Referring to FIG. 1, a bracket shell commonly used for bicycles in the prior art includes tubes 11 and 12 to which frame tubes are fixed to and welded to. Said tube 11 may have to serve as a joint for the seat tube 13 (or head tube). To facilitate adjustment of the seat height, a slot 14 is generally provided formed in the wall of the tube 11. On both sides of the slot a pair of jugs (lugs) 16 and 17, having respective joining holes 15 formed therein, are provided. Each of said jugs 16 and 17 are symmetrical to the other. One fixing bolt 19 and one nut 18 are also provided, the bolt 19 being received through the said joining holes 15 and the nut 18 being adjustably, threadably received on the forward end of the bolt 19, thereby securing said bolt 19 in place. As the seat tube 13 is inserted into the said tube 11 to a predetermined depth the tube 13 is clamped therein by said jugs (lugs) 16 and 17 being forced tight by means of the said bolt 19 and nut 18. The inner wall of the tube 11 will in turn force the seat tube 13 fixedly tight. However, since this form of structure does not provide firm fixation of the tube 13, the seat, being locked by the said bolt 19 is apt to slide down, especially when the road the bicycle is riding on is bumpy, causing injury to the bicycle rider.
In view of this, the inventor, based on his years experiences in designing and manufacturing bicycle's parts, found that such defect is caused by the shell's inner wall lack of overall tightening forces which are needed for the adequate tightening of the seat tube. All that it could provide is no more than one or two linear contact to the tube. Detailed analysis shows that the tube, being pressed or formed out of steel sheet, does not have such a good roundness (or shape) that perfectly matches (is complementary with) the seat tube. Due to the property of steel sheet to have low flexibility, the shell, while being tightened by means of bolt and nut, cannot provide enough roundness needed for the tightening of the seat tube. To solve this problem, the inventor, after a long time research, finally came up with the present invention.