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).
Technical analysis of wheel shimmy, frame stiffening, and torsional frequencies in vehicle chassis design.
| Identifier | ExFiles\Box 170\3\ img036 | |
| Date | 14th July 1933 guessed | |
| -7- the two wheels, each of them can build up a shimmy cycle independent of the other. The difference in cycle is like the difference between a single-acting and a double-acting engine. The wheel is shot up into the air as described, but is brought down again by gravity and spring reaction only instead of being forced down by the lift of the other wheel. In actual practice we have seen very few independents which would break into a real old fashioned shimmy. Some early 1934 phaetons came very close to it. Stiffening the frame, and taking other steps (engine mount etc) to "tune up" the nose of the frame to a high natural frequency, is resulting in gradual escape from this whole tendency. Stiffening the frame has a similar tendency on axle cars, though since the coupling of the two wheels through the axle is more or less rigid, the effect of frame stiffening is less marked. As a matter of fact if a front axle is examined under conditions of bad shimmy, it will be seen that the amount of front spring movement is small. The shock absorbers at such high velocities act almost like rigid couplings between frame and axle and the car vibrates torsionally about a nodal point somewhere near the centre of the wheel-base. Under such conditions stiffening the frame raises the natural frequency of rotary oscillation in a transverse plane, until it is out of step with the wobble. The chief distinction between axle cars and independents is that some fairly successful axle cars have been produced in which the natural torsional frequency of the frame-body assembly was low (around 300 per minute) whereas successful independents have only been produced by keeping this frequency high (700 per minute or more). The modern tendency is to build torsionally stiff body-frame assemblies even on axle cars, since it is found that such cars, if tuned high enough to be above all probable road frequencies, are much more free from shake and road shock. | ||