as a rocker it is desirable for safety reasons to prevent relative rotation between the front frame 14 and the rear frame 3 with a lock for example see fig4 an insertion hole is formed in the front frame 14 and a corresponding insertion hole is also formed in the back frame which is aligned with the former hole a rotation preventing pin 22 is inserted in the insertion holes formed in the front frame 14 and the rear frame 3 the pin 22 is attached to the front frame 14 by an attaching rope 21 a storing hole 23 for the inserted pin is formed in the front frame 14 when the front frame 14 is not locked with the rear frame 3 the pin 22 is kept in the storing hole 23 other kinds of locks are usable with the cycle of this invention a protection sheath 15 can be provided on the arcuate bottom rack 1 so as to prevent the arcuate bottom rack 1 from being worn out see fig6 and 7 there is an opening 16 formed on the sheath 15 which is open longitudinally for replacing the sheath 15 spaced ribs can be formed on the sheath 15 and extend peripherally this design reduces the amount of the materials needed to make the sheath 15 preferably the arcuate bottom rack 1 is u shaped and the ends of its opening are articulated with the rear portion of the rear frame 3 the opening of the arcuate bottom rack 1 becomes gradually.