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Monday, January 22, 2024

Tire inflation pressure, not the sidewall, supports the load

 Every few weeks I run across someone on an RV forum that makes the claim that it is the stiff tire sidewall that supports the load. As "proof" of their belief, many times they present a demonstration of pushing on a couple of unmounted tires. One is identified as a "cheap China bomb," while the other is usually a light truck tire that was made in the USA. In their "test" they show how the "China bomb" tire can be collapsed with minimal effort while the light truck tire does not collapse. This demonstration is supposed to be the "proof" that "China bomb" tires can't support the heavy load of a big RV, so you should run right out and buy some "12-ply" truck tires. Of course, it just so happens that the person giving the demonstration has some good heavy-duty tires on sale this week.

The problem with this "demonstration" is that it has little to do with the facts of tire "strength" or "load capacity".

Tire Load & Inflation tables

Since you are reading this post, I trust that you have used or at least heard about "Tire Load & Inflation" tables. Here is an example:

These tables are available for almost every size and type tire made. Consumer-type tires definitely are included in these tables. In the U.S., these tables are published for the entire tire industry. The data presented in the "U.S. Tire & Rim Association" yearbook covers all the tires sold at retail in the U.S. Many tire companies publish their own versions for their tires. The tire company may offer some additional information. But if you look at the data, you will see that the loads reported by a tire company for any given tire are identical to the loads in the industry publication and follow the format seen above.

You will note that for any given tire the load capacity changes with inflation. This is because a tire's load capacity is basically air volume X psi. (Note that is not the actual formula. The formula is quite involved and complex and includes exponential function and other adjustments.)

No tables for tire load capacity relating to sidewall construction

There are no tables for tire load capacity as a function of sidewall construction. The load rating (aka ply rating) is just a method of ranking the ability of any given tire to retain a given inflation. If you look at any Tire Load & Inflation table you may see more than one load range for a given size.

In the example above, we can see that a 255/70R22.5 can support 5,205 pounds when inflated to 110 psi. If that size tire is inflated to 120 psi, it can support 5,510 pounds. BUT to hold 110 psi, you only need Load Range G tire, but you need Load Range H if you want to inflate to a pressure above 110.

A tire company may make a given size in more than one load range. Therefore, when replacing tires it is critical that you order and purchase a tire of equal or greater load range than the specified original tires.



Clarity is needed on a question about "consumer grade" tires.

 Here's another question on tire inflation and "ply rating" on an RV forum, and an example of a "failure to communicate." This one is a bit complex as the person asking the question about tires has an F-150 and wants to be sure his tires are "good enough" for his plans.

Hi. I have a 2022 F-150 with 20-inch tires. The tires seem to be "consumer grade" 4-ply tires. I have a relatively light Real Lite SS-1600 that weighs in at about 1200 lbs. dry. I've read somewhere you need E-rated tires, which are 10-ply. I'm taking about a 2-month trip on this rig this spring. Do I really need to replace my almost-new tires?

I assumed that by "consumer grade 4-ply tires" he meant the standard "P"-type tire that would probably come standard on an F-150. Based on that assumption, my initial reply was:

Tires are rated to carry a certain load and the max load capacity is based on the tire inflation. You need to confirm your actual tire load by going to a Truck Scale. Many truck stops have scales. You can check Google for "Truck scale near me."

The "dry weight" of your RV is not important unless that is how you will travel, that is, with nothing in it. You need to confirm the weight on the trailer tires AND the load on the F-150 tires with the trailer fully loaded and hooked up as you intend to travel when "camping." Again, you need to get on a scale as you cannot estimate the weights by looking at the F150 or the trailer. Once you learn your actual weights you can consult tire Load & Inflation tables to learn the MAXIMUM load capacity and minimum inflation required for your set-up and your tires.

The rest of the story

Then the next day we learned "the rest of the story":

The "RV" he was talking about was a "slide-in" camper for the bed of his pickup. This confusion and the delay in getting an answer shows the importance of providing all the appropriate information about your RV and/or tow vehicle when asking a question the first time. Not everyone is familiar with the names of all the different models of RVs in the market. This results in a waste of time of the people reading the initial question if you do not include all the appropriate and needed information about the RV or your tow vehicle, or your tire type, size and load range.

Anyway, after learning that we were talking about a camper that slides into the bed of his pickup, I pointed out "if your truck load capacity is 1716# and the camper weight is 1200# when empty, that only leaves 516# for you, your passenger, clothes, food, water and camping supplies. Even increasing the tire pressure to their maximum will not increase the F-150 load capacity above the 516#. You will be very tight on what you can carry and even changing tires may not solve your problem. It is important that you not exceed the GAWR for the F-150 vehicle as increasing tire capacity will not increase the GAWR.

It's now been over a week since our camper asked his original question. Despite there being 43 posts, he still doesn't have an answer to his original question.