Steering strategy for a multi-axle wheeled vehicle

Owing to their superior off-road mobility and manoeuvring capability of multi-axle vehicles are used by Armies world over. There are only few studies related to selection of appropriate steering strategy for an eight wheeled vehicle. Multi-axle-steering increases the manoeuvrability by reducing the turning circle diameter at low speeds. Vehicle model is developed using Simulink software to perform simulations. Six practically implementable steering strategies using combinations of front wheel steer (FWS), FWS with intermediate wheel steer (IWS) and FWS with rear wheel steer (RWS) for an eight wheeled vehicle have been considered in this study. Bicycle model has been considered assuming left and right wheels steer at almost equal angles for large turning radius. Steering angles at intermediate and rear wheels are considered in proportion with front wheel steer angles. For constant forward speed, higher yaw velocity and lateral acceleration indicate capability of vehicle to take sharp turns. Under-steer behaviour is preferred in vehicle handling and smaller vehicle sideslip angles are associated with under-steer characteristic. Simulations have been carried out for step-steer input at constant forward speeds on a random road profile. It is observed that time required to attain steady state increases with increase in speed and increase in number of steerable axles. However, first and last axle has more effect achieving steady state than intermediate axle. Steering strategies with rear wheel steer (RWS) result in high yaw velocity and lateral acceleration responses, but they also produce undesirably large vehicle sideslip angles. Front two axle steering strategy is adopted for the eight wheeled vehicle based on this study. Objective evaluation of steady state handling behaviour in terms of steering angle as a function of lateral acceleration has been carried out through MBD simulation and actual test of vehicle on steering pad at NCAT VRDE. Good correlation between simulation and experimentation has been obtained.