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Undesirable low frequency vibrations experienced on vehicles equipped with semi-active suspension are observed: when road inputs have significant repetitive component, when there are large control gains in the suspension control algorithm, and when the velocities of the body are estimated by filtering the measured suspension deflections. Stability analysis performed on simple vehicle models demonstrates that interaction between the dynamics of the vehicle and the estimation filter is the root cause of the problem. A vehicle with an active suspension becomes unstable while a vehicle with a semi-active suspension exhibits limit cycles when the above mentioned conditions are present. A solution is proposed in which the control gains are adaptively changed depending on the frequency content of the road spectrum.