On the Development and Verification of High Fidelity Agricultural Tire Models

Multi body simulation of agricultural vehicles such as tractors has relied on the representation of tires by equivalent spring-damper combinations. Historically, the stiffnesses assigned to various modes of deflection have been established on the basis of tests on a limited population of tires. Vehicle accelerations obtained by simulations of transport over typical terrain and rectangular obstacles using these parametric characterizations have deviated significantly from measurements. This paper reports on an attempt to develop finite element tire models, which are capable of generating quasi-static spindle forces and moments for prescribed displacements over irregular terrain and capture phenomena such as enveloping of obstacles. Though based on the tire manufacturers knowledge of constructional details, they are presented as ‘black boxes’ to the vehicle analyst for imposition of boundary conditions and loads, including inflation pressures. Outputs of stand-alone models, typically spindle forces and moments and spindle displacements are compared with measurements to establish validity of the models for non-rolling as well as rolling performance. Using these models it is shown that carpet plots and similar parametric characterizations of tire stiffness can be generated for use in multi-body simulations.