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Tuesday, September 24, 2013

The tire was "Defective"

I see this statement or somethnig similar in many posts on various RV forums. The problem I have is that too often the person making the statement offers evidence to back up their claim.

 The mere fact that a tire failed, simply does not mean it was defective. If someone wants to make that claim they need to provide a plausable theory on which material componenet was made incorrectly or which part of the tire manufacturing process was done incorrectly in such a manner that it would result in a tire failing in that specific manner.

Most radial tires made today have about 20 to 25 different components.

 This Wikipedia page identifies nine major components

 Watch this video and you will see a manual process used for a farm tire.
Hankook has a nice graphic showing more steps in detail for radial tires
Here is a promotional video from Michelin. The part about automatic tire assembly is very similar to what you would see in just about any modern tire manufacturing plant making Passenger, Light Truck or TBR Truck/Bus-Radials .
You can see that from manual to fully automatic the steps are very similar. It doesn't make any difference who makes the tire as all tires share very similar construction features.
So now that you have a little better idea of how tires are manufactured Try this exercise. Identify a step in the process that could be done incorrectly on one tire or a small group of tires. The out of tolerance step or material needs to be close enough to spec to allow the tire to finish the manufacturing process and to also allow the tire to function properly for a few hundred or thousand miles but then this error must somehow cause the tire to suddinly cause the tire to come apart but first provide no warning and second leave no evidence of the out of spec part or material being present in the failed tire.

I think you will find that it is very difficult to have a material out of spec enough to function acceptable for a short time but then to catistrophically fail a few miles later and to also leave no physical evidence.

I am not trying to imply that mistakes are never made but since the manufacturing process makes tires and even the components in batches if a mistake is made it should affect all the tires in that batch which would be from a couple hundred to a few thousand. If in fact those defective tires somehow managet to pass through final inspection unnoticed and make it to the tire store where they are mounted on your RV I think you can see that in all likelyhood all the tires with the same defect would fail in identical manner.
If a group of tires fail and if dealers or owners report the fialures to NHTSA then there is a high probability that action would result.

There have been a few cases recently where thousands of tires were recalled because a small number had been found with a defect. What I haven't seen or heard about is a recall because a single tire failed.

When we see a single tire fail and it leaves evidence such as worn sidewall or melted polyester body cord

or fatigued steel body cord as seen on the right.

Tire engineers can be quite certain the failure was not caused by a manufacturing error but in fact the tire was operated under-inflated at highway speed for a few miles. Also when a tire has managed to perform satisfactorily for thousands of miles before failing it is again very unlikely because of a manufacturing error.

All to often those that jump to the conclusion that the tire must have been defective because they checked the air a few hours of days before the tire failed. These folk fail to realize that tires can fail with sidewall "blowout" in less than 5 miles of running at significant under-inflation.

Here is an example of a tire that had been run under-inflated in a controlled test of 3.9 miles at only 10 to 15 mph. You can clearly see the internal damage.
This tire still had 20% of the inflation needed to carry the load so it was not completely flat. That circumferential line is where the body cord would melt or fatique if the car had been driven at highway speeds and only a mile or two further. However dollars to donuts most people would say that since they had checked the air 100 miles prior to the failure the tire must be defective.

Thursday, September 12, 2013

TPMS Accuracy & I had an air leak!

I sometimes see people worrying about temperature or pressure readings they see on their TPMS monitor. Based on the reported numbers, I have never been concerned with the minor variation but I felt I needed data to see if my opinion was justified.

As you can imagine I can get pretty focused on tire inflation pressure so it may not surprise you that in addition to two digital pressure gauges,
which have been checked and found accurate to +/-0.5 psi against ISO laboratory certified pressure gauges, I also have two TPMS on my RV. I am probably the only individual in the US and possibly the world to have an RV so equipped. The reason for having two systems is so I can give you this test report as I have relatively high confidence that with my frequent inflation checks with my digital gauges and adjustments to inflation I feel I would identify any problems with my TPMS. More on that later.

I installed my internal TPM about two weeks after buying my RV in 2008. At the time I was quite familiar with internal systems as I had been spending a significant portion of my work time involved with the OE versions of TPMS sensors. Some like this one had been damaged by improper mounting practice.

 My internal system is actually a car based system as it has an upper pressure range of 75psi but since I set my tires to 65 psi which is about 10psi above the minimum needed based on actual tire load, I felt I would not be seeing a big rise in inflation pressure. I was of course more concerned with the potential of a puncture and loss of air pressure. I subsequently learned that I would occasionally get a "high press" warning on one or two tires but have confirmed that my tires only get up to about 77 psi max.

I started this blog in 2011. After that I also increased my reading of various RV forums where I started to see more and more comments and questions on TPMS accuracy, I decided I needed to do some testing.

Here are the results from a number of tests.
 This data compares the two TPMS with my accurate hand gauge with the tires all set to 65.0 psi cold

 As you can see that while there is measurable difference in the readings I do not consider these meaningful and would consider the +/- 3 psi acceptible as I consider a TPM to really be a warning system based on a change in pressure and not a substitute for an accurate pressure gauge.

Running pressure shows a simmilar range of variation as seen above.
Running temperature is a slightly different story.Since each tire has a slightly different load and there is a significant difference in air-flow around the fronts vs the dual rears as well as a difference between the inner and outer dual, I would expect more variation and we see this in the test results shown here.
I note that the internal temperature is consistently higher than the external temperature reading which is to be expected with the external sensor both further from the heat source and being cooled by the external air flowing over both the sensor and in the rears over my extender hoses.


I previously mentioned that I felt I would be able to see a failing TPM sensor and I can report that I was able to discover a leak of  about 8 psi a month because I pay attention to the range of readings of my TPMS. I do not write down the numbers every time but I do pay attention to the range of numbers observed. I can also confirm that my external system gave me a warning as it is set tighter than the internal system. The leak was at the TPM sensor itself but since my system has a three year warranty I have been able to get a free replacement sensor.

You can expect some variation in both pressure and temperature between different TPM systems. If you use a good hand gauge to set your pressures and just make a mental note of the range of pressure readings right after setting the tire pressures and on your first couple of trips, I think you will be able to recognize when there is a leak or an unusual reading that warrants additional investigation.

Wednesday, September 4, 2013

Why do tires fail?

There are basically two reasons that explain the vast majority of tire failures.
1. Over-load / under-inflation which are almost the same thing since it is the inflation air that carries the load not the tire.
Under-inflation: A tire operating at less than 80% of the inflation needed to carry the load, is considered to have been run flat and there is a good chance that there has been permanent internal structural done to the tire if driven on in that condition. Related to this is having the tire inflated to a level that just barely is rated for the actual load on the tire. Obviously you can be under-inflated because of cut or puncture or a valve leak or if you use an inaccurate gauge. If you run sufficiently low in pressure at highway speeds for a just a few miles you can have a Run Low Flex Failure or more commonly a "Blowout". Steel body tires can have "Zipper" failures due to fatigue of the steel body ply.

Over-loaded: Few people realize that by design, most passenger vehicles have 13% to 20% or more "Reserve Load". That means that they are actually under-loaded by that much for a vast majority of the time. Most RV trailers on the other hand have tires selected that are at the tire max load and can just barely carry the actual load while motorhomes seem to have some reserve load but not as much as passenger cars.
Data on actual loads measured shows that over half of RVs measured (out of many thousands) have one or more tires overloaded based on actual inspection.
While at a campground last week I got into a discussion with the guy parked next to me with a 5 week old RV.  I bet him a beer that at least one of his tires was at least 10% low. We found all his tires were between 18% and 21% below spec. I did enjoy the beer. :-)

2. Heat. Heat damage occurs at the molecular level and degrade the ability of rubber to flex and stretch and not break the chemical bonds. Once cracks form the rubber does not repair itself, the cracks just continue to grow. If they grow enough eventually you may have components come apart. Heat comes from a few different sources. This heat is generated by the flexing of the tire with the hottest region being at the belt edges (edge of the tread) in radials, not the sidewall if the tire is properly loaded and inflated. Increased speed generates more heat. Sometimes the heat is generated faster than it can be transferred to the surrounding air. Over-loading generates more heat. Under-inflation generates more heat. Having 0% reserve load generates more heat than having 10% reserve load which generates more heat than being 15%  under-loaded etc.  This heat can soak into the structure of the tire and actually accelerate the aging of the rubber in the tire. As rubber ages, it looses it's flexibility so this contributes to the breakdown of the rubber at the molecular level mentioned above.

Part of Organic Chemistry is chemical reaction rate.
For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.

Here are some specific references on tires

I provided temperature data in my post on  white tire covers which protect tires almost completely from this heat damage. Many times the cumulative damage from excess heat can result in a separation of the belts and tread from the rest of the tire. Remember aging of rubber makes it less able to tolerate flexing.

Hope this helps others understand the causes of the vast majority of tire failures.