Radial tire

A radial tire (more properly, a radial-ply tire) is a particular design of automotive tire (in British English, tyre). The design was originally developed by Michelin but, because of its advantages, has now become the standard design for essentially all automotive tires.

Tires are not fabricated just from rubber; they would be far too flexible and weak. Within the rubber are a series of plies of cord that acts as reinforcement. In the past, the alternating plies were laid within the tire at angles of +45 degrees and -45 degrees from the direction of travel so that they criss-crossed each other at 90 degrees. Because the plies were laid "on the bias", such tires were referred to as "bias ply tires".

By comparison, radial tires lay all of the cord plies at 90 degrees to the direction of travel (that is, across the tire from lip to lip). This design avoids having the plies rub against each other as the tire flexes, reducing the rolling friction of the tire. This allows vehicles with radial tires to achieve better fuel economy than vehicles with bias-ply tires. It also accounts for the slightly "low on air" (bulging) look that radial tire sidewalls have, especially when compared to bias-ply tires.

As described, a radial tire would not be sufficiently strong and the surface in contact with the ground would not be sufficiently rigid. To add further strength, the entire tire is surrounded by additional belts that are oriented along the direction of travel. First made of tire cord, these belts were later made of steel (hence the term "steel-belted radial") and subsequently aramid fibers such as Kevlar. This outer belt also helps the tire resist punctures from road debris.