A two-wheel drive bicycle includes a main frame having a steering head, a seat tube, a cross bar and a hollow downtube. A main drive mechanism includes a housing, a driving spindle, at least one driven ring gear, a first driven shaft and a second driven shaft. The first driven shaft is rotatably connected to the housing and operably coupled to the at least one driven ring gear. The second driven shaft is, likewise, rotatably connected to the housing and operably coupled to the at least one driven ring gear. Front and rear wheel drive mechanisms include a series of shafts that are themselves coupled to and between the first and second driven shafts and the front and rear wheels. The main drive mechanism preferably includes a plurality of driven gears comprising a series of concentrically spaced ring gears. Each ring gear of the plurality of driven ring gears has an inner diametrical surface and an outer diametrical surface. Each outer diametrical surface is configured to slidingly engage the inner diametrical surface of an adjacent ring gear. Further, the main drive mechanism preferably includes a biasing means for biasing each ring gear away from the first driven shaft and the second driven shaft and a selective engagement means for selectively engaging one ring gear of the plurality of ring gears with the first driven shaft and the second driven shaft.