This paper deals with state estimation of Powered Two Wheeled (PTW) vehicle and robust reconstruction of related unknown inputs. For this purpose, we consider a Proportional two Integral (P2I) observer. Based on LMI technique, we recover vehicle states and part of its dynamics (rider action) within steering angle disturbances and system uncertainties. To prove the effectiveness and the robustness of the proposed observer, two different scenarios are considered: first we observe the system with disturbances without system's uncertainties. The second case is more general as the observer synthesis is conducted in the presence of steering angle disturbances/system's uncertainties. Both the state and the rideraction, on the handlebar, are recovered in the two cases.