A comparative test of various makes of tyre is something one rarely sees being carried out by one of the tyre manufacturers themselves, at least not publicly, a reservation which might be attributed either to a shyness of competitor reaction or to an incomplete faith in their own products. When we were invited by Firestone to participate in such a test of radial-ply tyres we therefore accepted at once and, on a rainy day in mid-December, I was one of two motoring journalists who drove the test car, a standard Ford Cortina 1500 saloon, on smooth, wet tarmac at the Fighting Vehicles Research and Development Establishment at Chobham. The same car was used throughout the test, which consisted solely of straight-line stopping from an indicated 40 m.p.h., the wheels and tyres being changed after each succession of runs. We both made between two and four runs on each set of tyres, the mean being taken for the final figure. Cornering abilities were not compared. [Why not ? There is a skid-pan, steering pad and zig-zag track at Chobham ?—Ed.]
The same test was carried out privately by Firestone in October, when they achieved slightly shorter braking distances, but as we were using the same batch of tyres our longer distances can be attributed to the fraction of tread depth which was scrubbed off on that occasion. All tyres we used, however, appeared to be equally worn.
The timing was effective but would not have produced an absolutely accurate figure if used for a single set of tyres. A cartridge, mounted outside the car close to the ground, was activated by an extremely itensitive switch fitted to the brake pedal itself. This was fired by the slightest pressure, fractionally before braking actually began, but as the test was purely comparative the device was acceptable since the tolerance was the same for all the tyres used. In the tables below we present both the figures obtained by Firestone themselves and those achieved in our independent test. We do this purely as a matter of interest and we make no comment on their be-all-and-end-all accuracy.
We did note, incidentally, that the Firestone P.R. man had Avons on his Bristol 404.—G. P.