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Friday, October 6, 2017

"Safety Margin"

Some people ask "How much Safety Margin should I have with my tires?" While this concept is simple, the reality is quite complex.


If you want to skip over the "Why" safety is complex just jump to the "Bottom Line" below.

In engineering it is more proper to talk about "Safety Factor" and Wikipedia covers the topic quite well. " Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load". For tires this can become difficult to establish for unlike many materials such as steel or aluminum, tires being made of a number of complex organic compounds both natural and synthetic that have properties that can vary from batch to batch. Even how the raw materials are handled and stored can affect the end product. Also the "strength" of the tire rubber varies with both time and temperature history and as I have previously pointed out the temperature history is not established by just considering the ambient temperature as tire load, inflation and operating speed as well as even storage conditions play a part in establishing the temperature of the more critical components of a tire. Some of these factors can be controlled by the vehicle owner while others can not.

Another part of the calculation concerns the consequences of failure. With some products, the consequences are just an inconvenience say as when a pencil breaks or the ink in a pen stops flowing. With tires the failure can range from an inconvenience if the tire wears out faster than expected or property damage may occur or in extreme cases personal injury can result.

Over the past decades the tire industry has developed a series of guidelines as they try to anticipate the variation in service the vehicle operator will subject the tires too, but even here outside factors such as changes in speed limits or legal load limit changes can affect tires made years before these operating conditions were contemplated.

Top line tire companies have staff of engineers, chemists and statisticians who constantly monitor variations in raw materials and in the finished product. Different plants have different requirements as even something as mundane as the water source can have an affect on the end product. Test labs at each plant are constantly monitoring the quality and consistency of the products that plant makes. not every tire plant makes the same type of tires so along with sales volume requirements plant capabilities are taken into consideration.

Ya but you are thinking "So what? I just want to know the Safety factor of my tires."

Basically I and other tire engineers have tried to consider all of these factors and are constantly looking at tires that have been run on both test tracks and by end users such as yourself. We adjust our specifications to allow our tires to meet and exceed a list of special tests that over time have proven very reliable at predicting the potential for tire failure. While we shoot for zero failures we also know that due to factors out of our control that goal is never possible given the constraints of real life tire use.

I have seen some figures that show a failure rate in the range or 0.05% for many tires but I have also heard of some specific tires (brand, size, design) having a rate closer to 5% or even 10%.

The bottom line
The best I can do is to suggest that you obtain and read the product maintenance manuals for the brand tire you have or are considering of buying. You will probably find that the information across brands is pretty constant so I suggest you at least take a look at a couple different documents. Some of the top line tires have RV or truck application specific reference materials such as can be found from Michelin or Goodyear or Bridgestone or Maxxis or you can check some of the links on THIS post.
- If you have a Motorhome or pick-up slide-in camper you need to confirm the load on each tire position and using the highest loaded end of each axle and the Load Inflation tables from your tire company learn the MINIMUM cold inflation pressure.  I suggest you add 10% to the table number and use that for all tires on that axle for your minimum. 
- If you have a towable (trailer or 5th wheel) also confirm that no tire is loaded to more than 85% of the max load molded on the tire sidewall. AND inflate to the inflation molded on the tire sidewall associated with its maximum load capacity.
- Get and use a TPMS. I have written on how I would set the TPMS warning levels HERE.

- Inspect your tires. Motorhomes can have your tire dealer do the inspection. Trailer owners can follow THIS procedure at least once a year or every 5 to 7,000 miles if you travel that much.


- Never exceed 75 mph with any tire in RV application and if you have ST type tires with no speed symbol never exceed 65 mph.











In my opinion if you follow these guidelines I believe you will have a reasonable and realistic safety margin for your tires.

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