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 analysis of the dynamics of wheel shimmy, covering its starting and finishing conditions.
Identifier | ExFiles\Box 170\3\ img034 | |
Date | 14th July 1933 | |
-5- Shimmy Starting When the shimmy starter starts, however, the loop is like C, which means that the wobble is leading the tramp by something less than 90° of phase. Here it is evident that the outward precession of the wheel is caused not only by camber and rolling resistance as described above but also by slip angle torque (see "Guiding of Independently Sprung Vehicles." July 14, 1933. The wheel meets the ground toed in and a very strong precession torque due to the slip angle swings it outward. It attains maximum velocity outward at D in the above sketch, which is therefore the position of maximum upward gyroscopic acceleration of the axle. As the wheel swings past D, it is already lifting off the ground so that a contrary precession torque due to outward slip angle (such as occurs in the case of the vertical loop) is no longer present. It will be seen that in condition C precession torque due to slip angle leads the wobble by about 135° of phase, and gyroscopic torque due to wobble velocity leads the tramp by 135°. Self-energising effects are at a maximum. Shimmy Finishing Before the shimmy dies out the loop generally reaches condition E.{Mr Elliott - Chief Engineer} It can be seen that this represents a dying condition of self-energising. The precession torque due to slip angle (at F) is opposed to the torques due to camber and rolling resistance in their effects on the wobble. At the time when the wheel is being lifted gyroscopically (at G) it has almost its maximum downward velocity. The whole cycle is therefore going 'out of tune'. If the shimmy is being excited by unbalance of the two wheels, there will be a slow 'beat' to the phenomenon due to slight difference of rolling diameter of the two wheels. As the weights approach 180° apart on the two wheels, starting conditions C appears. This builds up to greatest violence when the loop is vertical. The weights have then passed the 180° point. As the weights become nearly in phase, the shimmy passes to the final condition E and suddenly stops. In case of great excitation (much unbalance) there appears to be a tendency for the weights to stay either in-phase position with no shimmy, or in the 180° position with violent, shimmy. | ||