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).
Comparison of high pressure and balloon tyres for 20 HP and 40/50 HP models, including data on effective circumference.
| Identifier | ExFiles\Box 158\1\ scan0018 | |
| Date | 14th January 1927 guessed | |
| contd :- -9- corresponding high pressure tyre. Thus for the 20 HP. and 40/50 HP. sizes we have :- | | EFFECTIVE CIRCUMFERENCE 10 M.P.H: INS. | |---|---|---| | | 40/50 HP. | 20 HP. | | High pressure. | 105.8" 33 X 5 S.S. 45 lbs/sq.in. | 101.25" 32 X 4.5 45 lbs/sq.in. | | Balloon. | 103.2" 33 X 6.75 30 lbs/sq.in. | 97.5" 31 X 5.25 35 lbs/sq.in. | | Percentage} Balloon. | 2.7% | 3.75% | | decrease } | | | However, owing to the greater deflection and lower inflation pressure of the balloon tyre, centrifugal force has more effect on the diameter than on a high pressure tyre. Consequently at high speeds the balloon tyre circumference becomes the same as that of the high pressure tyre. The appended graph shews the actual effect of speed on the effective radius. It is therefore hardly necessary for speedometers to be altered for balloon tyres in the case of the R-R cars. contd :- | ||