By Roger Marble
I recently read a question on a blog about tire “safety margin.” I pointed out that the term “safety margin” really doesn’t apply to tires, but rather a “durability margin” might be a better way to think about tires.
Tires seldom, if ever, fail as soon as they are run 10% overloaded or run 20 psi low or 10 mph over their stated speed rating. Damage is cumulative and I don’t know of any automotive parts that “repair” themselves.
In some mechanical parts the damage is in the form of accelerated wear. For a moment, think of running your engine low on oil. Will the engine fail if you run it 5% low or 20% low or 80% low on oil? In how many miles? I don’t know the specific answer to the number of miles based on how low on oil you are. Does the fact that you might not have an immediate engine failure from running 50% low on oil mean it is OK to do that?
In tires, the damage starts at the molecular level with chemical bonds breaking (cracking), which can grow and may become microscopic in size and eventually might result in cracks that grow large enough for the structure to fail, e.g., a belt separation.
Overload or low inflation or over-speed – let’s call these damaging events – can each initiate or contribute to an accelerated growth of these “cracks.” In rubber there is both an initiation phase and a growth phase.
It is conceivable for tires to tolerate “X” level of a damaging event for many thousands of miles and the tire never “initiates” a crack. But also there can be a single damaging event such as hitting a pothole of a certain size, depth, length, angle of incidence and speed that is the “initiation” event. This can result in a belt separation 5 miles or 20,000 miles later, with the variation being the result of different growth rates. This was covered in detail in my post in January 2020. I examined a Tire Industry technical paper on tire forensics and impact damage that identified a 100% correlation between impact damage (such as you might get hitting a pothole) and belt separation failure.
Many times I read reports from RV owners saying, “I was driving 55 mph down the Interstate and had checked the air just an hour previous and was not overloaded when the tire failed for no reason.” Sorry, but there is always a reason. That reason might be because of the speed, load, inflation, road surface, i.e., a damaging event, that the tire was driven on 1,000 miles previous to the actual failure.
Forensic tire analysis is a very complex science and there are very few engineers that have the decades of experience it takes to accurately identify the root cause of tire failure. I dare say that each major tire company has a handful of engineers capable of correctly identifying the actual root cause of a tire failure 70% of the time. While they may not be able to identify the actual root cause, they probably have been trained and have enough experience to know if the tire should be “adjusted” as a “customer satisfaction” case. But maybe only one engineer can properly identify 90% of tire failures and be able to issue a technical “white paper” offering the evidence to support their detailed opinion of the originating root cause for the eventual failure. Too often there is too much missing evidence or accurate reporting of tire operation history for anyone to identify the reason why each and every tire failed.
Since learning all the important facts of the tire’s operating and life experience is impossible, it is very difficult to be able to identify the reason for every failure. Even if there is physical evidence of the tire hitting a pothole, you can see in the data of my January 2020 post that you still can’t provide a reliable answer such as: You hit a pothole 5,678 miles ago and that is why your tire failed.
Back to the question of “safety factor or margin.” All tires sold for use on U.S. public highways have to meet a series of tests such as speed, load endurance, staying on the wheel with low inflation, etc. Tire companies also have their own minimum performance standards that tires are supposed to meet. Tires are not graded such that they pass 85% of the tests or that 90% of all tires made must be capable of passing all the tests. The laws for tire safety are written such that 100% of the tires sold must be capable of passing 100% of the tests.
Each tire company tests tires beyond the minimums. Using statistical data analysis, they can predict the performance required on any individual test tire such that 99.96%, or some similar number, of all tires made will be capable of passing the DOT testing when new. Tire companies have whole departments constantly looking at test tire results, and result variation, in an effort to be confident they are making good tires.
What tire companies can’t do is predict how much any individual RV owner will overload or under-inflate their tires. So it is impossible to come up with some specific “margin” of acceptable overload because the other factors such as speed, ambient temperature during operation and level of under-inflation are unknown and too variable to allow prediction of durability.