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Friday, May 27, 2022

Interply Shear and 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). While 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 interplay 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".
Since I was mentioned if the forum post along with the suggestion I might be spreading fear, I felt the need to reply.

Sorry, but Fear is not and never has 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 in 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 vehicles applications. What we discovered after the overnight computer run, was that in  simply S turns (similar to seen in the video) 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 application 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.


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