Bike with belt transmission and its frame

A bike comprising:

• a first frame element extending from an upper end carrying a steering tube of a front wheel to a lower end;
• a second frame element extending from an upper end receiving a seat unit to a lower end rigidly fixed to the lower end of the first frame element;
• a rear wheel fork comprising at least one chainstay element extending from the lower end of the first frame element to a rear wheel spindle support and a seat stay extending from said support to the second frame element;
• a crankset having a crank spindle borne by the frame and a driving chainring arranged laterally outside with respect to the first and second frame elements;
• a rear wheel driven by a coaxial sprocket arranged inside with respect to the rear wheel fork;
• and
• a drive belt forming a continuous loop engaged with the chainring and the sprocket,
• wherein the chainstay element has a major portion arranged below a straight line extending from the lower end of the chainring to the lower end of the sprocket, wherein the belt has an outward strand and a return strand both extending through the triangle formed by the second frame element, the chainstay element and the seat stay, these three elements forming an irremovable stiff assembly, and wherein the axis of the crankset is situated at a vertically measured distance from the upper face of the chainstay element of the first frame element said distance being at most slightly greater than the radius of the chainring and
• wherein the rear wheel fork is suitable for receiving a wheel having multiple sprockets, a derailleur with an idler pulley and a chain having a return strand passing through the fork triangle and an outward strand extending below the chainstay element irrespective of the position of the derailleur.