Axiomatic Design of Automobile Suspension and Steering Systems: Proposal for a Novel Six-Bar Suspension

The existing vehicle designs exhibit a high level of coupling. For instance the coupling in the suspension and steering systems manifests itself through the change in wheel alignment parameters (WAP) due to suspension travel. This change in the WAP causes directional instability and tire-wear. The approach of the industry to solve this problem has been twofold. The first approach has been optimization of suspension link lengths to reduce the change in WAP to zero. Since this is not possible with the existing architecture, the solution used is the optimization of the spring stiffness K to get a compromise solution for comfort (which requires significant suspension travel and hence a soft spring) and directional stability (which demands least possible change in wheel alignment parameters and hence a stiff spring).

This paper presents an axiomatic design solution to this problem and an attempt to remove the coupling in the steering and suspension systems by making the WAP independent of suspension travel. The four-bar linkages used in the existing independent suspension systems are incapable of satisfying their FRs and cause coupling at a higher level. The proposed solution uses a six-bar Watt-I linkage suspension, which removes the coupling. It offers other advantages like the hardening characteristics for the suspension. A new steering system conformal to the new suspension system has been proposed.

FR/DP decomposition of the vehicle systems is presented. This indicates other couplings and DP redundancies in the vehicle system and also provides the framework for design of novel vehicles.