Semi-active suspension systems aim to improve the stability and comfort of vehicles. Several semi-active suspension control strategies require the vertical velocities; however, the instrumentation of the vehicle normally does not include sensors for these variables, typically only accelerometers are available. Direct integration to estimate the vertical-velocities fails to provide accurate velocity estimations. In particular, the dependency of the system on the road profile, which is an unknown signal, complicates the use of traditional observer schemes. In addition, the behavior of semi-active dampers is highly nonlinear. Two estimation strategies are proposed: an Unknown Input Observer (UIO) and a Robust H
∞ Observer. During the design of the UIO the unknown road profile decoupling induces observability problems, which are overcome with an approximated observer. An experimental comparison between the estimation methods in common perturbation conditions is carried out. The experimental results, obtained with a 5:1 scale car equipped with ER dampers, show that it is possible to archive good estimation performance with the proposed schemes.