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Monday, March 13, 2023

How are tire load ratings calculated?

 I had a question asked recently on an RV Forum about tire load ratings.

Mike posted:

Tireman9 — I’ve seen your posts with this “minimum inflation” statement in other threads in addition to this one. I never was able to find documentation in the Michelin files that states that, UNTIL I finally realized that the “Maximum load & pressure on sidewall” statement in their Michelin Truck Tire Data Book and Inflation Charts establishes the relationship between weight and psi for each 5 psi increment in the chart.

I like to weigh our MH at least once a year because we’ve been making some changes to it, and our trips vary from short to fairly long with different loading as a result. Long story short, I use a simple Excel sheet to determine psi requirements based on scale weights (it’s just easier for me and I like using Excel). Well, this simple little Excel lookup routine turned into a bit of a pain until I realized that the weight increments for each 5 psi increase are not consistent. In fact, they are all over the place.

The way I found this out is I was calculating psi # for our MH’s weights in two ways. One was to find the corresponding psi for my actual scale weight, and then add 10% to the psi. The second way was to increase my scale weights by 10% and then find the corresponding psi for that weight. When I compared the resulting psi’s between the two methods, they were different in many instances, but not consistently so.

So, the question is why do the single tire increments vary from 140 lbs. to 230 lbs., and the duals vary from 230 lbs. to 410 lbs. Can you explain? Does setting tire pressures really have to be this exact? And which way is correct? You would think that both would have the same results, but nah-Baby-nah.

My response:

The simple answer is. The load formula is not linear, as you can see here.

As you can see, there are some values that are exponential.

Adjusting the load for dual position

On the question of adjusting the load for dual position, there are more instructions we tire engineers must follow:

I suggest:

1. Learn actual tire loading on a truck scale when the RV is loaded to your expected heaviest.

2. Assume one end of an axle is supporting 51% to 52% of the total axle load (this estimate is not exact). This is why “4 corner weights” are preferred if you are near the load limit.

3. Consult load and inflation tables to learn the MINIMUM cold inflation.

4. Add at least 10% to the inflation in #3 and use this new number for your “cold inflation goal.”

5. Set your TPMS Low-Pressure Warning level to the inflation in #3 above.

I think that if you follow these instructions you can stop using your Excel calculations, which can be misleading … and Go camping.

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