The steering arrangement of a 6x6 off-road military vehicle was investigated, with the aim to determine if a variable steering ratio between the first and second steering axle of the vehicle will make an improvement in the steady and transient state handling of the vehicle. Low speed manoeuvring was evaluated, comparing the vehicle steering geometry with Ackerman geometry. For steady state handling, a bicycle model was developed, and constant radius simulations at various track radii, vehicle speeds and steering ratios (ratio between the first and second steering axle) was performed. For transient dynamic simulations, a mathematical model was developed that included a simple driver model to steer the vehicle through a single lane change, again at various speeds and steering ratios. The vehicle was instrumented, and actual constant radii tests, as well as single lane change tests were performed. The measurements enabled the comparison of simulated and measured results. Although basic mathematical models were used, acceptable correlation was obtained for both steady state and transient dynamic behaviour. The results indicated that for this specific vehicle geometry, where the centre of mass is above the second axle, no marked improvement would be obtained by implementing a variable ratio steering system. The mathematical model was changed to simulate a vehicle with longer wheelbase and different centre of mass. With the new geometry, theoretical slip angles (and therefore tire wear) reductions were more noticeable It was concluded that a variable ratio system between the front and second axle would not be an economically viable improvement for this vehicle, since the improvement achieved will not warrant the additional cost and complexity added to the vehicle.
Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2007.