bridge structures mounted on radial arms the shape and function of these bridges is better illustrated on fig1 and fig1 looking at fig1 the two bridges 72 and 73 can be seen in superimposed inactive position as one possible execution of this embodiment the lifters are single pieces of flat spring steel stamped and bent to a shape resembling the letter l the short portion of the l is furthermore bent to form parallelograms 72 and 73 with cutouts in the corners 76 81 these cutouts weaken the corners allowing the parallelograms to collapse under pressure the arms of the lifters and the bases of the parallelograms are stiffened by forming ridges or creases slots are cut in the upper surfaces of the parallelograms designed to engage the chain laterally one of the parallelograms is slanted and collapses towards a larger sprocket downshifting while the other is slanted and collapses towards a smaller sprocket up shifting fig1 illustrates a phase of downshifting where only the lower lifter 71 is active the chain 36 is caught in one of the slots and is displaced from sprocket 35 to the plane of sprocket 34 by the collapsed parallelogram prior to meshing the process is better seen in its entirety in fig8 fig7 illustrates an up shift where both lifters move in tandem with.