A stabiliser (25) provided for a ball-and-socket coupling device especially for a vehicle/trailer combination has a hollow piston (27) slidably mounted within a housing (28) which is fixed to the socket member (14). The hollow piston (27) contains a hydraulic fluid (29) which is pressurised by rotation of a threaded rod (31) connected to a second piston (30) such that the surface (33) of the hollow piston (27) imparts a selected force on the ball (12) minimising pitching and yawing movements of the support (11A) (and thus the trailer, in the vehicle application). <IMAGE>
