a hydraulic drive system and a bicycle having such system as its drive train , the system including a pump , preferably a vane pump , operably connected to the bicycle pedal crank , the pump delivering pressurized fluid to a hydraulic motor operably connected to the wheel or mechanism to be driven , e . g ., the bicycle drive wheel . the pump has an impeller eccentrically disposed in a housing , and a piston - cylinder mechanism is provided to change the eccentricity of the pump impeller with respect to the pump housing as a function of hydraulic system pressure , to thereby maintain a constant input force , e . g ., pedal pressure , and rotation rate , for a wide range of speeds . the motor also has an impeller eccentrically disposed in a housing , and a spacer bar connected between the pump and motor housings automatically adjusts the motor impeller eccentricity with changes in the pump impeller eccentricity , thereby further maintaining constant pedal pressure and rotation rate for a wide range of speeds . a pretensioned spring opposes the movement of the piston in the cylinder , thereby requiring that a predetermined pedal pressure level be exerted before the pump and motor eccentricities will change . a lever is provided for manually setting , and varying , the pretension of the spring to thereby set the pedal pressure level at which the pump and motor impeller eccentricities will be charged .