DEVELOPMENT OF AN INSTRUMENTED REAR SUSPENSION TO MEASURE THE TIRE FORCES OF A RACE CAR DURING TRACK DRIVING

The maximum achievable performance of a race car is strictly related to the tire characteristics which determine the forces available for the vehicle control. The measurement of the wheel forces during racetrack driving allow to characterize the tires in real operating conditions involving the influence of the wear, temperature and pressure variations. In this work, a combined geometric and sensitivity matrices method has been applied to the rear multi-link suspension of a rear-wheel-drive race vehicle to estimate the tire forces (Fcx, Fcy, Fcz). The geometric matrix allows the calculation of the tire forces from the equilibrium of the wheel assembly, starting from the spatial orientation and the loads acting on the suspension arms. The suspension is composed by four link arms and by a lower arm having a complex geometry. The link arms are constrained by two spherical joints at their ends, therefore the axial loads acting on them have been measured through strain gauge full-bridges in axial configuration. The lower arm is constrained with four spherical joints: two of them are connected to the vehicle frame, one to a link arm and one to the upright. At the joint between the lower arm and the upright, three mutually orthogonal forces are exchanged, which have been measured with two strain gauge full-bridges and an half-bridge configurations by adopting a sensitivity matrix approach. The measured reaction forces are then multiplied by two geometric matrices which take into account the spatial orientation of the link arms and of the lower arm respectively, giving the force components acting in the global reference system to which the tire forces are referred (Fcx, Fcy, Fcz, respectively). After a sensitivity FE analysis, the strain gauges have been applied and the instrumented suspension system has been calibrated in a specifically designed test rig. In the test rig, the suspension assembly is connected to a fixed support by reproducing the constraints to the vehicle chassis; known forces can be applied at the ideal contact point between the wheel and the ground by means of four servo hydraulic actuators. A simulated racetrack load history of tire forces has been reproduced in the test rig for the calibration of the sensitivity matrix of the lower arm and for the validation of the geometric matrix. To minimize the estimation errors, two different sensitivity matrices for the lower arm have been evaluated depending on the sign of the Fcx force applied. The errors achieved on the estimation of the wheel forces are lower than 5% for driving conditions (positive Fcx) and lower than 7% for braking conditions (negative Fcx). After the calibration, the instrumented suspension has been installed in the car and multiple racetrack acquisitions have been successfully performed. The experimental tire forces have been evaluated by implementing a post-processing procedure that considers the braking pressure to switch between the two experimental sensitivity matrices of the lower arm and calculates the geometric matrices from the signals of a suspension potentiometer. The measured wheel forces will be used in future works for the characterization of the tire performances.