The present invention relates to an adjustable drive mechanism that converts a force supplied from an operator or other means along an adjustable curve path into rotary motion. More particularly, the present invention relates to an adjustable drive mechanism configured for an operator driven or motor driven exercise apparatus such as a stationary bicycle, recumbent stationary bicycle, cross trainer or other devices such as a bicycle. The present invention relates to the kinematic motion control of pedals which follow more complex curves that can be changed with the adjustable drive. More particularly, an adjustable drive mechanism based upon a pair of crank linkages and belt assembly can be incorporated into several exercise apparatus to drive a load resistance.