How are loads calculated

 Had a question asked on an RV Forum

Mike Posted

 

Tireman9 - I've seen your posts with this "minimum inflation" statement in other threads in addition to this one, and 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 DataBook and Inflation Charts establishes the relationship between weight and PSI for each 5psi 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 5psi 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, or 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 140lbs to 230lbs, 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.

Simple answer is, The Load formula is not linear as you can see here.

 

 

 

 

  there are values that are exponents

 

 

 

 

 

As to adjusting for Dual position there are more instructions we tire design engineers have to 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 & 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.


Go camping.

 

 

Can I change from 65 psi trailer tires to 80 psi GY Endurance tires?

 Denise asked:

My TT came with the cheap tires out of China. I did a few short trips and one long, NJ to FL. I had no issues but decided to change to the Goodyear Endurance. The old tires had a psi of 65 but the Goodyears have a psi of 80. I did use the TT with the tire psi at 80 but I have seen that even though it says 80psi I should have deflated them to 65psi. Which is correct?

The answer is... It depends.  Ya, this sounds like a cop-out but a key bit of information is missing. 

 

You can skip to the bottom and just read "The Bottom Line"  but for those that want to understand how I got there here are the details.

 

That being How much load are you actually placing on your tires? The proper way to learn that is to get the RV on a truck scale with the RV fully loaded. From your comment, I am guessing you might be in NJ or spending the winter in FL so it might not be easy to get the RV fully loaded (all your clothes, books, tools, food, water, and propane as it might be when you are starting a trip). Our goal is to learn the maximum load you might ever place on the tires. Also, it is best if you get the weight for each axle separately by parking with one axle on one scale platform and the other axle on a different pad. If the scale you use only has a single pad you would need to get a weight reading with only one axle on the scale and then move the trailer so the other axle is on the pad.

Once you learn the load on the heavier axle you should assume the load on each tire on that axle is not exactly half the axle load as there is a lot of data from thousands of RV that suggests one end is always heavier than the other but without individual tire load readings, we can not know which end is heavier. So our option is to multiply the heavy axle weight by 0.51. With that number as our estimated max tire load, you would go to the Goodyear Load & Inflation tables.  Select your size and Load range tire. Then find the box with a load number that is equal to or greater than the 51% number you just calculated. The inflation shown for that box is the MINIMUM cold inflation pressure you should ever have in any of your 4 tires when you start to travel. I recommend that people running TPMS set the low-pressure warning level to that level of inflation.

I also recommend you add 10% to that minimum inflation to learn what pressure you should be setting your pressure too. The 10% is a small margin that allows for day-to-day temperature changes that would affect your inflation number as tire pressure changes by about 2% for every change in temperature and I want to save you the effort of having to adjust tire inflation every day.

You may ask yourself why all this work and calculation. What we are trying to do is to ensure that no tire is ever run in overload or underinflated. The 65 psi inflation on your RV Certification sticker is based on the GAWR (max axle load) your RV is designed to carry. In theory, tires inflated to 65 psi should be able to support the load but actual measurements of thousands of RVs have found that a majority of RVs have a tire or axle in overload and this is one reason so many RVs have tire failures. Another reason for the high failure rate gets technical but it has to do with the fact that trailer tires are being dragged around every turn and corner. This side loading results in extreme forces on the tire that is trying to tear the tire apart. I have covered the Science of this force in my RV Tire Safety blog.

Having given you more information than you wanted, and knowing that you may not be able to get your RV on a scale in the near future, I will recommend that until you can get the loads measured that you run your Goodyear Endurance tires at 70 psi. Once you do the calculations you may be able to run lower than 70 or you may need to run higher.

 

 

 

 

  That's a good start. the 6060# would be both axles so we need to be conservative.

I start by assuming one axle has 51%  .51x 6060 =  3,091 (always round up)

and .51x 3091 = 1,577# 

I need to know the tire size for the next step

 

 

Tire size: ST 225/75 R15 so the GY chart says 30 psi can support 1,600#

 

 

 

As I covered in my post on "Reserve Load" you can see that normal motor vehicles run about 30%. Since you have such a light weight RV I suggest you shoot for at least that level, so that gets us to 50 psi minimum cold inflation.  Your Certification sticker says 65 psi so I would run 65 psi and set your TPMS Low Pressure warning level to no lower than 50 psi. You will need to review your TPMS info as some systems can be set to a specific level while others automatically "warn" after a certain percent pressure lost.

Bottom Line

Inflate the GY Endurance to 65 psi cold. ensure the low pressure warning level of your TPMS is no lower than 50 psi.