Abstract: A hydraulic activator operable to change a suspension characteristic in response to changes in the velocity of damping fluid flowing through a flow passage provided in flow control valving within the shock absorber. The hydraulic activator comprising a pressure cylinder forming a working chamber operable to store damping fluid and a reservoir cylinder coaxial therewith. A piston is disposed within the pressure cylinder defining an upper and lower portion of the working chamber. The activator further comprises a first transducer for emitting ultrasonic waves across the flow passage. The ultrasonic waves emitted by the first transducer are received by a second transducer. A frequency detection circuit determines the difference in frequency between the emitted and received ultrasonic waves and generates an output in response thereto. A central processor calculates the fluid velocity and causes a control circuit to change a suspension characteristic of the shock absorber.