The most common ply fabric is polyester cord . The cords in a radial tire run perpendicular to the tread. Some older tires used diagonal bias tires , tires in which the fabric ran at an angle to the tread. The plies are coated with rubber to help them bond with the other components and to seal in the air. A tire's strength is often described by the number of plies it has.
Most car tires have two body plies.
By comparison, large commercial jetliners often have tires with 30 or more plies. The Belts In steel-belted radial tires, belts made from steel are used to reinforce the area under the tread. These belts provide puncture resistance and help the tire stay flat so that it makes the best contact with the road. Some tires have cap plies , an extra layer or two of polyester fabric to help hold everything in place. The sidewall provides lateral stability for the tire, protects the body plies and helps keep the air from escaping.