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).
Explaining the practical basis for speedometer scale contraction to compensate for the increase in tyre diameter at high speeds.
| Identifier | ExFiles\Box 59\2\ Scan122 | |
| Date | 23th March 1931 guessed | |
| -2a- (1) The practical basis for the law of speed scale contraction. As the speed of the road wheels, from which the speedometer drive is taken, increases, the effective diameter of the tyres also increases so that the revolutions per mile made by the wheels actually decreases and, as the information on this point stands at present, an estimate of the amount of this decrease would be about 3% from dead slow to 90 miles per hour. That means that the speed at which the speedometer is driven is progressively smaller in relation to the road speed of the car as the latter increases. By rearranging the speed scale in such a manner as to make a slight but progressive contraction of the scabe divisions as the scale reading increases, compensation can be made for the centrifugal effect on the tyres. A slight over-compensation is necessary to the extent to which we desire the speed indications of the instrument to be a little high (for the average case) at high speeds, and a specific proposal for the amount of the total contraction will be given in what follows. | ||