Mixed tyres not a safe combination

Press, 7 May 1981, Page 20

Mixed tyres not a safe combination

Radial ply tyres are fitted to most new cars as original equipment these days and. because they cost more to replace than the older style cross-ply tyres, a tyre company fears that some car owners might be tempted tc replace their worn out radials with cross-ply tyres. “Because of the inherent handling characteristics, a mix of cross-ply and radial tyres can endanger safety in a quick reaction situation," said the manager of quality assurance, Mr Ross Tillman. But he did concede that some car owners could be feeling the economic pinch and, perhaps, were not in a position to replace radials with radials. “The decision to replace a radial with a cross-ply tyre should not be made purely on the basis of the price difference, especially if it is-only a question of replacing one tyre,” he said. “Under no circumstances should a radial and a crossply be . mixed on the same • axle. That means that two cross-ply tyres must be bought to replace one radial, assuming the car is running on radial tyres. "If radial and cross-ply tyres have to be mixed, the 'radials’ should be kept to-

gether. In other words, radials to the rear and cross-ply to the front. That always applies, whether the car be driven from the front or rear,” said Mr Tillman. “When one looks at the money saving associated with replacement on a medium or long-term basis, it is doubtful if the price differential is of much significance. Radial ply tyres have less rolling resistance than cross-ply tyres and our own test procedures and consumer surveys show that a car on radials consumes between 4 and 6 per cent less fuel than the car shod with cross-ply tyres. “While, we have not attempted to quantify fuel consumption in a car running on a mix of radial and cross-ply tyres, it is logical to suspect that fuel consumption would be greater than if the car. was shod with radials.” "There are also the inevitable problems an owner will encounter if it is decided to mix radials and cross-ply tyres and there is a puncture. In that; event, ideally, the owner.would have two spare tyres because cross-ply and radial tyres should never be mixed on the same axle."

Mr Tillman-also said that ’ a radial tyre, had a much

longer life than a cross-ply, all things being equal. He said that diagonal rottion, standard practice with cross-ply tyres to obtain even wear and longer life, was unnecessary with radials, and that radials should be moved from the front tc back and back to front on the same side of the car, if they were moved at all. "Good driving and car maintenance habits will most likely ensure that radials will wear evenly. Rotation will be required only if periodic tyre checks indicate uneven’wear. If the wheels are properly aligned and the tyres are properly balanced, it is unlikely that there will be uneven wear.” On the subject of wheel balancing, Mr Tillman said that checking was impractical because it needed almost as much labour as balancing itself. < \ “As tyres wear down, balance <?an change. Wheels should be rebalanced when noticeable vibration occurs on smooth roads. If balancing does not eliminate the vibration, something in the car. itself is the cause and this should be pinpointed and remedied,", said Mr Tillma’n.