The adsorption on Cu(100) of a long molecular wire constituted of a 2.5 nm conjugated board equipped with four lateral legs is investigated using scanning tunnelling microscopy (STM). The legs are free to rotate when the molecule approaches the surface. Experimental images are analysed and compared to calculated images based on an optimisation of the geometry of the legs in order to extract the conformation of the adsorbed molecule. The adsorption is dominated by the interaction of the molecular board with the metal surface which forces the legs to rotate and to twist around their attaching bond to the board.