A PASSENGER need ride only a short distance in a coach
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before deciding whether the suspension is good or bad. To a designer, seeking the most satisfactory layout, such a decision would be a gift from heaven if all the occupants were of the same mind. Unfortunately for the engineer, mankind is not yet " assembly-line " produced. Until that time comes, the inevitable compromise must be resorted to, and the vehicle most nearly meeting the requirements of the majority will stand as the best.
Having designed and produced a good chassis, work . is commenced on a later model which, for production reasons or some other, may possibly have totally different suspension in which it will obviously be the desire to retain and, if possible, improve upon the characteristics of the first system. The question will then be, "bow may one reproduce this performance, or assess its efficiency against an accepted yard stick? "
Periodicity, which is the most usual characteristic chosen for such a comparison, is the number of vibrations of the sprung mass of the vehicle per minute after it has been set in motion by some outside force (Fig. I). Periodicity depends on the static deflection, or spring deformation, incurred by adding the weight of the vehicle to the unladen springs. It is a characteristic of any kind of spring that, in theory, when deflected by a supported weight and then vibrated by an outside force, the frequency will be dependent purely on this dimension.