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Tuesday, April 14, 2015

Causation or Correlation Tires may be too good. Even "China Bombs"


The fundamental problem with the idea that just because the plant that built the tires is painted green, or has two floors rather than one, or has a Zip code beginning with 9 rather than 4, or is built by people that speak Spanish rather than German, etc., that we should expect that all tires made at one location to fail at a high frequency, is not based on sound problem analysis. I could just as easily claim that RV workers that live in Ohio are poor workers because over 95% of every RV problem reported on an Airstream RV forum was built by workers living in Zip code starting with 45.

Lets break this down:
It is a fact that of the 30,000 or so RVs that have been weighed with individual tire position scales, over half have one or more suspension component (tire, wheel or axle) in an overload condition. It is important to remember that tires are almost always the lowest rated item in the suspension. In any population of products, if over half are being used beyond their design intent (max load based on measured inflation) why would we be surprised if there is a relatively high failure rate? Maybe even as high as 1 or 2%.

Well, what if I said that 90% of the tires that are put on RV trailers are built in tire plants painted green? Does it seem logical that the color of the paint used on the building would result in a tire being weaker or to age faster? If that were true, maybe the color we paint the RV might have an effect on the life of your tires.

Let's also look at the design goals for many RV tires. Low cost is what the RV industry wants, so that is all the tire importer asks of the manufacturing plant. After all, they only give you a 12-month warranty -- if they give you any warranty at all. If you check around I think you will find that most major tire companies give you a 3- to 5-year warranty on their standard tires. Many even offer a road hazard warranty for a few bucks -- which means any failure for any reason is covered. Now, maybe if RV owners demanded, say, a 3-year warranty on the entire RV, including tires, or they won't buy the unit, then RV dealers would demand better quality from the RV assemblers, who in turn would specify better quality components. Such an improvement in quality would not result in a dramatic increase in price. It is even possible that the actual cost of ownership might be lower if the RV was built better to start with.

If you haven't studied Statistics, you might invest a few minutes and watch this video "Correlation does not mean Causation". You might see how improper analysis is many times used in advertising, on TV and even in political sound bites.

But back to tires.

Another fact is that other than with a complete loss of air, tires almost never fail as soon as they are overloaded or under-inflated or run faster than their designed speed. The reality is, in my opinion, that the problem with most tires produced today is that they are perhaps too good. Since they don't fail as soon as they are abused, the owner isn't immediately "punished" (by having an immediate tire failure), so the owner incorrectly assumes their actions are OK.

There have been documented cases of tires failing days and even weeks after being damaged from significant overload. A few cases have been well investigated as both personal injury and even deaths were involved.

This is both good and bad news. The good news is that tire failures happens infrequently. The bad news is that owners receive negative reinforcement that incorrect actions (over-load, low inflation and high speed) do not result in a failure, so the owner does not associate the failure with the incorrect behavior. Any parent knows that when a child does something wrong it does no good to chastise them days later. Now, I am not calling everyone that has had a tire failure a "bad" person or insinuating they are behaving like children. I am using this example as this is just basic psychology.

In a number of posts on failure analysis on this Blog, I have documented some tire failures that occurred and were not properly associated with the previous damage, so an incorrect conclusion was reached by the RV owner.

It is important to remember that if you do not properly identify the real reason for a product failure, simply changing the brand or painting your RV a different color will probably not prevent a recurrence of a future failure.


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1 comment:

  1. Often, the RV manufacturer put an "optimum/marginal" tire on their fifth wheels (E rated), when an upgrade to a higher rated tire would give a better safety margin (although more expensive). I had to upgrade to a better rated tire after 3 blowouts on a new fifth wheel within the first year. If you are a new buyer, insist on a G rated tire.

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