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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.