secure the connection of receptacle 5 and connecting piece 10 is also possible and thus can raise the connecting strength of the aluminum bar 1 and the side stay 8 further because of the construction and function of the embodiment of the present invention as stated above and because the receptacle and connecting piece are each fitted to the aluminum bar or side stay and fixed rigidly repetitive assembly and disassembly of the side stay will not cause any looseness and the structure is sufficiently strong to bear long term service the attaching and removing of the side stay is very simple and completed in a short time because it can be accomplished by only attaching or pulling out the side stay horizontally to or from the end face of the aluminum bar and screw tightening or loosening the clamping bolt further because the projecting piece of the connecting piece is fitted in the fitting groove and then screw tightened by a clamping bolt passing through the fitted portion to make them one united body the connecting strength of the aluminum bar and the side stay is quite high and the rigidity of the frame is also very high further the construction of the aluminum bar and the side stay which have hollow parts and the small receptacle and.