Figure 4.
The two-root tooth as a second example of a suspension without a CR. Figure on the right is a magnification of the circled object on the left. The wire-framed sphere is the region in which points can be found so that some properties of a CR hold true, but not at the same point. The surface on the right is that of all base points for screw axes arising from pure torques in all directions. In the case of the counterexample, the upper and lower burrs of the surface would correspond to the lines of action of the elastics.
CR: center of resistance.
