Although wheel wabble, even with high-pressure tires, is of ancient origin and the general methods of controlling it have been well understood, its importance among present-day problems is due to the fact that the recognized specific for its treatment, namely, increasing the air-pressure in the tires, has been denied. Shimmying, as generally applied, includes wabble, or the sidewise vibration of the front wheels about the knuckle-pin, and tramping, or the bouncing of the wheels vertically, alternately on the two sides.
In addition to discussing the advantages and disadvantages of the low-pressure tire, the author has enumerated the results of tests, some of which have been obtained from original research work by himself, others from the literature on the subject, with a view to determining whether shimmying is caused by defects in design, and what are the effects when certain modifications are introduced. Among these modifications are the use of moderate pressures and flexibility, wider rims, reversed caster-angle, increased weight on the front wheels, shock-absorbers and hydraulic dampers on the steering-mechanism, and changes in the geometry of the steering-gear.
The conclusions reached are that the low pressure and the thin side-walls of balloon tires are the chief factors contributing to front-wheel shimmying and that no acceptable changes in chassis design can be made to control it. A perceptible improvement in riding comfort and freedom from shimmying can be obtained, however, by the use of a moderate design of balloon tire, not too thin a side wall, proper width of rim, and medium air-pressure.


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