HYDRAULIC shock-absorber characteristics are analyzed by the authors with the aid of indicator cards made on a machine designed and built for the purpose. The machine is shown in a diagrammatic drawing and is stated to have been used with much satisfaction for more than two years.
Curves of the action of dry and lubricated springs with and without shock-absorbers attached are shown and the statement is made that the resistance of the shock-absorber does not increase fast enough, as the speed of link movement increases, to damp the spring suitably at both large and small deflections. Indicator cards from shock-absorbers of several types reveal the effects of incorrect design of the valves and of dirt in the oil passages.
The effect of change in viscosity of the working fluid as a result of temperature changes is discussed and attempts to obtain a fluid that is not thus affected are declared to be fruitless. Efforts to design shock-absorbers that compensate for temperature changes have taken the form of (a) a relief valve to prevent excessive link force at low temperature and (b) a thermostatic element to control restriction of the fluid flow. An inherent disadvantage of the relief-valve type is its limited ability to increase the link force with increase in link speed.
The design of an orifice-control valve which embodies a thermostatic element is shown and described and is stated to have shown remarkably satisfactory characteristics on the laboratory machine and that these have been confirmed on the road. Numerous indicator cards are presented for comparison of the characteristics of a shock-absorber provided with this valve with other types of shock-absorber at various temperatures and rates of link motion. These show that the former gives as rapid a build-up of link force with speed as may be desired, and no manual adjustment on the road is required.