Mechanical linkage which allows for a limited degree of relative vertical motion between its endpoints while not allowing them any relative motion at all in one horizontal direction and hardly any in the other horizontal direction.
The solution differs from a basic, swivelling or rotating, fixed length connecting rod or arm in that it does not need to exchange horizontal separation for vertical separation, through rotation, in order to facilitate a limited degree of freedom of relative vertical travel for the components it mechanically connects.
By changing the orientation the linkage is deployed in it can instead be used to allow horizontal separation in one direction to vary while keeping both vertical separation and separation in the other horizontal direction fixed or nearly fixed.
The linkage has a wide range of possible applications, wherever the relative position of two components needs to be kept constant or nearly constant in one plane but allowed to vary, to a limited degree, in another. Immediately apparent applications are suspension and steering systems of wheeled vehicles to eliminate bump steer and axle track gauge variation.
When used as suspension system, the linkage connects the wheels of a vehicle to its chassis in a way which permits little to no freedom of travel in either the vehicle chassis's lateral plane or its longitudinal one but allows a useful degree of relative movement in the vertical plane.
When used as steering tie rod, the linkage would connect the vehicle's steering rack to its steered wheels' steering knuckles or spindles, replacing pivot jointed tie rods of fixed overall length (used in such applications today) with compound ones whose overall length remains constant in the horizontal plane but is allowed to vary vertically.
The way the linkage works is by the relative positions of its hinges/joints and points of attachment (to the components it links) as well as the length of its upper (blue) and lower (yellow) control arm and overhanging connecting rod (red) being carefully chosen specifically such that the rotation motions of the two control arms and the translation and rotation motion of the overhanging connecting rod combine to mostly cancel out length variations in the horizontal plane over a limited degree of relative freedom of travel in the vertical plane, of the parts the linkage connects to each other.
When used as steering tie-rod, the linkage cannot attach to the end of the steering rack directly and requires an additional, intermediary component for its control arms connect to via hinges and which itself swivels either transversally or axially on the end of the control rack.
When used as suspension system the linkage requires an additional component to serve as attachment point for the kingpin (for the wheel steering knuckle) since the linkage's overhanging connecting rod's own rotation motion (as it travels through its range of vertical motion) precludes it being able to mount the kingpin itself directly. This additional component can either be part of the shock absorber or would also attach that too.