Improving Semi-Truck Torque Overlay Steering Systems to Boost On-Center Performance: Hardware Design Investigation

In class 8 semi-trucks, the hydraulic steering gear and torque overlay system are critical components affecting the steering feel design and vehicle control. Transitioning from traditional hydraulic gears to hydraulic gears with torque overlay steering (TOS) systems for increased enhancement of driver comfort is beneficial but has also resulted in drawbacks for on-center steer feel, especially at high vehicle speeds (60+ km/h). This article evaluates the impact of three design mechanisms within hydraulic steering gears of a TOS system that have shown improvement in on-center performance for traditional hydraulic gears. The study compares a standard assembly of TOS, i.e., baseline, and a design-optimized ideal prototype, to evaluate the effectiveness of the three design mechanisms: valve curve performance, on-center friction, and torsion bar stiffness. The two samples underwent high-speed vehicle testing to gather driver feedback and assess potential enhancements to the on-center steering feel. The final design changes on the ideal prototype were based on the best valve curve and on-center friction, as limitations in the torsion bar modification process precluded its use in the vehicle. The vehicle qualification team found insufficient evidence linking these design features to improved overall steering performance. Further research will be conducted to analyze the impact of torsion bar change as well as software controller performance within the TOS as a follow-up study.