26 is provided between the second ear 320 and the middle part of the axle 25 a first notch 260 and a second notch 262 are defined at the circumference of the positioning disk 26 wherein a width of the first notch 260 is smaller than a width of the second notch 262 other two pairs of gaskets 24 are respectively provided between the other damper 23 and the second ear 320 and between the second ear 320 and the positioning disk 26 the positioning member 10 has a rod 12 longitudinally movably mounted between the second ear 320 and the third ear 323 the rod 12 has a positioning end 120 extending through the fourth aperture 324 of the second ear 320 and an installing end 122 extending through the third aperture 325 a lug 124 is formed on the rod 12 and a resilient member 14 is provided on the rod 12 and between the lug 124 and the third ear 323 a collar 16 is mounted on the installing end 122 outside the third ear 323 to fasten the rod 12 the positioning end 120 is alternatively located in the first notch 260 or the second notch 262 of the positioning disk 26 the leg 40 has a shank 400 a tube 42 and a leg 44 are respectively formed at two ends of the shank 400.