Inasmuch as the forces that act when a vehicle passes over a road obstruction are very complex, comparative analysis of the riding-qualities of the different parts of a vehicle is difficult; hence, to obtain even an approximation of them, measurement of the different displacements that occur must be confined to a given representative condition or series of conditions.
The displacement that causes the most discomfort to a passenger is probably that which takes place in a vertical plane. Its three leading characteristics are the amplitude of the vertical movement, the velocity of the motion and the rate of change of the velocity. Of these the last mentioned is the most important. It is sufficiently exact to assume that tires and springs which reduce motion increase riding-comfort. Since the angular displacements having both vertical and horizontal components in transverse as well as longitudinal planes have comparatively long radii, the difference between the horizontal or vertical component and the actual path covered is slight and, for all practical purposes, can be neglected. It was found that measuring the vertical displacement and the accelerations occurring on the displacement curve would give all the data needed for the desired computations.
To approximate road conditions in the laboratory, a specially designed apparatus was constructed that included an accelerometer capable of translating rotary into simple harmonic motion. With this it was possible to determine definitely the acceleration at any portion of the displacement-time curve.
The relations found to exist among velocity, acceleration and displacement indicate that measurement of the displacement and acceleration, or at least the acceleration, of the frame and the axle is desirable. The reasons that a high natural free period of vibration and a large damping coefficient should be incorporated into the construction of an accelerometer are discussed and the details of construction and method of operation of an instrument embodying these features are described.