The aim of the presented work is to estimate the vertical forces of heavy vehicle and identify the unknown dynamic parameters using sliding mode observers approach. This observation needs a good knowledge of some dynamic parameters such as damping coefficient, spring stiffness...etc. We propose in this paper, to identify some of these parameters which are, in practice very difficult to obtain and to measure. This identification will improve the quality of vertical forces estimation. Some experimental results are presented in order to show the quality of the estimation and identification. These estimation results are then compared to the measures coming from an instrumented tractor.