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).
Progressive scale contraction on A.T. speedometers for the Phantom II chassis.
| Identifier | ExFiles\Box 59\2\ Scan116 | |
| Date | 24th March 1931 | |
| X6051 Ry. from EFC. c. FN. C. c. Wor.{Arthur Wormald - General Works Manager} Hs.{Lord Ernest Hives - Chair} EFC1/AD24.3.31. X.6051 A.T. SPEEDOMETERS. Further to our EFC1/AML8.3.31 and as referred to in the last paragraph of that note, we now attach for your information a copy of some sheets dealing with the argument for and method of speed scale contraction for such instruments. As you will see from perusal of these sheets the argument for such progressive scale contraction in place of the proportional scale so far used is very cogent. As is shown by the mathematical working, it is not merely a case of adopting a proportional contraction of the scale by which the divisions are still of equal width, but a case of what is suitably called a parabolic contraction. The whole matter has been fully gone into in the sheets referred to in so far as the general case is concerned. It is our intention in this memo to quote the figures which specifically refer to the case of the Phantom II and subsequently to provide a specification of the instrument that is required for that chassis. On immediately up-to-date information, the Phantom II wheel revolutions per mile at slow speeds with normal inflation are 616, this figure being for the present standard tyres namely 7" x 20" Dunlop Silent Crossed Tread Fort Reinforced tyres. If larger tyres are fitted this figure may be reduced to 600 or even slightly below. It being impossible to cater exactly for different sizes of tyres, it must be sufficient for our purpose to consider only our standard tyres so that the figure 616 must be considered to hold the field i.e. for normal inflation and slow speeds. | ||