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 mounting strategies, frame stiffness, and the prevention of torsional twisting.
| Identifier | ExFiles\Box 137\4\ scan0299 | |
| Date | 31th March 1931 guessed | |
| -2- four point Diamond mounting - with this I do not agree. I can now see that modern motorcars to have a frame rigid enough (torsionally) we are forced to use the engine crankcase to prevent the frame twisting and if so we must make our engine fundamentally as free from vibrations as possible? It follows that even with the 28 HP. we must criticise this with a four arm mounting with or without front and rear support and with rubber not too flexible to enable us to let the engine keep the frame from twisting. So that I cannot agree to deleting the rear engine feet and any of our schemes of three or four point mounting which leaves the large panel where the engine is located without torsional support. P.2 depends upon these four engine feet and I think that perhaps the large area sandwich mounting is the best that can be devised but it might be still satisfactory with softer rubber, the rear central support (gearbox) is also as good as the rear engine feet rubber can be normally in an unloaded condition. The front central support is of less value and more costly for room and inconvenience so should not be used unless it can be proved to be of great help. The only way to make three point support passable is to trust to the body to keep the side members from twisting. This can be tested with suitable box like open experimental body. Long ago it occurred to me that we ought to test the stiffness of frame as they are ready to rec. the body, i.e. opposite the B.A. we support on steel girder horizontally and rigidly. At the front we support on central point so that the front is free to put the frame in torsion when loaded out of centre. Now we know that powerful shock dampers and the change of spring pressure due to deflection may by the front axle's action put a torsion on the frame of many hundred lbs. at say 20" each side of the centre, the two modern shock dampers alone at 100 lbs. each would give torsion of 100 x 20 x 2 = 4,000" lbs. so one can see how rigid the frame must be, and one cannot get it without help from the engine carcase. I am writing this to say don't cancell the rear feet but try softer rubber, and that they are better the further they are apart front ones as far forward as possible rear on gear-box would be preferable and could be tested by rigidly bolting side girders to engine carcase and attaching these through rubber feet to frame wherever possible even to the first cross-member. something like Northcliffe underframe idea but in this case they must transmit the torsion of the front axle from end | ||