The contact height (44) "h", is defined as the radial distance from the point of contact on the tooth 30 of the flexspline 26 to a point on the neutral surface 38 of the flexspline 26. The neutral surface 38 is a term describing the surface between the outside diameter (OD) and the inside diameter (ID) of the flexspline 26, (ignoring the teeth). It is assumed that the neutral surface does not change length during the distortion of the flexspline 26.