and Load range D. I had to go to 215/75 R14s. They are about 1/2" larger diameter. I've been on one trip, and no problem so far. There is no excuse for tires that blow up, but Firestone got away with it for years until someone noticed it was killing people. In the Forest Service, we doubled the axle rating for dirt road use. Rough roads will overload an axle in a hurry. I broke the axle off my hunting trailer that way. If you boondock like we do, you'll want to take it very slow.

[Gyrogearloose]

Yup. I also had my eye on the Taskmaster 215/75R14 LRD.
2200lbs @ 65psi sounds a lot better than 1760lbs for the factory tire.

[Gyrogearloose]

I just received an email from an engineer at NHTSA.
I had sent them the analysis I did on my tire loading.
She's asking for the details of my trailer and tires...photos of the rating labels, of the trailer itself, and the like.


Hopefully this mess will soon be cleared up....
Stay tuned....

[Iwannacamp]

Originally Posted by Gyrogearloose

Well, I heard back from Dexter on two questions:


I asked whether the 0.9" larger diameter of my Kumho 205R14s would cause any problem with the axle or spindle. The answer was no. This makes me suspicious that the earlier warning I received from a Dexter engineer concerning increasing the tire diameter may have been a little overly conservative. I suggest going to the Dexter website and use their tool to ask similar questions.


I also asked whether overloading of the axles due to weight shift on curves might be a problem. Technically, this doesn't exceed the axle rating because as weight gets added to one end of the axle, it comes off the other end. The answer to this was also 'no'. I was told that the axles have extra capacity to handle this kind of overload. This begs the question 'how much extra capacity?'


So at the moment, my slightly bigger tires aren't a problem for the axles, and I can take curves without worrying about axle failure.


I'm still trying to get a response from credible tire manufacturers about whether a 580lb overload on curves is ok.

Just remember that's .45 per side. So you really only added .45 to the bottom. I assume you actually measured the tire. Specs could be a hair off depending on how you measure. May have to break out the geometry and use circumference and 3.142


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