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 wheel wobble (shimmy) and steering reactions, detailing the causes and the adoption of an elastic shackle as a solution.
| Identifier | ExFiles\Box 28\5\ Scan027 | |
| Date | 1st April 1930 guessed | |
| X457 WHEEL WOBBLE. FILE UNDER Considerations which Led to Adoption of Elastic Shackle [29598.]—It is necessary to distinguish between two different phenomena, both generally called “shimmy.” What we call shimmy is vibratory movement of the wheel synchronous with the vibration period of the front wheels and with the lateral oscillation period which affects the whole front of the car at right angles to its general direction. It is necessary for these three movements to be in synchronism before the genuine shimmy occurs. Another movement which is often called by the general name “shimmy” is the sudden and rapid oscillation of the front wheels around their pivots, which makes itself felt as a violent movement of the steering wheel. We call this last movement “steering reaction” in order to distinguish it from “shimmy.” For two years we have investigated both shimmy and steering reactions. Shimmy has appeared at the same time as the fitting of front brakes and low-pressure tyres because these features make smaller the oscillation period of the pendulum formed by a wheel and of another pendulum formed by the axle itself, which is able to oscillate around a horizontal axis parallel to a longitudinal axis of the car. After very exhaustive trials we have arrived at the positive conclusion that there is usually only one initial cause of shimmy, that is, imperfect equilibrium, dynamical as well as statical, of each of the front wheels. There are, however, amplifying causes, such as a very slight error in the equilibrium of a wheel which can bring about a tremendous shimmy in certain circumstances. Among amplifying causes one of the most important is the inclination of the steering pivots. When a wheel is turned from the straight ahead position, the fact that its steering pivot is inclined causes the car to rise and to fall each time the wheel is deflected from and returned to the straight ahead position. As the axle is attached to the chassis by a spring which is essentially a part capable of periodic movement, it is easy at a certain speed of the car to produce a movement of the wheel synchronous with the period of the spring. Some of the amplifying causes already mentioned have such important results that they have hidden the initial cause, which is bad equilibrium of the wheels. We have always been successful in suppressing shimmy, even with cars fitted with inclined steering pivots, by attention to the statical and dynamical balance of the wheels. We have also made cars with vertical steering pivots, and on these, in order to avoid shimmy, it is sufficient to obtain statical balance of the wheels. It is not necessary to consider dynamical balance, because the principal amplifying cause, the inclined pivot, is not present. On these cars, however, we can produce shimmy at will by fastening to one of the spokes of the wheel a weight of from 300 to 400 grammes. M.P.H. : PRESERVING LONDON'S GOO Our cars, therefore, have been exempt from shimmy and have had perfect steering when new, but as wear occurred the steering became bad and the steering wheels reacted to such an extent that it was impossible to drive the cars on bad roads. It should be noticed that shimmy appears usually on good roads having but small undulations, whilst, on the contrary, steering reactions appear every time one of the front wheels encounters an obstacle. These are the phenomena which induced us to study an elastic shackle, allowing a certain displacement of the front spring horizontally in a forwards and backwards direction. The theory of the elastic shackle can be stated briefly as follows : The two front springs, being connected one with the other, can be considered as two pendulums connected by a shaft having for axis points the steering pivots. A pendulum can only maintain its movement (neglecting the effect of friction) under the express condition that the axis point gives it at every moment the necessary reaction. If the axis point is able to move the pendulum movement cannot be maintained. The elastic shackle, by allowing the axis point to move, causes the pendulum movement to cease. DELAUNAY-BELLEVILLE MOTORS, LTD. London, N.W.6. | ||