Why are tire ply rating and load range different?

Here is a message from a reader on his confusion on a tire’s “load range,” “load capacity,” and “ply rating”:

“Well, I finally figured out the problem I was having in the process. For some reason I was not thinking of the ply. 10/14/16 ply. Not sure why it wasn’t registering but [I have it] sorted now. So now I will have to look at my tire options in order to meet the load rating.”

Here is my reply.

Tire ply rating

“Ply” started to go away in the late 1960s, when stronger nylon went into bias ply tires and we were making “6 for 8 ply” tires. (I recall that as my first assignment on 8.25×20 truck tire.) With the “Truth in labeling” movement and the switch to radial construction in the ’70s, “ply rating” went away officially to be replaced with “load range,” which uses a letter rating such as C, D, E, etc.

However, many people in the RV community insist on using the term “ply” without including the word “rating,” so sometimes they can confuse themselves.

Almost all radial tires have a single (one) body ply. Tire engineers use increased strength cord for the body “ply” as there are some industry standard tests used to measure the “strength” of a tire. The DOT testing has a series of tests with ever-increasing levels of “force” required to break or rupture the body of a tire.

If you read the material list molded on the sidewall of all tires, you will see that most “P”, “LT” and “ST”-type tires have just one layer or ply of polyester in the sidewall. For load range C, D, and E tires, you will also see two layers of steel.

There are some tires that also have one or two layers of nylon on top of the steel belts, which can result in higher speed capability. In ST-type tires, these steel “cap ply” can help reduce the effects of interply shear forces.

There are some tires that use materials other than polyester for the body cord, but that isn’t something that should be used in the RV world.

Larger tires

In general, moving to larger-size tires with 19.5″ and 22.5″ size wheels and load ranges such as F, G, H, etc., you will see the sidewalls will be one “ply” or “layer” of steel, and the belts will be three, four, or even five layers of steel. There may be some of these larger tires with nylon cap ply, but I do not remember ever seeing one.

No matter the number of layers of the materials used, all tires of a given load range must pass the same “strength” level with a specific number of (inch-pounds) for each load range.

Load capacity

You also need to realize that the “strength” test is not directly related to a tire’s load capacity, as “load capacity” of a tire is based on the air volume and the inflation level.

Yes, this can be confusing. I suggest you focus on the load range and the load capacity found in the “load/inflation” tables to learn the minimum level of inflation you need to support the load you are placing on the tires on your truck or RV.

That phrase “xxx Pounds Max Load at yyy PSI” molded on the sidewall of tires is just telling you that you cannot safely exceed the “xxx” pounds load even if you exceed the “yyy” level of inflation. Every tire has a specific maximum load capacity which should never be exceeded.

As you can see, none of this is simple. That is why there are “Tire Design Engineers” who spend their careers learning, developing, designing and testing tires to meet the needs of our customers and the goals of the tire companies.

Roger Marble

*****

If you have tire questions, check out Roger’s Blog as well as his posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.

If you still have a question for Roger after searching the above posts, send your inquiries to him using the form below.

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