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 document explaining the findings on wheel fight, wobble, and shimmy in steering systems.
| Identifier | ExFiles\Box 170\3\ img024 | |
| Date | 26th March 1936 | |
| COPY. WHEEL FIGHT IN STEERINGS. Mar.26th/1936. As we make more tests and get the picture more clearly, I think we are gradually enabled to state our findings in simpler terms. (1) Wobble & Shimmy. These are fully described in my notes of June 1935. I see no reason to make any change in those notes except that we did not then fully appreciate the importance of the natural flap frequency of the front wheels. Both are self-induced cycles which, once started, will maintain themselves indefinitely on a smooth surface. We knew at that time that individual cars had marked differences in tendency. Some tend to wobble, others to shi my. Few, if any, cars show any strong tendency to do both. Low speed wobble in its pure form occurs about 360-380 cycles/min. in all cars examined. Shimmy occurs close to 600 cycles/min. because this is the hop-frequency of the wheels. (2) Wheel Fight, on rough roads, is simply a forced vibration of the wheels and steering. If the road bumps are spaced regularly and alternately, and are encountered at about the right frequency, there is a tendency to set up wobble. But almost any haphazard arrangement of bumps which throw the wheels into the air will set up shimmy. If one wheel is thrown up it will ignore all intermediate bumps and only come down to receive a fresh impulse at a time interval determined by the natural frequency of wheel hop (about 1/10 second). I think this explains why the wheel fight which we generally complain about is definitely the second or high frequency type. Also why one can drive a car which is known to have a wobble tendency for days and weeks without encountering the particular set of conditions to start a real wobble on the road. Forced vibrations of the steering will only occur to any noticeable extent in a narrow range of frequencies, and this range is determined by the tendency of the car with regard to wobble or shimmy. Thus, if conditions are such that the car tends to wobble, forced vibrations of the steering wheel may occur between 340 and 420 cycles/min. If the car tends to shimmy, forced vibrations will occur within say 550-700 cycles/min. | ||