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).
Engine design decisions for the 25HP chassis, including bore, stroke, and crankcase specifications.
| Identifier | WestWitteringFiles\V\October1930-February1931\ Scan138 | |
| Date | 11th December 1930 | |
| SC. ) FROM R.{Sir Henry Royce} HS.{Lord Ernest Hives - Chair} ) C. to WOR.{Arthur Wormald - General Works Manager} RG.{Mr Rowledge} C. to DY.{F R Danby} RECEIVED 15 DEC 1930 25HP. CHASSIS. R1/M11.12.30. x5?70 x5010 x5000. (1) It has been decided to keep this in its class by keeping the bore 3.250 so that the chassis shall not need any changes or tax increase, but if we increase the stroke to 5" we can bring the proportion to the original. This will have 10" con. rods, and crankcase .750 deeper. (2) Definitely we do not want the cast iron crankcase because of the extra weight, this being in an undesirable position, affecting pitching, steering, etc. If it were behind the rear axle I should not object so much. The crankshaft will be strengthened from 2.125 to 2.5 with 2" to 2.250 pins, and arranged for counterweight on centre and ends to reduce couples, but not to bring up master period - i.e. balance crankshaft only, not the big ends of con. rods. (3) The crankcase will be stiffened by extra webs and box beam at bottom: aluminium permits of greater stiffness than cast iron with very much less weight. Also it permits us to put the extra stiffness where we want it - i.e. we can get much increased stiffness for very little increased weight. There may be some saving in cost with cast iron but much loss of class distinction. Even the comparatively inexpensive Austins have all aluminium crankcases. (4) We thank Mr. Tresilian for his suggested crankshaft which has good features, but we think it better to have journals and pins more nearly the same dia., and that the counterweight would be more possible as 2 pairs at greater centres, leaving the 4 intermediate bearings with their present load and reduce the 3 heavily loaded ones very considerably. (5) Nitro-hardened shafts should be used: we much prefer to bore out crank pins instead of fitting heavier balance weights. (6) It is understood that the improved engine primarily suggested to bring it in line with P. 2. should have the following virtues: (a) Saving in construction costs by less, and easier, machining, and a saving in many parts. (b) Less labour to get a smooth and quiet engine. (c) The camshaft runs in an oil tunnel, the valve gear is roller-less and simpler, and top adjustment of tappets only. (1) | ||