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).
The testing and specifications of A.T. speedometers for 20 HP and 40/50 HP models.
| Identifier | ExFiles\Box 50\4\ Scan304 | |
| Date | 7th July 1929 | |
| By. {R.W. Bailey - Chief Engineer} from EFC. {E. Fowler Clarke - Electrical Engineer} c. Hs. {Lord Ernest Hives - Chair} X4308. EFC {E. Fowler Clarke - Electrical Engineer} 4/ADL7.7.29. X.4308. UP-TO-DATE A.T. SPEEDOMETERS. 20 H.P. SPEEDOMETER. We recently reported to you in our EFC {E. Fowler Clarke - Electrical Engineer} 6/AD9.7.29 results of test on a recent 20 H.P. A.T. speedometer No.C.727-40 11/50. In this 20 H.P. case it is understood that the estimated revs. per mile of the speedometer drive on the chassis is very nearly 740 which estimated figure happens to be a round figure which can be adopted by the speedometer makes as the number of revs. per counter indicated mile to which 20 H.P. speedometers may be produced (though usually the next lowest figure possible is adopted). In regard to the miles per hour readings it will be remembered that the makers were asked to make these correct at an instrument temperature in the neighbourhood of 30°C. In this particular instrument the miles per hour readings were found to be correct at 35°C. with an increase of reading to the extent of 2% at a temperature of 22°C., the indications of this instrument can therefore be said to be satisfactory as the counter miles on the road should be almost exactly correct and the speed indications mainly slightly high. 40/50 H.P. SPEEDOMETER. We have also recently tested one of the new S.S. Phantom speedometers No.C855-810 11/41. In this Phantom case, and referring to particulars given us by your department, the estimated revs. per mile of chassis drive using the standard tyres are 826. The speedometers however, have been ordered and are being supplied for 810 revs. per mile as being the next lowest round figure acceptable to the makers. In line with our request that the instrument shall be correct in itself at 30°C. the reading of the instrument at this temperature when driven at 800 revs. per minute should be 44.5 miles per hour. If an instrument were thus correct when at 30°C. on a car, it would be reading counted miles and speed at 30°C. both 2% high, and the speed at lower temperatures would be more like 4% high. This would probably be considered satisfactory. contd. | ||