The main objective of this paper is the reconstruction of lateral dynamics and both roll angle and steering torque of single track vehicles (motorcycle, scooter, etc.). For that purpose, the well-known motorcycle model developed by Sharp in 1971 is used. This model characterizes the lateral dynamics of a motorcycle [16]. The roll angle is not observable in the obtained structure, for overcoming this problem, the model is transformed in order to take into account the roll angle as an unknown input as well as the steering torque. A Proportional two integrals (P2I) observer is then proposed for estimating simultaneously all the variable states, the lateral forces and both roll angle and steering torque. This study is a part of the ongoing work of the research team on the design of preventive safety systems for motorcycles users. Simulation results and discussions are given in order to illustrate the effectiveness of the proposed observer.