Abstract

The ride, handling and dynamic pavement loading properties of road vehicles are strongly influenced by the tire pressure. Although the central tire inflation systems (CTIS) have been implemented in many road and off-road vehicles, the assessments of variable tire pressure have been limited to field measurements in the context of ride and dynamic tire loads transmitted to the pavement. This dissertation research explored the role of variable tire pressure on the ride, pavement loading and handling dynamic characteristics through development and analysis of comprehensive three-dimensional models of an urban bus. The ride dynamic model was formulated based on nonlinear component models derived from the laboratory measured data, including the two-degrees-of-freedom driver-seat-suspension model. Considering that the passengers' load in an urban bus could vary from nearly none to the full passenger load, a tire pressure scheme in accordance with the passenger load was formulated and integrated in the model. The ride and dynamic pavement load properties of the vehicle were evaluated under random road roughness excitations corresponding to different tire pressures and passengers' loads. The influences of forward speed and road roughness conditions were also investigated. The results suggest that the use of nominal tire pressure under light passengers load would be detrimental to the ride vibration transmitted to the driver and the passengers, dynamic forces transmitted to the pavement and the forces transmitted to the chassis structure. A variable tire pressure in accordance with the load would thus be highly beneficial, provided that the tire deflection is controlled to reduce the tire wear and heat buildup. The limited available tire data on the cornering properties as a function of the pressure were analyzed to propose a regression-based tire cornering force model in conjunction with the widely used Magic tire formula. Two- and three-dimensional handling dynamic models of the vehicle were developed and analyzed to investigate the influence of tire pressure on steady-state and transient directional performance of the vehicle. The responses of the two models revealed reasonably good agreements in the steady-state handling, while the three-dimensional model accounted for the vehicle roll motion, which was observed to be considerably larger under lower tire pressures. The directional responses attained under different steering inputs and forward speeds suggested that a lower tire pressure increased the vehicle roll motions slightly due to reduced effective roll stiffness, while the high inflation pressure revealed greater oversteer tendency at higher tire pressures.