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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.



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