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 in 1946 [1] 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 act as reinforcement. All common tires (since at least the 1960s) are made of layers of rubber and cords of polyester, steel, and/or other textile materials. This network of cords that gives the tire strength and shape is called the carcass.

In the past, the fabric was built up on a flat steel drum, with the cords at an angle of about +60 and -60 degrees from the direction of travel, so they criss-crossed over each other. They were called cross ply or bias ply tires. The plies were turned up around the steel wire beads and the combined tread/sidewall applied. The green (uncured) tire was loaded over a curing bladder and shaped into the mould. This shaping process caused the cords in the tire to assume an S shape from bead to bead. The angle under the tread stretched down to about 36 degrees. This was called the Crown Angle. In the sidewall region the angle was 45 degrees and in the bead it remained at 60 degrees. The low crown angle gave rigidity to support the tread and the high sidewall angle gave comfort.

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.


Construction

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 made of steel (hence the term "steel-belted radial") by 1948, and subsequently aramid fibers such as Kevlar.

In this way, radial tires separate the tire carcass into two separate systems:

* The radial cords in the sidewall allow it to act like a spring, giving flexibilty and ride comfort.

* The rigid steel belts reinforce the tread region, giving high mileage and performance.

Each system can then be individually optimized for best performance.