Independently Sprung Front Wheels a Remedy for Shimmy 280002
THIS paper is an attack on the usual system of front suspension of an automobile and a proposal to substitute independently sprung front wheels for the conventional assembly of wheels, rigid axle and semi-elliptical springs.
Mounting of the front wheels on a solid axle is a survival of horse-drawn-vehicle days which the author asserts is so unsatisfactory from the standpoint of rational mechanics that it should long ago have become a memory in the automotive industry. To the complex action and reactions of the axle, oscillating between the springs and the tires, and of the wheels, with their gyroscopic effects, are attributed the phenomena of shimmy and wabble that have become so disconcerting and even dangerous since the advent of low-pressure tires and front-wheel brakes.
The origins of the various abnormal vibrations are analyzed, the effects of damping and of friction are discussed, and the part played by the conventional steering-gear and steering connections is considered. A distinction is drawn between damping and friction; the stabilizing effect of rake of the steering pivot and of deformation of the tires is explained, and the ineffective part played by friction shock-absorbers and by interleaf friction in the springs is described.
Rigidity and irreversibility of the steering mechanism are said to be most likely to induce shimmy at normal speeds with balloon tires and front-wheel brakes; hence the author advocates increasing the reversibility to the fullest possible extent and making the steering linkage sufficiently elastic.
The only specific remedy for abnormal front-end vibrations is, the author asserts, the adoption of correct independence of the front wheels, and he shows and describes a design that he has developed for such a construction.
He does not regard a similar independent suspension for the rear wheels as very desirable, but if such a suspension is to be used, he thinks it is better to adopt a split rear axle.
In the discussion, some of which was oral at the Annual Meeting and other, including that by the author, was submitted subsequently in writing, the findings from studies of front-end phenomena are declared to check closely with those of the author. One research organization found that shimmy frequently occurred when the wheels were in synchronism with the natural torsional frequency of the frame and body; also that, with the steering tie-rod removed, the same shimmy effect was obtainable as with the tie-rod in place.
Another organization found that shimmy and tramp were completely eliminated if the steering-gear was bolted to the axle and connected rigidly to the steering-knuckle arms. By restraining either tramp or shimmy, both are restrained, as was proved by road tests as well as on the dynamometer.
A third investigator agrees that the author's mathematical analysis is excellent but disagrees with his assumptions, as no front-wheel suspension has been devised, he asserts, that completely avoids a rocking motion of the front-wheel plane with relation to the car frame. He thinks that independent-wheel suspension will not suffice to suppress shimmy and must be supplemented by steering-complications. It has been found that a simple modification of the conventional steering system completely eliminates shimmy.
Objection is made to the author's design on the ground of expense of construction, and it is pointed out that European constructors have attacked the problem of shimmy without knowledge of the riding-qualities problem presented in this Country by certain types of road which set up small oscillations that can be absorbed only by suitable springs.
Replying to each critic, the author supports his theory and maintains that his construction is the only solution of the problem and is neither complicated nor expensive.