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, its causes, and how it is distinguished from shimmy.
| Identifier | ExFiles\Box 170\3\ img051 | |
| Date | 14th July 1933 guessed | |
| -17- maximum rotary speed and the right front wheel has minimum. The left front wheel is retarded between A' and D' and the right front wheel accelerated. At C' left front wheel has maximum retardation and right front wheel maximum acceleration. Retarding the wheel makes it push forward on the car and vice versa. Therefore at C' inertia torque about the kingpins is maximum clockwise. In other words increased wheel offset creates inertia forces opposing the gyroscopic torque and is equivalent to slightly softening the steering mechanism. From this it appears that increased offsets may make the wobble slightly worse. In actual tests on road and drums this is by no means clearly confirmed. The probability is that change in wheel velocity involves slippage between tire and road and acts as damping. The conclusion therefore is that, whereas offset x is pure disadvantage, offset y may have some advantage. In other words, extreme kingpin angles should be avoided, while the kingpin should be brought as close as possible to the wheel plane. Torques due to rolling resistance are higher at A' and C' than at B' and D' (due to slip angle). Fluctuation of rolling resistance therefore gives an impulse to the system at twice the frequency of wobble and may be ruled out as unimportant. CONCLUSIONS. (1) Low speed wobble is controlled by a rocking motion of the car. (2) It is self-energised, i.e. it is best produced on smooth roads or on smooth drums. (3) Within practical limits it is made worse by increased moment of inertia about the kingpins. A 'soft' steering gear. Increased caster. (4) The amount of self-energising is small. It is easily damped out, but the amount of damping necessary increases with the moment of inertia, and it is independently any damping at all is undesirable. (5) It is distinguished from shimmy by :- | ||