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).
Ratio of connecting rod length to stroke for 25HP & 45HP car engines.
| Identifier | ExFiles\Box 13\9\ 9-page5 | |
| Date | 18th January 1931 | |
| E.{Mr Elliott - Chief Engineer} HS.{Lord Ernest Hives - Chair} ) FROM R.{Sir Henry Royce} BY.{R.W. Bailey - Chief Engineer} ) (At Le CanadelHenry Royce's French residence.) X7030 R1/M18.1.31. C. to SG.{Arthur F. Sidgreaves - MD} WOR.{Arthur Wormald - General Works Manager} DA.{Bernard Day - Chassis Design} CAR ENGINES. 25HP & 45HP. CON. ROD LENGTH, ETC. X.7030 X.5030. I have thought over HS{Lord Ernest Hives - Chair}/R1's and 3's memos, concerning the ratio of con. rod to stroke. I think it would be dangerous to revert to a short rod, and that we had better go straight on with our L/S of 2. It is too long and difficult for me to make a complete investigation but I have thought there are several aspects of the subject, such as the actual energy change with a small improvement may be in proportion to some power greater than unity - (1) If the energy passing along the crankshaft in and out of the flywheel is great, then the forces would be great. (2) Also the shorter rods may need a larger flywheel to keep the speed of rotation sufficiently steady to avoid vibrations (torsional) behind the clutch. Therefore it may be that we should not be making the engine, plus flywheel, any lighter because we had shorter rods. The pistons are happier with a longer rod but this is not so important as we could make them right by other means. Thinking this over brings up the question as to whether or not we ought to provide for torsionally flexible coupling between clutch and gearbox, or in the clutch. Rubber would be preferable, and of the block pattern. If not provided for now, or thought out now it could be it may be found difficult later. There may be several other advantages of long rods. Assume that if the rod length was infinite there would be no change of energy between the front half of the crankshaft and the flywheel i.e. each set of the 3 cranks would run steadily, and have no direct reason for putting the crankshaft into torsional vibration even on the master period, so that there would be no torsional vibrations except those due to pressure in the cylinders. If exchange of energy between the front 3 pistons and the flywheel depends upon a high power of the ratio (as it says Prof: Sharp's formula indicates) then it may be more difficult to keep down the harmonics when using short rods. My views are that there are several known advantages of the long rods, (and there may also be some which we have not studied), to conclude that a 12% improvement is of sufficient importance to justify the extra weight. (1) | ||