Abstract: A two wheel drive bicycle has fixed rear wheel and a turnable front wheel, a chain ring rotatable by way of an external rotation force, and a bicycle frame comprising a top tube and a head tube. A front drive chain runs, in sequence, from the chain ring, about a chain tensioner, around a first pulley of a pulley system, around a freewheel of the front wheel, around a second pulley of the pulley system, and back to the chain ring. The pulley system is fixed adjacent the head tube. The chain tensioner is characterized by a retractable third pulley attached to a spring means, the spring means being biased in a direction away from the front freewheel.