A road-haulage semi-trailer having a sliding king-pin for lifting the landing legs to the retracted position, the landing legs being spring-biassed towards the retracted position and coupled to the king-pin through a rack and pinion to give rapid lifting of the legs with a small king-pin movement. The king-pin is disengageable to move independently of the landing legs over a large distance between front and rear clamps which enable it to be locked in either a front or a rear position for travelling.
