An adjustment device for controlling the aerodynamic load on a motorcar, comprising: - an underbody aerodynamic diffuser which comprises an aerodynamic surface, facing, in use, towards a ground surface, and an opening made through the aerodynamic surface, the diffuser being able to take in through the opening a flow taken in defined by at least one part of an air inflow received by an air inlet of the motorcar and produced by the forward motion of the motorcar travelling on the ground surface, so that the at least one part taken in by the diffuser becomes an internal flow inside the diffuser, the device being configured in such a way that the aerodynamic surface, in conjunction with the ground surface, defines, during the forward motion, a diffusion duct in which the internal flow flows, the adjustment device comprising - an aerodynamic element movable between a first position and a second position relative to the diffuser; - a movement system for moving the aerodynamic element; characterised in that: the movement system is configured to keep the aerodynamic element operatively at least at the first and the second position relative to the diffuser and to cause the element to move between the first and the second position; wherein, at the first position, the aerodynamic element is immersed in the internal flow and allows the internal flow to follow the aerodynamic profile of the aerodynamic surface in such a way as to generate a first aerodynamic load acting on the motorcar during the forward motion; wherein, at the second position, the aerodynamic element acts at least partly as a barrier against the internal flow, moving the internal flow away from the aerodynamic surface in such a way as to generate a second aerodynamic load acting on the motorcar during the forward motion, the device being configured in such a way that the second aerodynamic load is smaller than the first aerodynamic load. - an underbody aerodynamic diffuser which comprises an aerodynamic surface, facing, in use, towards a ground surface, and an opening made through the aerodynamic surface, the diffuser being able to take in through the opening a flow taken in defined by at least one part of an air inflow received by an air inlet of the motorcar and produced by the forward motion of the motorcar travelling on the ground surface, so that the at least one part taken in by the diffuser becomes an internal flow inside the diffuser, the device being configured in such a way that the aerodynamic surface, in conjunction with the ground surface, defines, during the forward motion, a diffusion duct in which the internal flow flows, the adjustment device comprising - an aerodynamic element movable between a first position and a second position relative to the diffuser; - a movement system for moving the aerodynamic element; characterised in that: the movement system is configured to keep the aerodynamic element operatively at least at the first and the second position relative to the diffuser and to cause the element to move between the first and the second position; wherein, at the first position, the aerodynamic element is immersed in the internal flow and allows the internal flow to follow the aerodynamic profile of the aerodynamic surface in such a way as to generate a first aerodynamic load acting on the motorcar during the forward motion; wherein, at the second position, the aerodynamic element acts at least partly as a barrier against the internal flow, moving the internal flow away from the aerodynamic surface in such a way as to generate a second aerodynamic load acting on the motorcar during the forward motion, the device being configured in such a way that the second aerodynamic load is smaller than the first aerodynamic load. the movement system is configured to keep the aerodynamic element operatively at least at the first and the second position relative to the diffuser and to cause the element to move between the first and the second position; wherein, at the first position, the aerodynamic element is immersed in the internal flow and allows the internal flow to follow the aerodynamic profile of the aerodynamic surface in such a way as to generate a first aerodynamic load acting on the motorcar during the forward motion; wherein, at the second position, the aerodynamic element acts at least partly as a barrier against the internal flow, moving the internal flow away from the aerodynamic surface in such a way as to generate a second aerodynamic load acting on the motorcar during the forward motion, the device being configured in such a way that the second aerodynamic load is smaller than the first aerodynamic load.