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Sunday, September 20, 2015

"Why do so many RV tires seem to be failing?"

 Well, that may seem to be a reasonable question but in my opinion it is based on what is often inaccurate failure analysis. In reality, tires fail for a relatively small number of Root Cause reasons that can be discovered, but it does require detailed and sometimes exhaustive investigation. By "Root Cause," I mean the initiating feature or condition that eventually led to the failure. Too often people confuse the tire's ending condition with the initiating or "Root Cause" reason for the tire to end up in the condition they are observing.

For the last few years of my 40-year career as a tire engineer, my primary job duty was to investigate failed tires that had conditions that were hard to understand or provided confusing and sometimes contradictory evidence as to the Root Cause for their condition. With that background, I have developed some guiding principles for the most likely reasons for tires to fail.

These are, in no special order:
1. External Damage. These include punctures, cuts, impacts, wheel and valve failures and similar conditions.
2. Run Low Flex Failure. This is best described by the failure of the tire sidewall due to excessive flexing. The flexing is the result of operation when the tire is significantly under-inflated.
3. Belt & Tread Separations. This is when the belts of a radial tire detach from the body or from each other and/or when the tread rubber detaches from the top belt.
4. Manufacturing Defect. For the purpose of this discussion this would be when components of a tire were not manufactured to intended specification.

The "Why" for some of these are obvious, such as the External Damage category.
Others can be further traced to various contributory reasons.

Run Low Flex Failure This is a more accurate description of what some incorrectly call "Blowout". When a tire looses significant inflation air they flex much more that the body cord can tolerate. Excess heat is generated which can, in extreme cases, result in the Polyester actually melting. I showed examples of what this can look link in THIS post. Polyester is used in most Passenger, ST type and LT type tires. Steel body tires are used on most Class-A RVs, the steel can fatigue. I covered "Zipper" failures and steel fatigue in THIS post.

Belt & Tread Separations occur primarily because the rubber around the belt cords or between the belt rubber and the tread rubber isn't strong enough. Now it can be weak for a variety of reasons. Some might be design, some might be manufacturing and some weakening can be caused by cumulative improper service conditions.

Now let's talk about tires made by a reputable company, i.e., one that has tire stores and dealers with physical stores and sells tires that have a warranty of two or more years.
If this tire is subjected to thousands of miles of lower inflation or higher loading or operation at higher speeds or stored is full sunlight it is reasonable to expect the strength of the belt & tread area rubber to loose a good portion of its strength primarily due to the increased operating temperature of the tire. If this tire is in service on a tandem axle trailer then there is also increased Interply Shear which can overload the belt area rubber. The combination of the above may result in a belt or tread separation.

Another possibility is a Manufacturing Defect. These usually occur in small numbers as tires are built in batches so the substitution of the wrong type of rubber may cause tires to fail. It is important to understand that in almost all cases this type of "defect" usually shows up at early life of the tire. It is also very important to understand that unlike some lawyers, engineers deal in facts and logic. Simply having a tire fail is not in it self proof of a defect but all to often that is the position that those in the legal profession seem to jump to. That approach may result in a nice payout to the lawyer but will not result in product improvement as there has been no determination or identification of the actual initiating "defect".

Finally there is the possibility of a design weakness. Now I do not believe that anyone is intentionally designing tires to fail but this sometimes may occur when the performance goals of the manufacturer are limited to meet the bare minimum for strength and durability and the focus is primarily on low cost. There is no absolute way to identify these tires but I do believe there are indicators when looking for tires that are made to a higher standard of quality and durability.
I would consider a tire company reputable if it had a chain of stores across the country. If the tires carry the name of the manufacturer and have a multi-year warranty the longer the better then they probably have higher durability requirements than those established by DOT.

 For ST type tires I would look for tires with a Speed Symbol of "L" (75 mph) or faster. Now a side point about speed. I did a POST a little while ago on the topic and strongly recommend you read it. I do not think I would recommend any tire for general use that does not have a speed rating molded on the sidewall.

I do hope this post will help some to have a better understanding of what can contribute to tire failure and consider what you can do to lower the chances of having such a failure.

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