PLY RATING

How is the load capacity of a tire determined?

Let’s begin by reminding ourselves that it is not the tire that carries the load, but the air inside it. The tire is just the container for the air. So, if you need to carry more load, you need more air. You might accomplish that with a larger air chamber or by forcing more air molecules into the same size air chamber you’d normally use for a tire with a lower load capacity. In other words, you might use a bigger tire or a tire running at a higher inflation pressure. Either way, the tire needs to be strong enough to handle the higher capacity. The traditional ways of defining this are “Ply Rating,” “Load Range,” and “Load Index.”

What is meant by “Ply Rating?”

In the early days of bias tires, casing strength was built up by adding layer upon layer of cotton fabric. The layers were placed with the thread in each layer at an angle to each other. That added strength, because the tensions would be distributed throughout the layers of fabric. The Ply Rating used to refer to the number of layers of cotton.

Ply Rating	Load Range
2	A
4	B
6	C
8	D
10	E
12	F
14	G
16	H
18	J
20	L

Why aren’t there any odd numbers?

Since you have to have at least two plies to have a “bias” arrangement, bias ply tires always had an even number of plies.

Is cotton still used?

Cotton went away a long time ago. One of the major improvements was making plies out of nylon. Nylon is so much stronger that if you’re old enough, you may remember seeing your tires stamped with the words, “2 Ply/4 Ply Rating.” That meant there were only two nylon plies, but they were so strong the tire was equivalent to one made of four cotton plies.

And that’s about when things started to get complicated.

How so?

Ply materials continued to improve, especially with the introduction of steel ply materials and radial construction, making the old Ply Number less and less meaningful. And that resulted in the newer designation we use today, called “Load Range.” 

On the chart, you can see how today’s Load Ranges correspond to the older Ply Rating system.

And why isn’t there an “I” or a “K” Load Range?

Just to avoid confusion. An “I” might, depending on the typeface, look like the numeral “1″ and “K” is a very common abbreviation for “kilo” meaning “thousand.”

What does Load Range really mean?

Load Range indicates the maximum load recommended for the tire. This varies by tire size and inflation pressure: A bigger tire can hold more air and can be rated for a higher load. Also, a given tire size at a higher air pressure results in a higher-rated load.

Load Capacities for Some “G” Load Range Single Tires

Size	Maximum Single Load (lbs.)	Inflation Pressure (psi)
295/75R22.5	6175	110
285/75R24.5	6175	110
11R22.5	6175	105
11R24.5	6610	105
12R22.5	6610	105
12R24.5	7160	105

And here are the same tire sizes, but in a higher load rating:

Load Capacities for Some “H” Load Range Single Tires

Size	Maximum Single Load (lbs.)	Inflation Pressure (psi)
295/75R22.5	6610	120
285/75R24.5	6780	120
11R22.5	6610	120
11R24.5	7160	120
12R22.5	7390	120
12R24.5	7830	120

So what’s the difference between tires of the same size but different load ranges?

It’s no longer the number of plies. Most radial truck tires, for example, have a total of five plies. There’s one steel body ply and four belts under the tread. What is different today is the strength of the steel cables in those plies or the number of cables per inch. We’re now at the point where we no longer add more and more plies, but instead, adjust the strength of the entire casing to achieve the desired load capacity.

Will a tire with a higher Load Range last longer?

It might, but it also might not. What usually determines tire life is the rate at which the tread wears and whether or not the tread wears evenly. Or how many retreads you can get from its casing. As long as the tire has enough load capacity for the maximum load you will be putting on it, buying extra load capacity may not increase your tire life.

Source: http://www.sttc.com, www.finixx.com