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).
S.S. chassis speedometer accuracy, drive ratio calculations, and a proposal for alternative instruments.
| Identifier | ExFiles\Box 50\4\ Scan311 | |
| Date | 25th September 1929 | |
| X4308. By. {R.W. Bailey - Chief Engineer} from EFC. {E. Fowler Clarke - Electrical Engineer} c. Hs. {Lord Ernest Hives - Chair} c. Wor. {Arthur Wormald - General Works Manager} EFC {E. Fowler Clarke - Electrical Engineer} 1/AD25.9.29. X.4308. S.S. CHASSIS SPEEDOMETERS. With the normal standard S.S. tyres latest measurements show a figure for road wheel revolutions per mile of 608.7. Since the speedometer drive ratio is 41/11 x 26/8 x 2/18 = 1.346 the speedometer drive revs. per mile are 1.346 x 608.7 = 821 A number of S.S. speedometers have been obtained for 810 revs. per counter mile and are thus a suitably small percentage (viz. 1.3%) high on mileage. According to tests made on a recent S.S. production speedometer (arranged for 810), the indicated speed agrees with the counter miles when the instrument is at a temperature of 30°C. and is 1.4% high at 22°C. Therefore the indicated speed of the car would be 1.3% high at 30°C. and 2.7% high at 22°C., which is regarded as a reasonable setting. There has been a proposal to order and fit instruments of 765 revs. per mile, which will thus read over 7% high on miles, the same amount high on speed when the instrument is at 30°C., and nearly 9% high when the instrument is cold. We have disagreed with this proposal as not being in accordance with R.R. principles and reputation for accuracy. We have now been informed by Wor. {Arthur Wormald - General Works Manager} that this is not intended. EFC. {E. Fowler Clarke - Electrical Engineer} | ||