The present invention discloses a system having a particular electric motor being able to do bi-directional rotational input having its output ends for providing output to the input ends of the constant directional output transmission device, while the constant rotational directional output is used to drive the loading wheel train via the output end of the constant directional output transmission device, wherein the user can operatively control the rotational direction of driving inputs, thereby the constant rotational output of different speed change ratio in different directions of driving inputs can be made via the constant directional output transmission device of different speed change ratios.