Recirculating ball (RCB) steering gear with four-bar linkage steering system is widely used in commercial vehicles with rigid front axle and over slung suspension owing to their architectural arrangement. The usage of RCB steering gear with four-bar linkage steering system with underslung suspension is not commercialized globally due to complexity of packaging suspension and steering linkages. Few automotive OEMs have studied and implemented Y link steering system in place of four-bar linkage steering system for underslung suspension arrangement. Y link steering system has got its inherent disadvantages such as higher bump steer, poor self-centering, complex linkages, more number of parts and less tire life. Further this arrangement is comparatively costlier than four-bar linkage steering system owing to their higher number of parts. In this paper, an extensive analysis has been made to implement the four-bar linkage steering system for underslung suspension vehicle. In this arrangement, the track rod (one of the links of a four-bar linkage system) is packaged above the leaf spring unlike in vehicles with overslung spring. Due to this unique arrangement, there exists some challenges in critical parameter optimization and packaging which are addressed in this paper. A mathematical model was developed to arrive at optimum steering and suspension geometry inorder to reduce the Ackerman error for better tire life and improve vehicle handling characteristics. By solving this mathematical model, optimum hardpoints can be arrived to achieve less Ackerman error, lower steering effort, lower bump steer, better steering returnability and best in class tire life. The present paper shows a possible approach to define the optimized steering and suspension linkages from the existing complex geometry with underslung suspension. This attempt is first of its kind in automotive industry and if commercialized successfully will go a long way in improving steering performance and cost savings for the vehicles with underslung suspension.
Keywords: Underslung suspension, four-bar steering linkage, Y link steering, Ackerman, bump steer, overslung suspension, self-aligning torque, trapezoidal steering, pitman arm, Steering gear ratio, Kingpin torque, Scrub radius, caster angle, Axle lift, King pininc
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