a conventional segment is connected to the body of the back up ring at a junction 66 and basically pivots about a point 106 from a solid line position 108 to a dashed line position 110 to engage the inside of the casing string 40 upon expansion of the plug 10 the solid line position 108 represents the centerline of the unstressed segment 62 and the dashed line position 110 represents the centerline of the stressed segment 62 when it abuts the casing 40 the angle 112 is accordingly defined by the length of the solid line 108 and the distance between the pivot point 106 and the inside of the casing string 40 in fig7 a segment of the improved back up ring 80 is connected to the body at a junction 92 and basically pivots about a point 114 from a solid line position 116 to a dashed line position 118 to engage the inside of the casing sting 40 upon expansion of the plug 10 the solid line 116 represents the centerline of the unstressed segment 88 and the dashed line represents the centerline of the stressed segment 88 when it abuts the casing 40 the angle 120 is accordingly defined by the length of the.