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Friday, November 27, 2020

Claims of "defective" tires causing "Blowouts" continue to be posted on YouTube

 Here is a post from, July 22, 2012. The owners of this Airstream continue to use the word "defect" but also fail to identify the defect. I also see no signs of them running a TPMS.

"Blowout" A Real Life Experience

In the previous post, I covered Run Low Flex Failure, RLOF, and how there were telltale signs or physical evidence that while each taken individually are just circumstantial in nature but when taking as a whole make a very strong case for the tire having been operated for a number of miles at very significant air loss. In this post, I want to walk you through the thought process of a forensic analysis of the evidence that is available and show how an opinion or conclusion is reached.

First I need to tell you that I have not had the opportunity to personally inspect the subject tire, but there is a YouTube video and I will show you some shots from the video and tell you what I see in these shots. Second I cannot tell you about the individual as I have had only limited correspondence about this tire failure but according to statements in the video they called it a "Blowout", said they had checked the air pressure in the morning and had only driven about 45 miles before the loss of air occurred.

So lets see what the pictures show. I have highlighted the area of interest in each picture. In pic #112 we see in the yellow area signs that the interior of the tire was worn from abrasion on the inside of the tire under one side of the tread rubbing on the inside of the tire above the wheel. This occurs at 0 psi but takes more than the half mile it takes to come to a stop to wear this much. Also the tire must still be in one piece such that the tread can uniformly contact the lower interior of the tire so the wear occurred before the tread part of this tire broke away. In the red area we can see a few dark holes where the body cord has melted and shrunk back into the tire structure. This occurs when the body cord exceeds 390°F. To put this temp in perspective properly loaded and inflated tires may get to 160°F and ultra high speed tires such as seen at the Indianapolis 500 may see 200° to 210° but rubber starts to come apart an just a little more than that.

In pic 108 is another view of the same location of melted body cord. If the cord had simply failed due to high pressure the cord would be frayed. Also note the nice circumferential line the failure took. More on that later.

 Moving on to picture 057 note the areas circled in red. An alternate view is seen at the arrow in pic 426. Now if we compare the loss of the ridge that is easily seen in pic 121. What this means is that the upper sidewall was in contact with the road surface for a number of miles and the outer edge of the sidewall "Scuff Guard" raised rib has been completely worn down. What we do not see is the lettering of the "Scuff Guard" worn away so we know that the wear was not due to the trailer being repeatedly parked against a curb.


Now I am sure some are wondering how I can tell the difference between running a tire at 5 psi vs 0 psi. Well here are some shots from an experiment I ran myself a few years ago.

  This first picture here on the left shows the wear of the upper sidewall that occurred in only 3.9 mi while the tire had a minimum of 5 psi cold inflation. This is a bit like the wear on the RV tire sidewall
The next picture is of the interior of the same tire. You can see the nice circumferential crease/crack that has formed. This tire did not generate enough heat to melt the body cord as I was driving at less than 10 mph for the 3.9 mile test. That crease is at the location up the sidewall similar to where we see the circumferential failure in the "Blowout". Note there is no contact between the inside of the tread and the inside of the tire above the wheel as seen in the RV tire.
If you look at some of the examples shown in the previous post you will note that many show the same nice circumferential failure line.
Here are some examples that I use as ref pictures to help me understand the Root Cause of a tire failure.

Here are pictures of melted body cord, broken body cord and the holes sometimes left when the cord heats up and shrinks as it melts.
I hope this helps you see how easy it is to be mistaken when you think the failure is because of a defective tire when in fact the evidence shows the tire lost air, the Rv continued down the road at normal highway speed and after a few miles the tire "blew out". The only way to have some warning and to protect yourself against this all too often type of failure is to have a TPMS which will warn you as soon as the tire loses 10% of its air.

Bottom Line: I do not think this tire was "possibly defective" or a "crap tire". It did not "fall apart for no apparent reason". It came apart because it lost its air and the operator did not know till too late.  

Friday, November 20, 2020

"The tire is defective" More information

 I hear that statement quite often and will admit it is a bit of a "hot button" for me. For a good part of my 40 years as a tire design engineer I was tasked with doing Forensic level tire inspections and issuing "white paper" engineering summaries of the findings of the results of my inspection of tires that had failed. My inspection was not simply looking at the tire and saying "Yup it failed". I was expected to learn and identify the reason, the Root Cause for the failure, and to issue a report to management or even the auto company engineers. In a number of cases the report could be simple such as: "The tire suffered a belt separation at x thousand miles. The separation initiated between the two steel belts on the serial side and progressed until the tire failed. The bead area showed signs of under-inflation/overload."    or    The tire suffered a "Run Low Flex Failure which resulted in the melting of the polyester body cord as seen here.

It would never be acceptable for me to say "The tire failed so it obviously had a defect".

 I gave a short list of examples in my post of Sept 20, 2015. Improvements can not be made in tire production or in the manufacturing of any product if you only say "There was some Defect" for without knowing and clearly identifying the actual defect there is no way to know what material or process needs to be changed or improved to make the product better.

While simply saying the tire had a "defect" means you can't be sure that simply replacing the tire with a new one or even a new tire from a different manufacturer will prevent a re-occurrence of another similar failure.

Tire failures come from a number of different Root Causes. Some have nothing to do with the tire itself. These can include wheels that are porous like this one,

other wheels can develop cracks so they leak air, or are even the wrong size as seen here. I have previously covered the variety of "failures" that involve leaking valves.  Many failures are from external causes as seen in these examples. 

Now some may feel that because I worked in the tire industry I always want to blame the consumer but this is not the case. In fact I am personally responsible for one recall of over 8,000 tires based on my inspection and research that identified the root cause for the failure was a tire plant manufacturing error due to improper identification tag on a pallet of a specific rubber compound. Others had already seen the tires and decided they were "defective" but they stopped there and did not make the effort needed to properly identify exactly what the "defect" was.

While I understand that consumers really are only interested in getting back on the road. But if the reason for the failure was not something in the tire you can end up with another tire failure in your future.  If you have a failure and really care about the facts I may be able to help. First you need to keep the tire if possible. Get good overall picture IN FULL SUN LIGHT, Also some close-up pictures, Close enough that you have no more than about 12" of the tire in each shot.. Email me the pictures and I will be happy to work with you so we might learn the real cause. 


Friday, November 13, 2020

Max pressure

A clarification might be of help. The pressure number molded on the sidewall of tires is NOT the "Maximum Allowable" pressure. It is the Cold pressure necessary to support the Maximum Load capacity for that tire. In reality the pressure number molded on the tire sidewall is the Minimum needed to provide for the support of that load.
Increasing the cold pressure above the number on the tire sidewall will NOT increase the tire rated load capacity per industry practice, standard and guidelines.

If the pressure increases because of increased Ambient Temperature or because the tire gets hot from being in the Sunlight or the tire gets hot from being driven on, that increase is considered and accounted for by tire design engineers. The increase is about 2% for each increase of 10°F in tire temperature.  Even a temperature increase of 100F ( 20°F to 120°F for example)would only result in about 20% increase in tire pressure and undamaged tires can tolerate a greater pressure increase than 20%.

Note: I am not saying that you can heat a tire to above 190F and not have problems but those problems would be the result of high temperature degradation of the rubber and not simply due to pressure increase.

Friday, November 6, 2020

Tire Load Capacity is like your engine Red Line.

Many times some people point to regulations for tire inflation and point out that RV companies select the tire inflation and that is what we should run because they think that is the "optimum" inflation.

Well the reality is that the "Load & Inflation" tables actually are giving you the MINIMUM inflation needed to support the stated load.

First we need to remember that the the load is not your estimate or the load your Brother-in-law has in his RV but the actual measured load that you can only learn by getting on a truck scale.

Here is a comment on tire load capacity.

"Ya might have a valid point if the discussion were about OEM tires. However this discussion is about new tires on an existing TT. The owner should go through the process that the TT manufacturer did when determining tire pressures. Tire pressures should be set based upon the tires and their actual loads.

Unfortunately, TT manufacturers don’t have actual load information, so they base pressures on max load. The TT owner has (or should get) information that the manufacturer does not have: actual load. That information should be used to determine optimal inflation pressures.

As an actual tire engineer I would be happier if people understood that the inflation given in the Load tables are the MINIMUM needed to support the stated load.

In the non RV world tire inflation is not based on the minimum needed to support the load. You will find that most cars have inflations that provide a Reserve Load of 20% to 25% or more. That is one reason car tires have such a low failure rate that tires in RV application.

Some people want to believe that all is good as long as you meet the minimum standard, then they are surprised when tires fail.  Well they fail due to the cumulative damage done to the structure when used. This damage reduces the load capacity because damage weakens the structure.

It has been shown that a hard hit from a pot hole can effectively "kill" a tire with the only question being how long can you travel before it finally dies. I have also posted that you can hit a pot hole hard enough to "fatally" damage a tire yet have no recollection of the hit.

Your car or truck has an engine "red line" think for a moment about how long your engine would live if you ran it at 95 to 98% of red line speeds all the time. Well that red line is like the inflation number in the tables in reverse.
Running at the red line is like running at the maximum load capacity for your tire for the inflation shown in the tables.

I bet most of you run no higher than 75 to 80% of red line on your engine, if that high. That gives you a 20% reserve. Maybe you should consider doing the same for your tires.