FACTS ABOUT TYRE GRIP

Press, Volume CVI, Issue 31118, 22 July 1966, Page 9

FACTS ABOUT TYRE GRIP

'JESTS have shown that even a good tyre can lose up to half its adhesion on a smooth, wet surface at 60 m.p.h, says an article in the latest issue of “Rubber Developments,” the journal of the Natural Rubber Producers’ Research Association. A car driven with smooth tyres on a similar surface can be made to aquaplane at less than 50 m.p.h., and may need more than three and a half times the distance to stop, compared with a good tyre. Research has also shown that the common belief that grip in snow can be improved by reducing normal tyre pressures is not necessarily correct. Engineers suggest that if any alteration is made, pressures should be increased 51b to 101 b above normal.

If pressures are reduced a 25 per cent reduction is

needed to give as little as 10 per cent improvement in tyre contact area. But this reduces the power of the tyre to cut through the layer of snow, so the over-all effect on grip is very small, and the distortion of the soft tyre can cause a skid. Increasing the pressure makes a tyre more rigid and reduces skid-producing distortion, but even then the advantages are small, and engineers advise most motorists are best to leave tyre pressures alone and concentrate on avoiding harsh acceleration and sudden braking. With nearly every commodity on sale in Britain costing at least four times as much as it did in 1922, it is interesting that in the same period the prices of tyres have dropped by four-fifths, while giving about 10 times the mileage, the article says.

Forty-three years ago a 815 by 105 tyre cost £5 3s 6d. Today an equivalent tyre in Britain costs £6 Bs, a considerable price fall when the change in monetary values is taken into account.

A feature of the present tyre market is the greatly increased demand for radial-ply tyres, which give better grip, generally longer life, and safer high-speed rtmning than normal cross-ply tyres. However car owners should remember that radial tyres

must be fitted either to all four wheels or to the rear wheels only—irrespective of the type of car. Fitting such tyres to the front wheels only produces a dangerous “oversteer" effect, the article says. Whatever type of tyre they use drivers should always bear in mind that their car’s performance, from a safety viewpoint, is only as good as its tyres .... and the tyres are in contact with the road on four small areas, each a little bigger than the size of an average man’s shoe-sole.