A Methodology of Optimizing Steering Geometry for Minimizing Steering Errors

The focus on driver and occupant safety as well as comfort is increasing rapidly while designing commercial vehicles in India. Improvements in the road network have enhanced road transport for commercial vehicles. Apart from the cost of operation and fuel economy, the commercial vehicles must deliver goods within stipulated time. These factors resulted in higher speed of operation for commercial vehicles. The design should not compromise the safety of the vehicle at these higher speeds of operation. The vehicle should obey the driver’s intended direction at all speeds and the response of the vehicle to driver input must be predictable without much larger surprises which can lead to accidents. The commercial vehicles are designed with rigid axle and RCB type steering system. This suspension and steering design combination introduce steering errors when vehicle travel over bump, braked and while cornering. The errors are well known as bump steer, brake steer and roll steer respectively and can cause the unintended vehicle directional response. This work shows the methodology to optimize the steering system design to reduce the steering errors at minimum level as well predictable in nature.

The methodology includes use of newly developed test rig to get the trace of draglink to steering arm ball joint under bump as well as braking conditions. The multibody dynamics model of steering-suspension assembly prepared in MSC ADAMS is validated with test rig results to rightly represent steering system kinematics and compliance. This validated model is then used with novel methodology to get steering hard points which will result in minimum steering errors that are easy for the driver to predict and control the vehicle during high-speed operations. The results obtained from this methodology is further validated with vehicle level result and it is found that this methodology helps to improve the accuracy of prediction for the dynamic behavior of vehicle during braking such as brake pull/steer.