Tire ‘safety margin’ versus ‘durability margin’ – What’s the difference?

The term “safety margin” is used for many “structures” like bridges or sometimes in individual components like bolts or steel cables. However, I bet you have never used the term “durability margin.” When it comes to tires, this might be a better term to use.

Tires seldom, if ever, fail as soon as they are run 10% overloaded or even 80% overloaded, or run 20 psi low, or 10 mph over their stated speed rating.

Damage is cumulative

Damage is cumulative and I don’t know of any 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. But I think you will agree that the engine will fail sooner rather than later if you run it significantly low on oil. 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 in the rubber breaking (cracking). They can grow and become large enough to see with a microscope, and then larger so they are visible with a hand magnifier. They eventually might result in cracks that grow large enough for the structure to fail, e.g., a belt separation that leads to a “blowout.”

Damaging events

Overload or low inflation or over-speed—let’s call these damaging events—can each initiate or contribute to accelerated growth of these “cracks.” In rubber, there is both an initiation phase and a growth phase.

It is conceivable for tires to survive an “X” level of 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 blog post of January 2020, “I never hit a pothole”. In it 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 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, and inflation, or the road surface, i.e. damaging event, that the tire was driven on 1,000 miles previous to the actual failure.

Tire strength formulas even have a “T” or time function that few engineers even know about. One example that I confirmed involved over-inflation.

Able to “force” a tire failure in testing

One specific tire had been developed and in production for a number of years with no reported failures due to over-inflation until I received a report. After investigating and resolving the report, I had an opportunity to do some pure research and was able to “force” a failure. But the failure took more than two weeks to result in a tire failure.

Normally this tire could tolerate an overinflation of more than 400%, but if I over-inflated the tire to 250% AND heated the tire to 100F AND maintained the over-inflation by connecting to an air hose with pressure higher than you can normally get from your compressor, the tire would develop a failure after 425 hours. I confirmed the test results with a second tire failing in the exact same condition with this extreme test at 427 hours. When it comes to tire testing, this could be considered an almost exact match.

This test led me to some old engineering textbooks on structures that involved wood bridges built in the 1800s. Engineers had a “T” for time function in their calculations.

if you read my blog or posts here on RVtravel.com or any of my posts on RV Forums, you will note that I often recommend that people run an extra 10% to 15% air pressure above the minimums in the published tables. So, I do not consider there to be a “safety margin” in the normal use of the term. In fact, if you want to consider this to be a “safety margin,” that’s fine with me. But I consider this extra inflation to be a “durability margin” designed to account for the failure of most RV owners to ensure the tires are never overloaded and never underinflated even for a single mile.

Forensic tire analysis

As I cover in my RV Tire Seminar “Master Class,” forensic tire analysis is a very complex science. There are very few engineers who have the decades of experience it takes to accurately identify the root cause of tire failure. I dare say that each major tire company only has a handful of engineers capable of identifying the actual root cause of a tire failure 70% of the time, and maybe only one engineer that can properly identify 90% of tire failures. Too often there is too much missing evidence or incomplete reporting of tire operation history for anyone to identify the reason why each and every tire failed.

Here are some reference books you can study if you doubt my claims about tire failures.

• Price, V & Follen,G (2019). Road Hazard Impacts: Their Influence on Radial Passenger Tires and the Forensic Signs They Leave Behind
• Bolden, G. C., Smith, J. M., & Flood, T. R. (2001). Impact Simulations in the Lab. Tire Technology International.
• Bolden, G. C., Smith, J. M., & Flood, T. R. (2005). Impact simulations – what happens when a tire/wheel impacts a road hazard. Tire Technology International, 44.
• Bolden, G. C., Smith, J. M., & Flood, T. R. (2006). Structural Impact Damage Under Varying Laboratory Conditions. Tire Technology International, 10.
• Gent, A. N., & Walter, J. D. (2005). The Pneumatic Tire. Washington: National Highway Traffic Safety Administration, U.S. Department of Transportation.
• Giapponi, T. R. (2008). Tire Forensic Investigation Analyzing Tire Failure. Warrendale: SAE International.
• McClain, C. P., & DiTallo, M. A. (2001). Tire Examination After Motor Vehicle Collisions. In K. Baker, Traffic Accident Collision Investigation (9 ed.). Evanston, Il: Northwestern University Center for Public Safety.
• Tire Industry Association. (2005). Passenger & Light Truck Tire Conditions Manual.

Roger Marble

Check out my Blog www.RVTireSafety.Net

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