Car Parts and Accessories

Radial-ply tires

Radial-ply tires are fitted to all new cars. A large proportion of the tires replaced on older cars are also radial-ply.

The construction of a radial-ply tire reduces cornering wear and considerably increases the overall life of the tire; but this may be at the expense of slight harshness at lower speeds.

Radial-ply tires are made in two parts; the layers, or plies, and a belt of cords.

The cords in the plies run from bead to bead across the crown at right angles, not diagonally as in the cross-ply tire. This gives great pliability and comfort, but little or no directional stability. Stability comes from a belt of cords running around the circumference of the tire beneath the tread. Belt cords are usually spun from rayon or fine steel wire and are flexible, but do not lose their tautness. So the belt firmly restricts any lateral stretching of the tire’s tread.

A recent development in the manufacture of radial-ply tires is the use of glass fibre for breakers. Glass fibre is very strong and surprisingly elastic; but special techniques have to be used to prevent the chafing of the filaments, and to achieve a satisfactory bond between them and the rubber casing. To help it keep its stiffness against sideways forces, a belt is made of a least two layers of cord which are slightly diagonal to the circumference. The angle between the cord varies from 18 to 22 degrees. The number of layers depends on the material used, the lateral stiffness needed and the load the tire is required to carry.

If steel cords are used in a belt, there are usually only two; but as many as four, five or six cords may be used if they are made of rayon or other fibre. Where four are used, they are folded over and sometimes interleaved, to increase their lateral stability. Steel makes a stronger cord and does not stretch under inflation pressure.

Six layers may be produced by interleaving two or three plies, suitably folded. Nylon, first used for breakers in aircraft tires, is stronger, more elastic and more flexible than rayon and keeps the tire cooler. It is used in car tires for high speeds or heavy loads, but only in combination with other materials.

When cold, nylon cords tend to lose their flexibility. If nylon cords alone were used, the section of the tire resting on the road would change its shape and give a bumpy ride until the casing had warmed up enough to recover its flexibility.

Cross-ply tires

Cross-plytires have a casing of two or more layers of fabric. They are rarely fitted to modern cars, but are sometimes still found on commercial vehicles. Originally, the threads, or cords, were at right angles to the direction of rotation.

This design gave a comfortable ride but had side-effects on the steering. It was found that placing the cords parallel to the direction of rotation greatly improved the directional stability; but it also reduced the comfort.

As the tires have to provide a comfortable ride, help with the steering and cope with the different forces exerted in acceleration, braking and cornering, a compromise had to be found.

The plies were accordingly superimposed at an angle so that their cords ran across each other diagonally in a lattice pattern. This allows the layers to retain their strength while being stretched in different directions during rotation.

For many years, cords were placed at an angle of 45 degrees; but this was later reduced to 40 degrees, and even less in some high-speed tires where accurate and easy steering response is of more importance than comfort.

Some cross-ply tires are now being made with what is known as a depressed- crown contour moulding.

When new and uninflated, they have a concave tread. This becomes flat when inflated, to give a uniform pressure over the whole contact area.

In addition to a tire’s inner lining, various strips and fillers are incorporated to strengthen the casing.

A tire’s strength and load-carrying ability were at one time indicated by its number of plies: a four-ply tire indicated that the casing was built up from four layers of inner lining material and could carry a specific load with safety.

Modern materials and man-made fibres, such as polyester and glass fibre, embedded in the rubber, are much stronger than the cotton fibres originally used; and the ply rating can no longer be related to the number of plies.

Ply-ratings are still used, but only to indicate the strength and load-carrying capacity: a four-ply rating may well be given to a tire which has only two plies in the casing.

Two steel-wire hoops, or beads, which prevent the tire stretching and parting from the wheel, are common to all tires. But most tires vary considerably in casing and tread construction.

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