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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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Friday, April 3, 2015

How fast can I safely drive?

Recently saw a story on AP titled "Big rigs often go faster than tires can handle"

The basic point of the article was that over the past few years, states have increased highway speed limits from the 50 or 60 mph range to 75, 80 or higher but the tires in use are for the most part still rated for 75 mph and below.

I know I read many posts from RV owners stating they travel at speeds well above the max rating for their tires. Be that exceeding the 65 mph limit on ST type tires or exceeding the 75 mph rating whenever in RV service.

Some people incorrectly believe that since they drove faster than the tire speed capability last week and didn't have a problem they can drive that fast whenever they want. Nothing is further from the truth.

As I have pointed out in numerous posts both in this Blog and on various RV forums, tires can tolerate a certain level of abuse be it over-load, under inflation or over speed. But every minute a tire is operating outside its design capacity the driver is consuming large portions of the tire life and the tire will eventually fail. Sometimes in a catastrophic manner.
When that happens all to often the driver says "I was just driving 50 mph down the road and had checked the air that morning when the tire blew out. must have been a defective tire'. In reality it was the driver's conscious decision to ignore the specifications for the tire be it load, inflation or speed and many miles of improper operation finally caught up with the driver.

If you take a look at the Tire Selector link for Goodyear RV tire application and download the RV Tire Care Guide on pg 16 you clearly see the statement of 75mph max speed.

Checking Michelin web site we can see a similar 75 mph max as seen on THIS sample pg.

Firestone says 75mph as seen here.  and Bridgestone on their R250 says 75mph
and Toyo has a similar limit for truck/bus type tires.

The important thing to remember is that just like an engine Tachometer that has a "Red Line" engine rpm limit, it may be possible to exceed that limit for a short time but you will pay for that practice in shorter engine life or in the case of a tire shorter tire life.


Couple of comments. When using the above links you might need to do some additional searching as some web sites are portals to additional information.

Some tires such as LT type may have a higher speed rating than 75 but that would be for normal pick-up truck service and not for RV service which is different. Also ST type trailer tires have a lower speed limit of 65 mph and in some cased certain heavy duty trailer tires have even lower 62 mph limit.

I would strongly suggest you lower your actual normal travel speed to 5 mph lower than the tire limit and of course never exceed the posted limit or the limit that could be considered safe for the conditions.

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