Invariant Properties of Automotive Suspensions

Abstract

Dynamic systems which are composed of interconnected sub-systems are subject to dynamic ‘constraint equations’ which are independent of the nature of the interconnections. A dynamic constraint equation is developed for a quarter car model of an automotive suspension. It is shown that only one of the three transfer functions (acceleration, suspension deflection and tyre deflection) can be independently specified and that the first two contain ‘invariant points’ at frequencies within the frequency range of interest. These constraint equations lead to conclusions with respect to trade-offs between the three transfer functions. Improvements in all three can be obtained near the unsprung natural frequency; however, severe trade-offs are shown to exist at all other frequencies.