What does the 'r' in tires mean?

[Peyton]

The 'r' in tires stands for radial tires, where 'r' is the abbreviation of the English word 'radial'. Car tires can be categorized by structure into radial tires and bias tires, with bias tires denoted by the letter 'b'. Radial tires can be further divided into three types based on the materials used: all-steel radial tires, semi-steel radial tires, and all-fiber radial tires. All-steel radial tires use steel cords for both the carcass and the belt layers, and are generally used in heavy-duty and construction machinery vehicles. Semi-steel radial tires use rayon or other fibers for the carcass and steel cords for the belt layers; this type of radial tire is commonly used in passenger cars or light trucks, such as buses. All-fiber radial tires use rayon or other fiber cords for both the carcass and the belt layers, with the belt layer cords requiring low-elongation fibers; this type of radial tire is typically used in low-speed passenger cars or tractors.