ACCELERATION AND BRAKING

Evening Star, Issue 21993, 1 April 1935, Page 15

ACCELERATION AND BRAKING

VIEWS OF BRITISH EXPERT Tractive adhesion, called popularly the “ road-grip of the tyres,” is a subject which lias received much prominence in recent years for_ the very obvious reason that as engines have become far more powerful and speed and acceleration arc much greater, wheelslip and braking distance have assumed a now imporlancc. There seems, however, to he no little divergence between

theoretical calculations and the results actually recorded in tests, and that question would seem. also to be associated with the condition of the tyre treads.

Writing in the ‘ Autocar,’ a scientist, Mr A. Douglas Clease, refers to an amazing exhibition of acceleration registered recently by a Mercedes model, which from a standing start covered a mile at an average of over 117 miles per hour. This, lie points out ; is an astonishing performance, and raises the question, “what is the maximum degree of acceleration possible?” Hp adds; “ Assuming that the co-efficierit of friction between the tyres and road is its maximum value, that is, unity, and neglecting wind resistance and the rotational acceleration which must be imparted to the wheels, then it appears that the best possible time for the mile is approximately 18.1 seconds, equivalent to $ speed of 198.9 miles per hour. “ In brake tests, however, a stopping distance of 30ft from thirty miles per hour is the theoretical best possible performance, assuming a co-efficient of fricti6n of J, but this has frequently been .surpassed in practice, and figures as low as 21ft have been recorded. Such figures have, of course, been challenged ... but the explanation is that with rubber tyres on certain road surfaces there is a ‘ keying ’ effect, almost as if the patterned rubber tyres were large pinions and the road surface a rack with which they meshed.” “There seepis no reason why this should not obtain in acceleration also, so that the theoretical maximum acceleration figure, may also he exceeded to the same extent, to give speed for the standing-start mile of about 250 miles per hour. “ This is, ef course, neglecting wind resistance, which in actual fact could only be neglected for the first few feet. It would therefore be extremely interesting to have figures for the first few feet of the Mercedes’s run, so that they could be compared with brake test figures.”