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).
Standardisation sheet comparing the circumference and gear ratio effects of different tyres for the 20 HP model.
| Identifier | ExFiles\Box 58\1\ Scan030 | |
| Date | 21th June 1927 | |
| To B.J. from Hs.{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} X5820 Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}2/LG21.6.27. STANDARDISATION SHEET. 33 X 6" 20 HP. TYRES - X.5820 With reference to BJ7/E17.6.27. The original 20 HP. high pressure tyre 32 X 4.5" had an effective circumference of 101.25. The 31 X 5.25 tyre or 20 HP. medium pressure tyre, had an effective circumference of 97.5". The 33 X 6" or 20 HP. low pressure tyre has an effective circumference of 101.5". In other words the 6" tyre has brought us back to the gear ratio which existed with the original high pressure tyre, 4% above that with the 5.25" tyre or equivalent to about 3/8 of a tooth in the rear axle pinion. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||