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).
Vehicle dynamics, including shimmy, frame stiffness, and vibration frequencies.
| Identifier | ExFiles\Box 4\6\ 06-page51 | |
| Date | 12th May 1931 guessed | |
| -3- The shimmy shackle does more than the side steering tube springs in permitting the axle to walk to and fro and lowering the frequency of wobble, at the same time dissipating energy in tyre friction. (I think this is a correct rough description). Too soft a shimmy shackle will promote low speed wobble. Even though "drag links" are solid they are by no means rigid and actual frequencies of wobble would indicate a virtual stiffness of the whole assembly of around 2000# /inch measured at the forward drag link ball. If the solid drag link is actually used only with shimmy shackle the explanation is undoubtedly that having spent the money for the one it is not considered desirable to make the other. I know nothing about spark plugs at the moment. Frame. We call jellying "fender shake" and it is just the same sort of curse as with you. In addition to taking some actual tests on frame stiffness (which are not, however, in such a form that they can readily be translated into lbs.inches per degree) we have taken tests of frequency. For example the Buick that you mention (5 passenger sedan) has a torsional frequency of 720/minute and a "sag" frequency around 800. The rear axle road chart that you kindly sent me the other day shows clearly the high importance of this since all the charts are virtually made up of two S.H.M. frequencies of 450/minute and 71/minute. The front end criss-cross on all cars seems to be about 600-680 for the axle and 650-750 for the frame and whether stiffening a frame improves it or otherwise seems to depend on whether one approaches or departs from complete synchronization. This is just one aspect of the thing, and you probably know more about it than I do, but I should welcome your comments. We have built a "shaker" consisting of a fan-hub with all but one blade cut away and a steel weight bolted to the blade giving 8.9 lbs.ins. out of balance. | ||