Recently, an all digital simulation was developed for open and closed loop automobile maneuvers, including severe ones (see reference [1]). A basic philosophy in the development was an attempt to create independent "modules", each involving different aspects of the modeling. This, it was felt, would enhance user flexibility and allow relatively easy incorporation of any changes in modeling. The purpose of the present note is to report on work done in connection with the module on the forces and moments generated by tires. As a guide and aid to possible future research workers in the area of vehicle handling and vibration, a comparison between two different methods of computing the tire side forces will be presented.
The two methods compared are the APL-CALSPAN one (see reference [2]) which is based on curves fitted to the measured data, and a Partial Data Deck Model which was designed to use directly the measured data.
The Partial Data Deck Model works as follows: from the data an array of evenly spaced points giving the tire side force as a function of slip angle, camber angle and radial force was developed. Since there were insufficient data points available for non-zero camber angles, the APL-CALSPAN empirical equations were used to generate a modified slip angle for non-zero camber angles. For a particular set of values of slip angle, camber angle and normal load, the side force is then calculated from the array by means of threevariable, linear interpolation. If sufficient data points are used this procedure is certain to be more accurate than the CALSPAN one, in which a cubic curve is fitted to the data.
The procedure used in the partial Data Deck Model is as follows: First, the nearest points to the current tire operating conditions are found from the equations