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).
Car performance and gear ratio report detailing tests on valve bounce and speeds with different tyre sizes.
| Identifier | ExFiles\Box 108\1\ scan0381 | |
| Date | 22th February 1938 | |
| To. Rm.{William Robotham - Chief Engineer} c. Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls} c. Rm{William Robotham - Chief Engineer}/Swdl.{Len H. Swindell} 600a. CHATEAUROUX 2/.22.2.38 26.G.VI. - 25/30 HP. Car Performance & Gear Ratio. In view of the valve bounce experienced and general fussiness of the engine at speedometer speeds above 70 m.p.h. we have as already intimated, been running some of the mileage with 7.50 x 17" rear tyres. These when new, gave an increase over the worn 6.50 x 17" tyres removed of 7 to 8%. We found however, that we were still getting well into the valve bounce speed, and to-day wired you that the exhaust valves were stretching, .006" - .007" being noted after a fast run, on all except No II. We have in the course of the running taken performance and comparative figures between the 7.50 and 6.50 tyres, and also a check on the speedometer. New 6.50 x 17" tyres were used for these tests and the increase given by the 7.50 x 17" tyres against these was 6%. The speedometer showed a straight line error, the indicated speeds at 60 m.p.h. road speed being :- Standard 6.50 Tyres... 63.0 m.p.h. Oversize 7.50 Tyres... 59.4 m.p.h. The following figures have been taken, the results given being actual road speeds in M.P.H. 7.50 Tyres. 6.50 Tyres. Mean Timed Speed over kilometer 75.3 74.3 Approximate maximum speed under favourable conditions 87. 85 Cruising speed under favourable conditions 83. 79 Slight Valve Bounce 77. 73 Bad Valve Bounce 83. 79 ........ | ||