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).
Speedometer calibration calculations due to changes in tyre dimensions for Wraith and New Series Bentley cars.
| Identifier | ExFiles\Box 165\2\ img147 | |
| Date | 11th December 1937 | |
| To By.{R.W. Bailey - Chief Engineer} c. to Da{Bernard Day - Chassis Design}/Hdy.{William Hardy} c. to By/RD. c. to RM{William Robotham - Chief Engineer}/PD. 6065 Rm{William Robotham - Chief Engineer}/RC.{R. Childs}6/JH.11.12.37. re Speedometer Calculations. With the recent changes in tyre dimensions etc. and particularly in connection with the new series of Bentley cars, there has arisen discrepancies in the calibration figures of speedometers which appear to be due to the method of calculation. Similar disagreement arose some months ago in connection with the Wraith. We then discussed the matter fully with Hdy.{William Hardy} as a result of which, a method of calculation was arrived at and a statement issued accordingly. To avoid any misunderstanding, we give below the calibration values for cars which are of particular interest at the moment. These values of revolutions per mile, as determined by the constants of the car, are arrived at in the agreed manner. Both from the point of view of the speed indication at 30 m.p.h. which is so much used on the road, and the Hodometer portion of the instrument, it is desirable that the instrument makers should work as close as possible to the calibration value for the particular instrument. Any error that is desired can be introduced at other points by suitably distorting the scale. Wraith. Calibration revs. per mile of car 786. New Series Bentley. (Tyre 17" x 650" 10/41 axle). Calibration revs. per mile 772. Rm{William Robotham - Chief Engineer}/RC.{R. Childs} | ||