Tires as a Cause of Shimmy 270023

THE author enumerates and describes various inter-related movements of the front end of the car that are commonly known as automobile shimmy. A long list is given of experiments made in an attempt to correct the trouble. These did not produce consistent results but showed that caster angle acts as a considerable influence, while the influence of camber and toe-in seems to be more on tire wear than on shimmy. Lubrication of springs and conditions affecting the free motion of the steering pivots have some influence but the author sees imperfections in the tires as fundamental causes. The nature of the road is also important, particularly of concrete roads having regularly spaced depressions at joints.
Some of the tire imperfections are described and blame is placed on the tire makers for being less thorough in their methods of testing and inspection than are car manufacturers.
A number of shimmy causes are analyzed quantitatively and shown in curves that are arranged to indicate how shimmy is caused when these forces are synchronized. The author expresses the opinion that tire makers should try more diligently to eliminate causes of shimmy within the tires, rather than to suggest that the car manufacturers supply dampening elements to counteract their results.
In the discussion, several representatives of tire manufacturers take the opposite ground, describe some of their tests and experiments and state that tires are manufactured with a very high degree of accuracy considering that they are made from plastic material.
The importance of caster angle was emphasized by manufacturers of axle gages, who said that this angle varies in different car models and is not always kept accurate in manufacture. This item, they said, should be checked by the dealer when the car is delivered and the car owner should have it checked frequently.
The great effect of gyroscopic action as a factor in shimmy was mentioned and demonstrated by a model that showed the sidewise turning of the pivoted road-wheel resulting from gyroscopic action when the axle end is raised or lowered.


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