Improvement in the design of variable original radius chainrings for coupling to conventional bicycle cranks in order to apply traction, which primarily consists of its being provided with a plurality of possible anchorages to the crank, so that the user may adjust the relative position between the crank and the major radius of the chainring, and thus optimize the biomechanics of his own pedalling; furthermore, other design parameters are offered to endow this non-circular pedalling with leg speeds optimized within the limitation of pedalling with cranks at 180 degrees and therefore with a dead spot.