This article shows the bicycle as machine and merchandise. It outlines some ideas about the relationship between the bicycle's sharing and its disposal, the taking of a cycloid's image, and the interest for the city's public issues. The transit ends with the presentation of a machine called cycloid emulator. In three different parts the author attempts to answer the following questions: What kind of machine is a bicycle? What else is it? And what is the result of the exercise of spinning forward? Finally, the author presents a particular kind of bicycle, public and shared.