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).
Analysis of the causes, systems, and frequencies of low-speed wobble and high-speed shimmy in automobiles.
| Identifier | ExFiles\Box 170\3\ img060 | |
| Date | 15th July 1935 guessed | |
| cars with large masses swinging around the kingpin have a tendency to the first type, and cars with small or swinging masses have a tendency to the second. In the case of low speed wobble or shimmy on smooth roads one can distinguish two separate vibrating systems, which must have about the same natural fre- quency for the cycle to occur at all. The wobbling of the wheels forms one system in both cases, but in low speed wobble the second system is a rocking motion of the car, while in shimmy the second system is the tramping of the wheels. In both cases the forward motion of the car feeds energy into the wobble motion, and the wobble motion feeds energy into the secondary vibrating system, so that, once started the cycle will continue indefinitely. There must also be a detonation, i.e. something to start the cycle. In low speed wobble this is generally a road bump, though it can also be a snatch at the steering wheel. In severe cases it is impossible to distinguish what starts the wobble as it appears to grow from no perceptible input. In shimmy, wheel unbalance is the usual cause, but an isolated road bump may also start the cycle. The rocking motion of the car which is an essential part of low-speed wobble occurs around an inclined axis passing through the bottom of the windshield, and the centre of the rear axle. Its frequency in cases so far examined is about 36 cycles a minute. Natural frequency of wobble on "Dubonnet" jobs is about 380-400 cycles a minute. On Wishbone jobs natural wobble frequency is 200 a minute or more. Tramp frequency with modern tires is about 540 cycles a minute. Since more or less of a "match" between the natural frequency of the two oscillating systems is necessary before the self-induced cycle will appear, it follows that Dubonnet jobs have a low speed wobble tendency but less tendency to shimmy, while Wishbone jobs are in the reverse case. | ||