From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Letter to A.T. Speedometer Co., Ltd. regarding required modifications for speedometer calibration.
| Identifier | ExFiles\Box 165\2\ img330 | |
| Date | 17th June 1941 | |
| Elect: Rm{William Robotham - Chief Engineer}/RC.{R. Childs}13/SW. 17th June, 1941. A.T. Speedometer Co., Ltd., Chrones Works, North Circular Road, LONDON N.W.2. For the attention of Mr. H.R. Buckland. Dear Sirs, For some time, we have felt, that the existing method used for the calibration of our speedometers, does not completely cover our requirements. Recently, we received information from the tyre manufacturer, showing that the tyre radius increases as much as 4% at a road speed of 90 M.P.H. A curve computed from these figures is attached. Thus, the existing speedometers will be about 1 M.P.H. slow even if everything else conforms to specification, i.e., 3 M.P.H. fast at 90 M.P.H. Obviously, the scale required further modification, taking into account tyre losses. Also with the existing method of calibration, the hodometer only counts accurately at 30 M.P.H. We realise of course, that we must compromise, however, we consider that the average speed for the R.R. car is nearer 40 M.P.H. than 30 M.P.H. and that the gearing should be arranged accordingly. Thus we consider the following modifications necessary :- (1) Scale to be corrected so as to provide accurate speed indications at 30 M.P.H., and thereafter, a flatter of about 0.85 M.H.P. per each 10 M.P.H., increase in speed, giving a flatter of approximately 5 M.P.H., at a speed of 90 M.P.H. (Tyre losses to be taken into account). (2) The Hodometer to count accurately at a road speed of 40 M.P.H. (continued over) | ||