and contracts in the normal course of operation of the isolator 10 for receiving working fluid displaced from the upper chamber 21 for the purpose of providing fluid communication between the upper primary working chamber 21 and the lower secondary expansion chamber 22 a so called inertia track passageway 23 is provided as best seen in fig2 the inertia track passageway 23 is arcuate and has a substantially uniform cross section illustratively of circular shape throughout its entire length the inertia track passageway 23 has a port 23a at one end opening into the upper chamber 21 and a port 23b at its opposite end opening into the lower chamber 22 the dimensions of the inertia track passageway affect the mass of fluid contained therein and thus affects the resonant frequency of the fluid effect of the isolator 10 during operation oscillating movement of the upper wall 15 relative to the partition 20 as by movement of a member connected to its studs 16 17 causes working fluid to oscillate in the inertia track passageway 23 between the primary chamber 21 and the secondary chamber 22 a vibration isolator of this construction may have a minimum complex dynamic stiffness at a relatively low excitation frequency and over a relatively narrow range of frequencies for many applications it is desirable for the inertia track passageway 23 to be relatively inactive or decoupled at small amplitudes of excitation and.