member 72 is fixed to an underside of the footboard 17 and includes a tubular member 721 which includes an annular groove with which the swing link 2 is engaged a protrusion 722 is located on an outside of the tubular member and a gap is defined between the protrusion 722 and the tubular member 721 the protrusion 722 includes a notch 7221 defined in axially therein and a reception recess 7222 is defined through the protrusion 722 the protrusion 722 is located between the two lips 713 a lever 723 is located in the notch 7221 and has a stub 7231 extending from an inside thereof the stub 7231 is removably inserted into the reception recess 7222 and a spring 7232 is biased between the lever 723 and an inside of the notch 7221 two pins 73 extend through the guide slots 714 and are connected to the protrusion 722 so that the end member 72 is pivotable about two pins 73 as shown in fig7 and 8 two driving units 8 are respectively connected between the footboards 17 and the rear wheels 18 each driving unit 8 includes a pair of side panels 81 a pedal 82 and a driving device 83 the two side panels 82 are connected to the underside of the footboard 17 and extend toward the rear wheel 18 a pair of frames 811 are connected to an.