Dynamic behaviour of vehicles having castored wheels can be affected by wheel shimmy phenomenon, that is a self-sustained vibration of the whole castor around the steering axis. This article presents a shimmy investigation conducted on a laboratory castor characterized by low lateral stiffness; numerical models show that this characteristics leads to three different stability conditions depending on the forward velocity: at low speeds the system is unstable and the castor is affected by shimmy oscillations; then there is an intermediate speed range in which the system recovers stability; finally, above a threshold speed value, the system returns to be unstable. The experimental investigation mainly aims to compare the numerical results with the experimental ones and to define the steering damping required to stabilize the castor according to its main features.