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Thursday, October 20, 2022

The "safety factor" for tires


Many people ask about the "safety factor" for tires. The dictionary offers this definition for "safety factor": "The ratio of the maximum stress that a structural part or a piece of material can withstand relative to the maximum stress estimated for it in the use for which it is designed." While that sounds reasonable, it really only works when talking about items that fail from simply increasing the load placed on the component.

Items like tires do not really have a "safety factor," as tires generally do not fail by simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300% of the load marked on the tire sidewall if they never had to roll. However, as soon as you introduce rolling or time or operating temperature, the maximum load before failure is much closer to the max load number molded on the tire sidewall. The exception to Max speed is affected by temperature, time and load. With zero load, many tires can probably handle 200+ mph, but again, for how long and at what temperature?

So you see that tire durability is affected by a combination of load, temperature, speed, and time.

Since tires are basically a structure made of some steel but mostly "organic" components, time and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in a pyramid, we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire engineers prefer to use the term "Reserve Load" when talking about the load capacity of a tire. Here we would find a tire engineering definition as the difference between the tire's maximum capacity when inflated to the stated level for the specific application (the inflation on the tire placard) and the actual load to be placed on the tire.

Here are a few comparisons: First some normal car and truck applications.

As you can see, tires in normal vehicle service have their Reserve Load well in excess of the RVIA suggested 10%. I would suggest that in addition to the special, internal tire structure forces such as Interply Shear, this low level of Reserve Load is a significant contributor to the shorter tire life seen in RV service.

While you, the owner, have little input or control over the materials used in tire construction, you do have significant control over the Reserve Load on your tires as well as operating speed and tire inflation. By decreasing the actual load you place on your tires while ensuring the tire has the highest Load Capacity (the "Max Load" number molded on the tire sidewall), you will increase the "Reserve Load" for your tires.

You can also ensure the Reserve Load stays high by not exceeding the speed for the service. For LT and truck tires (19.5" and 22.5"), the max speed for RV service is 75 mph, except for a few tires that have a lower limit of 65 or 62 mph. Check the "Data Book" publication from your tire manufacturer. Goodyear, Michelin, and Bridgestone state their max speed in RV service in their data book (75 mph max). I see no reason to go faster than that unless you find a data book from your tire manufacturer that states faster. If you have a tire salesman that claims faster is OK, ask to see the speed IN WRITING from that tire manufacturer or ask for a signed letter from the RV dealer on their letterhead paper stating the max allowable speed of operation for the tires they sold you.

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