Shimmying is an oscillating motion produced by repeated impacts or forces in the linkage of a mechanism that lacks stability or has become loose because of wear. Although previously existent in chassis in which the steering-gear was imperfect, it has become particularly noticeable since the introduction of low-pressure or balloon tires. But increasing the rigidity means increasing the unsprung weight, which, in turn, means greater impacts, hence, more shimmying. This is apparent in the effect produced by front-wheel brakes. Consequently, as the amount of looseness that can be removed is limited, the periodic forces that cause shimmying must be overcome.
The propensity of low-pressure tires for assuming periodic rebounds when traversing bumpy roads and for causing shimmying and tramping, as well as pitching and bobbing, is illustrated by an example involving the use of springs; and the deduction is made that in order to minimize its ill effects, the kinetic energy stored in the tires must be absorbed. Another example proves that only 76 per cent as much kinetic energy is stored in high-pressure tires as in balloon tires.
Lateral stability in a tire is a very desirable feature. The lack of it in balloon tires has a profound effect on the steering and produces shimmying. Backlash will cause the same result in cars having center-point or near center-point steering.
The use of a dash-pot, which prevents the lateral forces from synchronizing and, therefore, from gaining momentum, and of friction-discs mounted on the king-pin and the knuckle-shaft, to absorb kinetic energy, is recommended as having a beneficial effect. The three remedies for shimmying suggested by the author are:
Design so that no slackness can develop
Design for rigidity
Use effective devices to absorb kinetic energy wherever it is likely to accumulate