Clamping assembly comprising at least one clamping device for fixing an accessory to a crossbar, at least one tightening knob (10; 30; 50) for tightening said clamping device, said tightening knob (10; 30; 50) being releasably attachable to said clamping device for tightening and releasing purposes, wherein said clamping device comprises a base portion configured to be supported on an accessory to be mounted on said crossbar, at least one movable clamping member, for example a pivotable clamping member, movable with respect to said base portion between a clamping position in which said clamping member at least partially embraces a lower portion of said crossbar and preferably applies a clamping force on said lower portion of said crossbar, and an opened position in which said clamping device is insertable on or removable from said crossbar, and a coupling section with a rotatable force receiving portion rotatable about a rotation axis and operatively coupled to said at least one movable clamping member such that a rotation of said force receiving portion is transferred into a movement of said movable clamping member, said force receiving portion at least comprising a torque transmitting profile defined by a wall portion, said wall portion at least partially extending about said rotation axis of said force receiving portion, and wherein said tightening knob comprises a coupling portion (11; 31; 51) for detachably coupling said tightening knob (10; 30; 50) to said coupling section of said clamping device, said coupling portion (11; 31; 51) being configured engageable with said coupling section in a torque transferable manner, and an operating portion (12; 32; 52) configured to be grasped and rotated by a user, said operating portion (12; 32; 52) being operatively coupled to said coupling portion (11; 31; 51), wherein in a detached state, said at least one tightening knob is stowable in a space saving manner, characterized in that said wall portion is cylindrical or conical and said torque transmitting profile of said force receiving portion is non-round and defined by a radially outer surface of said wall portion, and in that said coupling portion (11; 31; 51) comprises an accommodating portion (13; 33; 53) in the form of a recess for at least partially receiving said force receiving portion of said coupling section therein in a torque transferable manner, wherein said recess is defined by an inner surface which is shaped corresponding to said radially outer surface of said force receiving portion, wherein said inner surface defines a cylindrical shape or a conical shape, and wherein a cross section of said inner surface is non-round, and wherein said inner surface is configured so as to allow an insertion and detachment of said tightening knob on said receiving portion in axial direction. at least one clamping device for fixing an accessory to a crossbar, at least one tightening knob (10; 30; 50) for tightening said clamping device, said tightening knob (10; 30; 50) being releasably attachable to said clamping device for tightening and releasing purposes, wherein said clamping device comprises a base portion configured to be supported on an accessory to be mounted on said crossbar, at least one movable clamping member, for example a pivotable clamping member, movable with respect to said base portion between a clamping position in which said clamping member at least partially embraces a lower portion of said crossbar and preferably applies a clamping force on said lower portion of said crossbar, and an opened position in which said clamping device is insertable on or removable from said crossbar, and a coupling section with a rotatable force receiving portion rotatable about a rotation axis and operatively coupled to said at least one movable clamping member such that a rotation of said force receiving portion is transferred into a movement of said movable clamping member, said force receiving portion at least comprising a torque transmitting profile defined by a wall portion, said wall portion at least partially extending about said rotation axis of said force receiving portion, and wherein said tightening knob comprises a coupling portion (11; 31; 51) for detachably coupling said tightening knob (10; 30; 50) to said coupling section of said clamping device, said coupling portion (11; 31; 51) being configured engageable with said coupling section in a torque transferable manner, and an operating portion (12; 32; 52) configured to be grasped and rotated by a user, said operating portion (12; 32; 52) being operatively coupled to said coupling portion (11; 31; 51), wherein in a detached state, said at least one tightening knob is stowable in a space saving manner, characterized in that said wall portion is cylindrical or conical and said torque transmitting profile of said force receiving portion is non-round and defined by a radially outer surface of said wall portion, and in that said coupling portion (11; 31; 51) comprises an accommodating portion (13; 33; 53) in the form of a recess for at least partially receiving said force receiving portion of said coupling section therein in a torque transferable manner, wherein said recess is defined by an inner surface which is shaped corresponding to said radially outer surface of said force receiving portion, wherein said inner surface defines a cylindrical shape or a conical shape, and wherein a cross section of said inner surface is non-round, and wherein said inner surface is configured so as to allow an insertion and detachment of said tightening knob on said receiving portion in axial direction.