Sunday, September 25, 2022

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RV Tire Safety: Interply shear – Am I spreading fear?

On an RV forum I saw some posts about tire failures:

There were some posts that mentioned the RV total weight capacity or Gross Vehicle Weight Rating (GVWR). Others were mentioning the total of the tow vehicle plus the RV trailer or Gross Combined Weight Rating (GCWR). Others were talking about tire load capacity.

One person responded with: “You need to pay more attention to just what acronyms are telling you. GCWR has nothing to do with tire inflation pressures. The tow vehicle and towed vehicle are individual vehicles and answer to their own standards and regulations.

“Maybe you should ask Roger why, during his working years with tires, the interply sheer problem was never corrected. (Interjecting a fear factor for attention purposes might be Roger’s goal.)

“Tire tread separations are a fact that trailer haulers must respect. Fooling around with RV trailer tire inflation pressures by using less than what the vehicle manufacturer has recommended is, IMO, counter productive.”

My response regarding interply shear

Since I was mentioned in the forum post along with the suggestion I might be spreading fear, I felt the need to reply, as follows:

Sorry, but fear is not and has never been my intent. I covered interply shear and how I discovered that it was different for multi-axle trailers than it is in motor vehicles in my blog post of Nov. 20, 2013, when I did the first of multiple posts on Interply Shear as it pertains to multi-axle trailers.

The fact is that Interply Shear is well known in the tire industry but not the unique forces imparted on multi-axle trailers. It was pure chance that I noticed the irregular path taken by a trailer doing a 180° U turn on freshly smoothed gravel (picture in the Feb. 22, 2018 post). That was my Ah-Ha moment.

While I was retired by the time I first saw the gravel path, I still had friends that worked with the high power Finite Element and vehicle simulation software and called in a couple favors. I asked them to run a simulation of a truck pulling a tandem-axle trailer through a series of “S” curves. All 8 tires were identical with the same load and inflation so we could end up with a comparison of trailer effects on tire belts vs. the normal interply forces found in all radial tires in motor vehicle applications.

Interesting discovery regarding internal shear forces

What we discovered after the overnight computer run was that in simple S turns, the belts on a multi-axle trailer were developing 24% higher internal shear forces than the tires on the truck pulling the trailer.

This goes a long way in explaining why tires in multi-axle applications have much shorter life and significantly higher failure rate than do tires in car or truck applications.

So, you see, science and facts can be used to point out why blindly following 50-year-old design decisions and regulations that are the foundation for the current ST tire regulations can be significantly improved upon.

Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1054

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Tommy Molnar
3 months ago

I do my best to avoid any sharp turns with our two axle trailer, whether it’s in parking lots, RV parks, fueling stops etc. Not always possible, but I do my best. Wherever we are parking I get in a couple of pull up and back ups to be sure there is no ‘torsion’ on any of the trailer tires. Single axle trailers are immune to this worry.

John_Brown
3 months ago

I have owned both single, double, and triple axle trailers and I build them too. I am building both a single and double axle trailer right now.

What I noticed on sharp backwards turns using P235/75r15 tires on double axle trailers is the rear tire on the inside of the turn has a tendency to fold over on the sidewall. Going forwards the same tire in a turn tends to skip or skid. This is noticeable on triple axle trailers, you can feel it in the truck, and I would not want to be a tire on the middle axle.

I think this is probably aggravated by the use of short springs in the 20-25″ range.

I have not noticed this when using smaller single axle trailers in the 5×8 range.

I have no doubt interply shear happens and is a valid thing, though almost all my tires (especially ST tires) wear out before I suffer failures. I never had a trailer tire blow out, so far, but, I watch my inflation pressures and try not to run the weight limit.

George Paniagua
3 months ago

Oh yeah, we lived in a cul-de-sac. 360′ turn around and back that puppy in. Tread de-lamination, out side edge tread wear was off the charts. I could see the skid marks from my U-turns, duh! I started using my neighbors driveway for a 90′ to straight back in. Ta-Da! Needless to say I avoid tight turns.

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