What is meant by ply rating of tyres?

Automotive tyre consists of a number of rubberized fabric layers and the layer is called ply. A collection of such plys is called carcass or casing. It is this casing which is responsible for the tyre’s performance including its load bearing capacity.

Originally cotton tyre fabric was used. Irrespective of the nature of the fabric nylon/rayon/polyester), the strength of the carcass is equated to the number of ply. This convention is still followed in cross-ply tyres.

 For example, the 8 ply-rating of nylon fabric reinforcement implies that the strength of the carcass is at least equal to 8 cotton plies. But the actual number of nylon fabric ply could be 4 or 6 (because nylon is much stronger that of cotton). But this numbering system is beset with two misconceptions: One is that the tyre consists of that many of plys and the other one is that the strength of the carcass is limited to the number of plys. 

To overcome this, nowadays, the ply rating is also indicated in alphabetical letters. 

Ply rating of tyres is the number of layers of soft rubber-clad fabric below the tread (rubber layer) of a tyre. The fabric nylon or rayon is subjected to tension before calendaring. The tensioned fabric is then rolled between steel rolls with rubber on top and bottom of the fabric. The calendar is then cut at 45 degrees according to the width of the tyre to be manufactured. The cut calendar is placed on the drum with rims on both sides of the drum to make a skeleton of the tyre. The tread (rubber) is placed on the skeleton and pressure is applied to stick the tread-to the calendar. Later on it is cured and moulded to actual shape.