Device for transporting bicycles on motor vehicles comprising: - a small channel connectable through supporting means to a roof of a motor vehicle, - an arm connected rotatable to the small channel through a connecting element, - catching means connected to the rotatable arm for catching the bicycle and keeping it in a vertical position when its wheels are rested on the small channel, and - guiding means (33, 34) associated with the rotatable arm and the small channel and configured for guiding the translation of the rotatable arm along a direction of a rotation axis of the arm, when this latter one is in an assembly position to allow inserting or withdrawing the arm and keeping it on a loading part of the bicycle on the device, both when the small channel is assembled on the motor vehicle in order to transport a bicycle oriented along the traveling direction, and when it is assembled along the opposite direction for transporting a bicycle oriented along the opposite direction to the traveling direction, the guiding means (33, 34) comprising at least one inserting element connected to the rotatable arm and configured to be inserted into the connecting element when the rotatable arm is in the assembly position, in which the rotatable arm is free of translating along the direction of the rotation axis of the arm, characterized in that the guiding means comprise two inserting elements connected to the rotatable arm and aligned along the direction of the rotation axis, to allow inserting the rotatable arm in the seat from both longitudinal sides of the small channel when the rotatable arm is in the assembly position, to arrange it always on the loading part of the bicycle on the roof of the motor vehicle, both when the small channel is assembled in order to load the bicycle along the traveling direction of the motor vehicle, and when it is assembled along the contrary direction, for loading the bicycle along the opposite direction to the traveling direction of the motor vehicle. - a small channel connectable through supporting means to a roof of a motor vehicle, - an arm connected rotatable to the small channel through a connecting element, - catching means connected to the rotatable arm for catching the bicycle and keeping it in a vertical position when its wheels are rested on the small channel, and - guiding means (33, 34) associated with the rotatable arm and the small channel and configured for guiding the translation of the rotatable arm along a direction of a rotation axis of the arm, when this latter one is in an assembly position to allow inserting or withdrawing the arm and keeping it on a loading part of the bicycle on the device, both when the small channel is assembled on the motor vehicle in order to transport a bicycle oriented along the traveling direction, and when it is assembled along the opposite direction for transporting a bicycle oriented along the opposite direction to the traveling direction, the guiding means (33, 34) comprising at least one inserting element connected to the rotatable arm and configured to be inserted into the connecting element when the rotatable arm is in the assembly position, in which the rotatable arm is free of translating along the direction of the rotation axis of the arm,