Views of British Expert On Acceleration And Braking

Poverty Bay Herald, Volume LXII, Issue 18632, 16 February 1935, Page 9

Views of British Expert On Acceleration And Braking

TRACTIVE adhesion, called popu- *" Jarly the “road-grip ot' the tyres/' is a subject which has received much prominence is recent years lor the very obvious reason that as engines have become far more powerful anti speed and acceleration are much gi enter, wheel-slip and braking distance have assumed a new importance. There seems, however, to be no little divergence between theoretical calculations and the results actually recorded in tests, and that questions would scorn 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 tut average speed of over 117 in.p.li. This, he points out, is an astonishing performance and raises tinquestion, “what is the maximum degree of acceleration possible?” -H*' adds: “Assuming that the co-eliicient of friction between the tyres and road is its maximum value, that is, unity, and'neglecting wind resistance and tin rotational acceleration which must be imparted to the wheels, then it appears that the best possible time for tin mile is approximately 18.1 seconds, equivalent to a speed of 198.9 m.p.h. “In brake tests, however, a stopping distance of 30ft. from 30 m.p.h. is tintheoretical best possible performance, assuming a, co-cilicicat of friction Oi l, but. this has frequently been surpassed in practice, and ligures as low as 21ft. have been recorded. Such ligures have, of course, been challenged . . • but. the explanation is that,'with rubber tyres on certain road_ surfaces there is a ‘keying’ cfleet, almost as if the patterned rubbed tyret were large pinions and the road' surface a rack with which they meshed.’ “There seems no reason why this should not obtain in acceleration also, so that the theoretical maximum acceleration figure may also be exceeded to the same extent, to give speed ioi the standing-start mile of about 251 m. . “This is, of course, neglecting wind resistance, which in actual tact could only bo neglected for the first lev feet. It would therefore be extremely interesting to have figures for the “first, few feet of the Mercedes’ run. so that they could lie compared' with brake tesl ligures.”