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.”
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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So, to combat possible tire failures on our TT, I went from “D” rated tires to “E” rated tires. The sidewall calls for 80 psi, which I maintain religiously. I bump-check the tires every time we stop – for fuel, rest stops, or in the morning before heading out on the road again. I don’t drive over 60 mph regardless of the speed limit, and so far have only had one blowout about 20 years ago before I adopted my current driving style. Meantime, I see other RV’ers blow by me at crazy fast speeds. Downhill as well. God bless ’em.
“Bumping” or “Thumping” tires may be ok for telling you you have a flat once you are stopped but is of no value if you pick up a nail while traveling. Also you don’t know if you have a slow leak till it is too late. I suggest you get, program and use a TPMS.
Another “issue” with running at minimum is that it requires a lot more time fiddling around adjusting cold tire pressures based on any particular morning’s ambient temperature. The normally recommended 10% of minimum factor allows much more flexibility to be well within a safe margin day-to-day without constantly adjusting cold pressure.
Would you please give some examples of how someone knows what their safety margin is? You wrote … “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.”
The “OR” I capitalized is my question. I thought I read that simply installing a higher load range tire but NOT increasing the tire inflation did not give you more load capacity. If that’s accurate, what mechanism creates the safety margin that makes a tire failure less likely when just the load range is increased?
I know the RVIA added a mandatory tire safety margin for RV manufacturers of 10% for RV’s built sometime in 2017 or 2018 and later, but what does that mean for the average RV owner? Should owners of older RV’s without the extra 10% be looking at upgraded tires automatically?
Did you click on the link “aka reserve load to”? The link opens a post with some examples. You are correct simply going to a higher Load Range tire of the same size such LR-D to LR-E but not increasing the inflation will not increase your load capacity. It is the increase in inflation that gives the increase in load capacity. If you have an older RV (pre 2018) you probably need to look closely at your tire load capacity at the originally specified inflation and compare that to the load capacity of a tire with a higher Load Range(inflation). You can increase Load Capacity by either increase in tire size (if you have the clearance) or increasing Inflation. To run higher inflation probably requires an increase in Load Range.
You stated above: “the properties of the materials we specify have some level of variation in their properties”. This variation follows a distribution around the mean – both within a lot and lot to lot. Question: Are there acceptable upper/lower limits on those distributions (3-sigma)? And could a given lot that passes at the lower end of a specification contain substandard strength due to the normal distribution within a lot, leading to premature failures? OR, is that all taken into account when sourcing/specifying/approving materials for tire construction? OR lots of material falling on the lower half of the specification are rejected due to the potential for a portion of that lot being substandard? Also, how would DOT know that future lots of material and subsequent tire manufacturing would adhere to those minimums?
Material properties are subject to both purchasing and in-house testing at many times in the manufacturing process. The company I worked for uses “6-Sigma” quality standards in an effort to essentially eliminate the problem of “stacked tolerances”. DOT does not specify sampling or testing variation. Their standards simply say “all tires produced for sale must be able to pass the appropriate test”. DOT can and does select tires randomly for testing.