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).
Taming wheel wobble frequency by adjusting steering and suspension systems.
| Identifier | ExFiles\Box 170\3\ img014 | |
| Date | 25th February 1936 guessed | |
| -2- (2) Taming Wobble Frequency. However, either type of wheel fight can be suppressed at the expense of an increase in the other type by a suitable "tuning" of the natural frequency at which the wheels "flap" around the kingpin. For example on the T.W. job with cross steering, if the steering box is fastened to the frame fairly rigidly the low frequency fight will be slight, but the high frequency fight will be violent. As the mounting of the box is stiffened still further the "400" fight disappears, but the high frequency fight occurs at about 600 cycles/min. and is very violent. As the mounting of the box is softened the low frequency fight becomes more violent and the high frequency decreases. Tests on the road show very clearly that this last condition is preferable. In other words it appears necessary to produce on the cams a condition approaching low speed wobble in order to get a steering which is free from wheel fight on the road. This has been true for a long time (for example, the need of softening the steering mechanism on wishbone suspensions by springs in drag-link etc.) but I do not think it has been clearly recognised. (See rough chart). All the Dubonnet suspensions, from the very first type tested on the S.I.A. car No.311, have shown a very definite tendency to the low frequency wheel fight and practically no tendency to the high frequency. Consequently we do not need to soften the steering mechanism on these cars, but rather the reverse. The explanations of this basic difference between wishbone and Dubonnet are evidently - (a) Mass of boxes which amounts roughly to doubling the moment of inertia of the wheel alone, and therefore lowers the natural frequency of wheel flap in the ratio .707 (say 550 cycles/min to 390 cycles/min.). (b) The additional flexibility in the steering mechanism represented by lateral bending of the wheel-carrying arm. That is as compared with the wishbone suspension the wheel disc is not o rigidly connected to the steering mechanism. | ||