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).
Calculations for Phantom & S.S. speedometer drive ratios.
| Identifier | ExFiles\Box 50\4\ Scan297 | |
| Date | 31th May 1929 | |
| X 4308 31.5.29. X.4308. PHANTOM & S.S. SPEEDOMETER DRIVE RATIOS. Phantom 15/52. 52/15 x 16/29 x 21/33 = 1.217 S.S. 11/41. 41/11 x 26/8 x 2/18 = 1.346 Ratio S.S. to Phantom = 1.106 S.S. SPEEDOMETER DRIVE. In the S.S. chassis with 11/41 back axle ratio as compared with the Phantom with 15/52 back axle ratio, the speedometer is driven 1.106 times as fast for the same back wheel speed. Utilising 33" x 6.75" Balloon tyres on 21" dia rims, the speedometer revolutions per mile will therefore be Road revs per mile = 608.7 747 (as for 15/52 Phantom) x 1.106 742 x 1.106 = 821 618 rpm. = 826 Phantom speedometers on production were supplied for 730 revolutions per mile, and were therefore 2.3% high in mileage. S.S. speedometers on production are supplied for 810 revolutions per mile, and are therefore 2.0% high in mileage. 1.3% N.B. For S.S. experimental cars revolutions per mile figure for same tyres = 18/19 x 826 = 782.5 EFC. {E. Fowler Clarke - Electrical Engineer} | ||