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Monday, May 13, 2013

Defective tire or incorrect diagnosos? (Part 2)

Last time, I discussed how a defective tire could be made. Today I want to show how a failure that is the result of external cause can be easily misdiagnosed. Since there are so many different possible external causes I will only be able to cover a few of the multitude of possible external causes. Please note that these pictures are only a few of those collected during my investigation and many times an on-hand examination is necessary to arrive at the real reason.   Lets start with the example from last post.

Here we see a tire with a sidewall split.









Here is an X-Ray showing the broken bead wire
 
 Here we see the ends of the bead wire showing tensile (pulling) not cut ends.

 Here we see a special tool used to measure wheel diameters. If the wheel were of correct diameter the front piece would line up with the slot in the back piece.

This shows the same tool on a reference rim and you can see an in specification wheel.





While this is an extreme example I think you can agree that the engineers at the car company who are responsible for the wheel being made to specifications didn't want to accept the possibility that their manufacturing plant had made a mistake.  I liken this to RV owners who do not want to accept the possibility that by driving at speeds higher than the tire is rated for or at lower pressure or higher load they might have contributed to the tire failure.

Here are so other examples. 
This tire had physical evidence of puncture and run low but the owner wanted to claim the tire was 100% OK up until the instant the tread came off.








This tire had numerous marks of damage on the inside proving run low.







The tire with this tread throw had an old un-repaired puncture with signs of sidewall contact with part of the vehicles.
 In closing I think a review of an earlier post of a "blowout" will show you that it is a rather common mistake to incorrectly identify the reason for a tire failure.

Monday, May 6, 2013

Defective tire or incorrect diagnosis?

I have been following a thread on a forum for an iconic RV. The company makes trailers that are all silver, but I won't name names. A few of those posts insist the tires are defective because first they failed and second because the owner didn't want to admit the possibility of some external cause as they want to find someone to blame. They will insist that there's "not a scratch on the sidewalls anywhere, and they have never hit a curb or anything else with them."  But somehow two tires failed after 9,000 miles.

The tire pictured here with a very obvious failure was identified as "Defective" by engineers at a large car company. I will cover the real reason in the next post.

I thought this would be a good opportunity to discuss the realities of tire failures and possible defective tires.

While tires are built individually, the various components are made in batches. Depending on the component these batches may be used in 100 to many thousands of tires. So let’s discuss how a "defective" tire might be made.

If it's a design defect then we should see the failure occur in a large percentage of the tires that are manufactured to that specification. Given that in regular production it is not unusual to see 10,000 to 1,000,000 tires made to a given specification it should be easy to see thousands or tens of thousands of identical failures.

A specification identifies which type of rubber is to be used for each of the 15 to 20 different components as well as the type of steel, polyester or nylon to be used. Now it is actually pretty unusual to have a design go into production that results in tires that will fall apart in service. Dozens if not hundreds of tires made to a new specification are tested before a specification is released to production. Additional testing is done by the producing plant to "qualify" the plant to make a specific tire. Even after a spec is released at a plant, modern quality control methods dictate spot checks of production tires. If you research tire recalls over the past few decades you will find that they are not tire design related but either service related or occur in relatively small batches of tires where there was human error in the producing plant on a single batch or two of component rubber not being made to spec.

Another thing that is basically true about tires made with an off-spec material is that they have a very high failure rate, very early in life, sometimes approaching 100 percent. Also the condition of the tires will be essentially identical. You will not see some tires throw a tread off while others have sidewall bulges. Sidewall bulges would all be of approximately the same size and location. If the error was, say, something like the wrong rubber used for the sidewall then I would expect almost all tires made with the wrong compound to suffer the same fate at about the same number of miles and that this failure would be early in life.

Next time I will show some examples of failures that were improperly diagnosed as “defective tires” by tire dealers and engineers working for the auto companies. The tire above is but one example.