illustrated in fig2 the bumper 20 includes a flexible wrapping means 200 which has an outer tube 210 an inner tube 220 rotatably received in the outer tube 210 a spiral spring 230 positioned in the inner tube 220 and a first lid 240 and a second lid 250 a first elongated slot 212 is longitudinally defined through the outer tube 210 for the connecting member 40 passing therethrough the first lid 240 and second lid 250 are respectively mounted at two open ends of the outer tube 210 the spiral spring 230 has a first end attached to the first lid 240 in this embodiment the first lid 240 has a lug 244 formed at an interior wall facing the spiral spring 230 preferably at the center of the interior wall the first end of the spiral spring 230 is fastened on the lug 244 and a second end of the spiral spring 230 is fastened on the inner tube 220 a first recess 242 and a second recess 252 are respectively defined in the interior walls of the first lid 240 and second lid 250 and two ends of the inner tube 220 are respectively and movably received in the recesses 242 252 two bearings only one bearing 260 shown in fig2 at the second recess 250 are respectively provided between the inner tube 220 and the recesses 240 250 for a smooth rotation of the inner tube 220 in the recess 240 250 for preventing the spiral spring 230 from twisting a shaft 270 extends in the spiral spring 230 and has a first end positioned in a first aperture 246 defined in the lug 244 of the first lid 240 in this embodiment a.