Modern tires are made from a range of materials. The rubber is mostly synthetic, with carbon black added to increase strength and toughness. When used in the tread, this combination gives a long life.
Natural rubber is weaker than the synthetic version. It’s used mainly in sidewalls.
The plies are made from cords of fabric, coated with rubber.
Early tires used cords of cotton, but with increased vehicle speeds, and loads, rayon and nylon cords are now common.
Cords of synthetic fabric have high tensile strength. They resist stretching, but are flexible under load.
The cords are placed in parallel and impregnated with rubber to form sheets called plies. Plies have high strength in one direction, and are flexible in other directions.
When cotton was used as a cord, the number of plies or layers in a tire was a measure of the tire’s strength.
Newer cord materials use fewer plies. A modern steel-belted radial with a 6-ply rating may have just 2 plies in its sidewall.
Having fewer plies makes the tire more flexible.
Higher numbers of plies make a tire’s response to bumps harsher.
The bead of the tire is made of a cord of high-tensile steel, coated with rubber. The end of the ply is wrapped around the bead, which is then wrapped in rubber to stop chafing of the plies, and also to seal the bead against the rim.
The length of the wire used for the bead determines the rim diameter of the tire. Belts that reinforce the tread area of the tire are mostly made of braided, high-tensile steel wire, but they can also be made of rayon, or polyester.
The inner liner of a tubeless tire is made of soft rubber. The inner liner must be flexible, and airtight.