If you have read any of my posts here at RVtravel.com or on my blog RVTireSafety.net or on any individual post on the various RV forums I follow, you have probably seen me suggest you add a “margin” (safety factor aka reserve load to some folks) to either the load capacity or the minimum inflation recommendation, or even to both. You may wonder what are my reasons. I have seen some people try to chip away at my load margin or the minimum inflation I suggest.
Reason for adding extra margins
I do have a reason for adding these extra margins. It has to do with the basic nature of the rubber and other materials used in tire manufacturing. Adding margins to other items such as bridges, buildings, or even the cars and trucks used to haul a trailer or load in the bed, is also fundamental to the nature of Engineering Design.
Engineers and others in scientific fields such as chemistry, medicine or physics all know that the properties of the materials we specify have some level of variation in their properties. For tires, I know and have seen firsthand that you cannot have an absolute number for the strength of a material. Each component of a tire, as seen above, be it rubber, steel, polyester or others, has a set of properties depending on which specific material I select. If we take a sample of material, be it steel or natural rubber (NR) and ethylene–propylene diene monomer (EPDM), we are presented with stated strengths. Even what we call “steel” is available in more than 3,500 different “grades”, each with different properties and levels of strength ranging from 30,000 psi to 72,000 psi.
When designing a tire we can basically choose from one type for the wire used to make the “bead” (the part that keeps the tire on the wheel when inflated—see above) and a different type of steel used in the belts. But then we can also select from a variety of wire cables which is a configuration of individual steel strands. Each cable has its own set of properties of strength or flexibility or even the ability of rubber to adhere to the cable.
As you can see, there is an almost infinite variety of combinations of materials a tire design engineer can select from. There are other things that can also be selected such as the temperature and time to “cure” the rubber, which can also affect the strength and durability of a tire.
All tires for U.S. highway use must pass DOT tests
Now all tires sold for highway use in the U.S. must be certified by the tire manufacturer to be able to pass specific DOT tests. So passing those tests sort of established a minimum “strength.” Tire companies can select to exceed those minimums but generally to exceed the minimums we would need to use more expensive materials or constructions. Since the tire companies want to stay in business, they also need to keep the costs of making a tire in mind. It is also well-documented that if you take a sample of steel cord and pull on it till it breaks, you never get a single result but will get a range of numbers. The same variation is observed in the strength of rubber and polyester, as used in tires. Now to ensure we produce tires that are acceptable to the public and pass the DOT tests, tire companies also have their own internal minimum performance standards. BUT I do not think that all companies design or make tires that perform identically for every type of test possible.
It is also important to remember that the DOT requires that ALL tires be capable of passing the tests—not some or an average or even most. But 100% of the tires made must be capable of passing each and every DOT test. To ensure that all new tires are capable of passing the tests, tire companies use statistical analysis of test variation in an effort to be confident that production tires will pass the DOT testing.
Finally, we come to you, the user. We know that a significant percentage of RV users do not set or even maintain tire inflation necessary to meet published minimums. We also know that many have no idea of the actual load they have on their tires. While some may keep driving speed below 65 mph, some will actually boast of towing at well over 75 or even 80 mph.
A tire’s strength decreases with use and time
In previous posts here and on my blog I have covered the fact that after a tire is used and as time moves on, its ultimate strength decreases. In an effort to decrease this variation, I have been advising that people not run at the lowest possible inflation for the load on their tires. The lower the inflation you run, the more heat is generated, which in turn lowers the ultimate strength of the rubber.
Degradation of rubber strength is not an on-off switch but a continuous process. The more you drive at higher heat, the more strength is “consumed.” The more pot-holes you hit, the more damage you do to the tire structure. This, in turn, can result in a decrease in the maximum strength of your tires.
Load & Inflation tables give you a guide for the MINIMUM inflation for a tire loaded to the stated number and operated at a given speed.
More speed means more heat.
Lower inflation means more heat.
More load means more heat.
And it is HEAT that ultimately can result in a tire coming apart.
However, if you run more inflation than what the tables show, that will decrease the heat.
In a future post, I will cover what I feel is the improper use of the word “defect.”