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).
Typed letter discussing speedometer corrections needed due to tyre performance variations.
| Identifier | ExFiles\Box 165\2\ img335 | |
| Date | 19th August 1941 guessed | |
| -2- Thus neglecting tyre loss the indications of the existing instruments can vary between 93 and 95 m.p.h., for a true speed of 90 m.p.h. The additional correction as proposed on your drawing, AT.732, will be 1/3 m.p.h., per each 10 m.p.h., after the "30" mark. Thus neglecting tyre loss the indications can vary between 95 and 97 m.p.h. Subtracting the 3% increase in tyre loss which occurs between 30 m.p.h. and 90 m.p.h., the limits will be 92.3 - 94.3 (cold) for the 6 ply tyre, and 92.5 - 94.5 for the 4 ply. The values for the two tyres, both hot and cold, at the various speeds will be as follows :- Dunlop 650 x 16 Dunlop 650 x 16 True "Fort" 4 ply. "Fort" 6 ply Speed Cold Hot Cold Hot 30 30 - 32 29.8 - 31.8 30 - 32 29.8 - 31.8 40 40.7 - 42.7 40.4 - 42.4 40.7 - 42.7 40.4 - 42.4 50 51.1 - 53.1 50.9 - 52.9 51.2 - 53.2 50.9 - 52.9 60 61.7 - 63.7 61.3 - 63.3 61.7 - 63.7 61.3 - 63.3 70 72.0 - 74.0 71.5 - 73.5 72.0 - 74.0 71.5 - 73.5 80 82.3 - 84.3 81.7 - 83.7 82.2 - 84.2 81.6 - 83.6 90 92.5 - 94.5 91.8 - 93.8 92.3 - 94.3 91.6 - 93.6 In calculating the above values, the usual procedure of allowing for 1% increase in tyre radius at 30 m.p.h., has been adopted, also the additional dial correction shown on your drawing AT.732 has been taken into account. We feel that the correction should be increased to give an indication of 95 max., for a true speed of 90 m.p.h., when using the 4 ply type tyre, and in the hot condition. Yours faithfully, for ROLLS-ROYCE LIMITED, | ||