The tire ply cannot be seen just by looking at the outside of the tire. Tire plies are the internal layers that make up most of the inner workings.
These reinforcement layers are covered with rubber compounds and made with steel cables, nylon, or other sturdy materials that help a tire maintain shape and structure under pressure from inflation and load.
Tire ply technology was developed by the Michelin brothers in the form of bias tires. Constructing tires with tire plies immediately became the standard in tire design.
Radial Tire Ply
Tire plies that run horizontally to the tire bead are known as radial tires. The rolling resistance of tires with radial plies is lower, allowing for a more fuel-efficient ride. The roll of these tires is smooth, and a radial tire ply is most often incorporated into high-performance tires and all-season tires. Radial tire ply technology results in tires with additional stability and longer tread life.
Bias Tire Ply
A tire ply or a set of tire plies that run diagonally within a tire's construction can be found in tires known as bias tires. The plies in these tires are sturdy and are known for offering a very smooth ride on rough surfaces. On the other hand, the drawback of bias tire ply design is their poor rolling resistance. That's why these tires are common on entry-level offroad tires.