I. Introduction
A reliable and real time estimation of the vehicle lean (or roll) angle, i.e., the inclination of the vehicle with respect to the vertical direction, is the key to implement effective control systems that may enhance safety and performance on board of two-wheeled vehicles. As a matter of fact this variable greatly influences the tire-road friction forces, which are the means by which traction and braking forces are transmitted to the ground. As the lean angle cannot be measured directly on commercial vehicles (a direct measurement can be achieved only by means of optical sensors that may be used on racing bikes only, see e.g., [1]), it is important to devise effective estimation methods.