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).
Opinions and specifications for A.T. speedometers on Phantom II and 20 HP chassis.
| Identifier | ExFiles\Box 50\4\ Scan338 | |
| Date | 12th December 1929 | |
| To By.{R.W. Bailey - Chief Engineer} from EFC. " Hs.{Lord Ernest Hives - Chair} " Wor.{Arthur Wormald - General Works Manager} X4308. EFC/AD6.12.29. X.4308. A.T. SPEEDOMETERS. With regard to these instruments generally for both Phantom II and 20 HP. chassis it is our opinion (a) That the correspondence of the drive revolutions per counter mile of the instrument, which depend on its internal gearing, with the revolutions per car mile of the drive, should be arranged as nearly as possible, for the case of the largest tyres that it is possible to use, but at the same time in such a way that the possible figure reading of the former is less, but less by the least possible amount, than the figure for the latter. For example in the Phantom II case the minimum number of revolutions per mile of the speedometer drive on the car with the largest tyres understood to be possible, is 801 (for average tyres it is 819) with the present standard speedometer drive gearing. Correspondingly instruments of 800 revolutions per counter mile (which number appears to be a possible figure for the internal gearing) should be utilised. (b) That the makers should be asked definitely to set the correspondence of the miles per hour indications with the counter mile readings to be correct in the instruments themselves when these are at a temperature lying between 27°C. and 33°C. (c) That particulars of the tyre sizes and gear ratios should be no concern of the makers of the instruments, and should not be given to them; the only information given being the required revolutions of the instrument per counter mile and the condition that the instrument should read correctly in itself between 27°C. and 33°C. and have a temperature error at 18°C. not exceeding 4%. Of three instruments recently tested the temperature errors were satisfactorily within the 4% limit and one instrument was correct in itself at a temperature within the limits given. | ||