1.  Field of the Invention
This invention relates to an improvement of a foldable bicycle frame, and more particularly to a foldable frame of a portable sports bicycle. 
2.  Description of the Prior Art
Various kinds of foldable frames of portable sports bicycles have been proposed.  In general, however, these hitherto used bicycle frames have disadvantages in that in assembling, disassembling and folding these frames, they require particular tools and troublesome and complicated operation requiring much time and great skill. 
In these prior art bicycle frames, when they are folded or extended into a folded or running condition, it is difficult to ascertain whether halves of the frames assume desired positions to be eventually clamped.  Moreover, complicated clamping operation is needed, for example, such as tightening nuts by spanners. 
In the foldable bicycle frames of the prior art, furthermore, they do not have positive locking means for the halves of the frames, if not most.  It is, therefore, very dangerous for a cyclist, if the clamping between the halves of the frame is accidentally released when running. 
In the foldable bicycle frames of the prior art, as all frame members are rigidly fixed to each other so as not to permit any relative slight movement of the members, so that there is a tendency of compulsion forces to occur at the members when releasing the locking between the halves of the frame. 
In the foldable bicycle frames of the prior art, moreover, a support bar or stay rod which supports a bicycle so as not to fall down when not used in a running condition is fixed to a chain stay by clamping by bolts, mounting plates and a washer which are respectively on lower and upper sides of the chain stay behind a bottom bracket.  In other case, a support bar is fixed to a chain stay fork.  In each case, the mounting of the support bar is comparatively troublesome and the mounting members are undesirably exposed to the outside which would spoil the beauty of the bicycle.