it in which the cylindrical body of the centering head 58 is lodged the centering head 58 is fixed to the reinforcing structure 42 mounted on the front part of the trailer 1 by means of a seat 59 bolted on the structure 42 through one or several intermediate wedges 60 to allow height adjustment the centering head 58 consists of two elements the first element being intended for absorbing the opposite upward and downward movements of trailer 1 and motor vehicle 11 once these have been coupled to each other i e the 34 silentbloc 34 61 element made of reinforced elastic material fixed to the seat 59 which is bolted on the structure 42 and the second element being intended for centering and fixing the front part of the trailer 1 to the roof rack 43 of the motor vehicle 11 i e the body 63 of the centering head 58 which has a cylindrical shape also fixed to the 34 silentbloc 34 61 the body 63 is composed of three zones a first zone which consists of the free end 62 of the body 63 preferably having a conical shape to make it easier to enter the base 64 into the hole 57 the diameter of the base 64 being slightly smaller than that.