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Ply rating is a conventional term used to describe the strength of a tyres casing. Tyre casings all come in different strengths; with an example 6 ply rating.
The term comes from the time when tyre fabric was made of cotton; this was because this was the only material available at the time that would stick to rubber once it had been vulcanized. In order to contain air pressure within the tyre, two or more layers of this "rubberized" fabric were used on the bias, in a cross like fashion. Eventually once the tyre had worn out, this was described as being worn to the canvas. The last tyres made of cotton were tractor tyres produced in Australia in 1952.
In order to cope with vehicle loads and increased speeds, more and more layers were added to allow the tyre to be used at a higher pressure. However eventually due to the increasing number of layers the tyres became so bulky that they would overheat and explode.
As technology developed and stronger synthetic fibres such as nylon and polyester were introduced, less fabric had to be used for each tyre to achieve the same casing strengths. An 8 ply rating tyre may only have had 4 layers of fabric (piles) but would have the same strength as 8 piles of cotton. Further developing this concept, radial ply truck tyres only have one layer of fabric, which is made up of strands of steel.
The introduction of synthetic fabric in tyre construction is dependant on the development of suitable adhesives. These adhesives must be able to bond the tyres smooth surface to the rubber, as unlike cotton, none of the manmade fibres have the short staples sticking out of the tyre cord surface allowing the rubber to bond.
As technology has progressed, many tyre manufacturers abandoned the term ply rating as they felt it was misleading for the consumer. The term "service index" is now more popularly used, which utilizes a coded system of speed and load to determine the load at tyre can bear at a maximum specified pressure and speed. This service index appears on the tyre sidewall after the size branding.