By Roger Marble
I received this question about tire ply recently:
I’ve read many comments on various RV forums about truck tires vs. “motorhome” tires as it pertains to cost. Some also mention the firmness of the ride as one of the comparison differences. Upon reading several tires’ specifications, I’ve seen that some have 16-plies (for example: Michelin XA2 energy 295/80R22.5), whereas others have 18. This leaves me wondering if the number of plies can help predict ride firmness?
Does the advertised number of plies refer to the sidewall or to the portion of the tire that touches the road surface? (I’m guessing these can be different.)
Also, what other factors and/or specifications contribute to a tire’s ride firmness that may help us compare them?
When people talk about the “ply” they are usually referring to the sidewall, as there are usually two to four or more additional layers under the tread of radial tires.
I suggest you read the tire sidewall of your motorhome tires. I believe you will see that those tires say something like “Sidewall 1 Ply (or layer) of steel”. The use of “ply rating” in advertising is just perpetuating the confusion that started in the ’70s with “ply rating” such as “6 for 8” or even “8 for 12”, when better and stronger cords were introduced in bias truck tires.
With the switch to radial construction, you will find that most radials from car to light truck and even heavy truck have just one “ply” or “layer” in the sidewall. The letters for “Load Range” replaced the “ply rating” advertising because some people don’t understand the concept of “rating”. When you are looking at large radials with “Load Range” of F and higher, all that those letters are telling you are the inflation level the tires can tolerate.
If you look at the published Load and Inflation tables, you will see that some sizes are available in a number of different Load Ranges.
While these are LT tires, they show both Inflation, Load Range and Load Rating. (Click images to [slightly] enlarge.)
Here you can see that some sizes only come in one Load Range, while others come in many. You will also note that a Load Range C, D, or E are only rated for the same number of pounds if the Inflation is the same.
Comparing ride qualities of different Load Range tires
One tire I designed was a Load Range E version of a Load Range D tire. After running all the required tests, it was discovered that for this specific tire I only needed to change the number of strands of wire in the bead area (where the tire attaches to the wheel) to meet all the strength requirements. So you can see that it is impossible to make a broad statement on the tire construction to provide any useful information on the possible ride qualities of two different Load Range tires.
If you were to conduct a controlled ride test of 295/80R22.5 size tires with different Load Range but ran the same inflation level, you would not be able to feel a difference in ride. However, if you were to change the inflation level to achieve greater load capacity, you might feel a difference. But in that case, wouldn’t you expect a heavier loaded vehicle to have a different “ride”?
My original question in the title of this post is intenionally misleading as you will find that the load capacity of a “10-ply rated” tire and a Load Range E tire of the same size from the same drsign are really identical.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.