Balance

Balance in a bicycle is created by a number a factors.  The handlebars, pedals, tires, and rider all work together to keep the rider upright and balanced.  In order for the bike to stay balanced, the reaction forces have to be exactly equal to the forces applied by the bike.  Wikipedia uses the examples of gravitational if leaning, inertial or centrifugal if in a turn, gyroscopic if being steered, and aerodynamic if in a crosswind.  The balance depends on the geometry, mass distribution, and forward speed of the bike, and weight of the rider.  These will all influence how easy or hard it is for the bike to actually stay upright.