RADIAL-PLY TYRES HAVE MANY ADVANTAGES

Press, Volume CVIII, Issue 31750, 6 August 1968, Page 11

RADIAL-PLY TYRES HAVE MANY ADVANTAGES

Although radial-ply tyres have been available for several years, many motorists are only now learning of the important benefits they can bring. Compared to ordinary cross-ply tyres, the radial-ply tyres give both better road-holding and longer life.

Radial-ply tyres differ from conventional tyres in having very soft and flexible sidewalls, and also a firmly-braced, very rigid tread. Because of this latter feature, they are sometimes called “braced-tread” or “rigidbreaker” tyres.

Because of these features, i the area of tread in contact ■ with the road is not squeezed up or distorted during comer- i ing or braking, as happens : with conventional tyres, and 1 the individual pattern blocks i

do not shuffle or scuff against the road surface. Thus, the braced tread rolls over the ground with less resistance and less abrasive wear. As the tread bracing is incompressible as well as in-

extensible, the tyre circumference remains constant and any deformation is spread right around the tyre instead of being concentrated at the area of contact between the tyre and the road. LESS FUEL This near-constancy of the tread pattern in all circumstances is reflected in the handling qualities of any car which is fitted with radial ply tyres. More tread stays in contact with the road, and because the rolling resistance of the tyres is low, less power is needed to drive the car and less fuel is used—savings of up to 5 per cent have been recorded in supervised tests. In wet weather, the tread of radial ply tyres tends to clear water from the surface of the road better, because the tread pattern does not “close up” under stress. Further, a larger area of the tread is held in contact with the road. Similarly, because more of the tread stays on the road surface, radial-ply tyres have appreciably better braking performance than ordinarytyres. MORE MILES Although radial ply tyres are a little more expensive than ordinary tyres, they can give up to 70 per cent more mileage—and they also give safer cornering and braking. The more severe the conditions that cause low tread life with cross-ply tyres, then the longer radial-ply tyres will last in comparison. Quiet driving will produce a high mileage from cross-ply tyres because of the lack of distortion and abrasion of the tread. Radial-ply tyres give extra tread life because of the lack of movement and distortion of the tread pattern, and consequently will not give big increases in

The casings of ordinary tyres are made from successive layers of plies of textile cords, arranged so that the cords of each layer cross each other at an angle of 74 degrees, but in the radial-ply tyres the casing cords radiate out from the centre like the spokes of a wheel, and do not cross each other.

This gives the tyre its soft and flexible sidewall, and in addition a layer of special textile fabric—or in the case of one tyre, a layer of steel wire—runs around the circumference of the tyre under the tread. This layer, known as the rigid breaker, braces the tread pattern and prevents the segments of the tread pattern from moving in relation to each other. FITTING Because radial ply tyres give so much more grip than ordinary tyres, they must never be fitted only to the front wheels of a car. even if it is a front-wheel-drive model. If they are fitted only to the front wheels, the back wheels will lose their grip on the road at a considerably lower cornering force than that needed to make the front wheels lose their grip. This can result in dangerous oversteer, with the back wheels suddenly sliding wide on a corner. Manufacturers say one radial-ply tyre should never be mixed with three crossply tyres: and that they should never be fitted only to the rear wheels if the car is a front-wheel-drive model. Radial-ply tyres can be fitted only to the rear wheels if the car is a rear-wheel-drive model, but in any other case they should be fitted to all four wheels or not at all. “The car of today is characterised by a high power-to-weight ratio and the ability to corner fast in comfort and safety,” says the sales director of Dunlop

New Zealand, Ltd (Mr D. Smith). However, he says, the performance of which modern cars are capable can have the effect of reducing the life of conventional tyres. “The radial-ply tyre is an answer to this problem. It has been designed to allow motorists to enjoy the capabilities of modern cars to the full, with a' greatly increased margin of safety and without paying the penalty of rapid tread wear. “The prime cause of tread wear is the shuffle of pattern segments against the road when cornering. The bracing under the radial-ply tread pattern reduces this considerably and provides a pattern life of up to 70 per cent greater than conventional tyres. “Although radials cost a little more, the greatly increased life provides the motorist with a substantial reduction in tyre cost per mile.

“Driving a car fitted with radial-plies can be likened to a feeling of running on rails, so precise is the steering control. There is a high resistance to wander on any kind of surface."

mileage for the very quiet driver.