the development of a machine (scrim) for the routine measurement of resistance to skidding from the earlier research machines has inevitably led to design changes that have been made in the interests of greater reliability, greater precision of measurement and to overcome problems associated with commercial production. this report describes a study of the effect that these changes have had on test results. this study showed that sfc levels have increased by between 19 and 28 per cent and that three classes of factors have contributed approximately equally to the increase, namely: (a) change from test car to scrim, (b) change of test tyre and (c) improvements in test equipment. it was also found that it would not be practicable to reduce sfc levels by more than a proportion of this increase. alternative courses of action are available:- (1) revise published findings based on earlier results to bring them into line with present-day figures. (2) adjust present-day results to bring them into line with the published figures. the second course has the advantage that any further changes in scrim could be easily accommodated and published sfc levels will remain valid.(a) for parts i, ii and iii, see irrd nos. 224039, 224038 and 224037 respectively.

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