Tire Tech

Tire Tech

Ply Ratings vs. Ply Construction. We often talk about ply ratings during the sale of a tire. Light truck owners frequently demand a 10-ply tire on their vehicle, but this reference continues to be a confusing and often misunderstood concept, even among tire professionals.

What the heck is a ply? In the simplest terms, a ply is a layer. So, if we say that a tire has a 3-ply sidewall, we are saying that the carcass of the tire uses three layers of material, typically polyester. Because the carcass material is connected from bead-to-bead, the three layers also are part of the tread area as well.

Speaking of the tread area, if the tire has two steel belts plus a nylon cap ply, plus a 3-ply carcass, you would have six layers, or plies, in the tread area and three plies in the sidewall. This information can be found printed on the sidewall of the tire. If the construction contains belt edge covers, they are not counted, as they are not full plies.

As mentioned before, carcass material is typically polyester, however other materials are used, such as nylon and steel, depending on the application. Belt material in the tread is typically steel, while the steel belt overlays are typically nylon, aramid or Kevlar.

So, what the heck is a ply rating? Years ago, when pneumatic tires were first produced, manufacturers used cotton as the carcass material. However, they eventually discovered that cotton wasn’t very strong and required numerous plies (layers) to gain load capacity. The numerous layers added weight and also created friction. This caused tires to overheat and sometimes catch fire.

As a result, synthetic fibers such as polyester replaced the cotton plies in tires. Because polyester was lighter and stronger than cotton, it didn’t require as many plies to equal the strength of cotton. Simply by increasing the cord size, they were able to match the strength of multiple plies of cotton.

Therefore, when we say a tire has a “10-ply rating,” we are saying that the plies in the tire (two or three) provide the strength of 10 plies of cotton. In order to simplify the designation in the size, a letter was added to the size that corresponded to the ply rating. So, C denoted a 6-ply rating, D was an 8-ply and so forth.

Confused yet? To make matters worse, there’s a common perception that all 10-ply tires are equal.

Spoiler alert! They’re not. A 10-ply rated 35×12.5R22 does not carry the same amount of load as a 35×12.5R17. This is primarily due to the shape and size of the air chamber. The larger the air chamber, the more volume of air it will hold and thus more load it will carry.

So, how the heck do I know which ply rating is correct? The best way to ensure that you select the correct size and ply rating for a given application is to forget about the ply rating and start using the Load Index.

What the heck is a Load Index? As the amount of sizes grew in the 1990s, it became apparent the old Ply Rating/Load Rating system was becoming too confusing. Therefore, the Load Index system was introduced. This system used a number in the service description of the size to designate a load amount on the Load Index chart.

It’s a simple system, because you don’t need to know what load it corresponds to on the chart, you just need to make sure the replacement tire has an equal or higher Load Index number. For example, if the original equipment size has a 120 Load Index, then your replacement size would need to be 120 or higher.

It’s certainly possible for a customer who is seeking a large diameter tire (i.e., 37 inches or larger) to only need an 8-ply rated tire because the Load Index of that size may exceed the Load Index of the 10-ply rated original equipment tire.

It’s certainly possible for a customer who is seeking a large diameter tire (i.e., 37 inches or larger) to only need an 8-ply rated tire because the Load Index of that size may exceed the Load Index of the 10-ply rated original equipment tire.