Tstop4me, you said you had a different definition of "sweet spot" than I do. What is it? Mine is the place(s) that the rocker radius changes - e.g., between the main rocker and the spin rocker.
* As I previously posted, a proper response will require preparation of drawings, which I plan to do.
* In the meanwhile, I’ll provide you with a summary of major points. You’ll probably have follow-up questions. If you do, I’ll address them later in my more detailed response, which will include the drawings.
* You have previously referred to the transition point between the spin rocker and the main rocker as a “cusp”. “Cusp” has certain mathematical definitions, which I do not intend to get into [even mathematicians don’t agree on a single definition]. But I’ll simply give one example of a cusp that readers here are familiar with: that found in the trace of a three turn. In a well-executed three turn, the pattern formed on the ice looks somewhat like the numeral three:
3 (not to scale; not exactly this shape). The point at which the turn is executed (front-to-back or back-to-front) is a cusp. One characteristic feature of a cusp is that it’s a
sharp point.
In a skate blade, the only places in which we would want a cusp is at the tips of toe picks. We want the working edges of the blade (the edges that contact the ice) to be smooth along their entire lengths; hence, we do
not want cusps along the working edges.
* Your definition of a sweet spot leads to the following consequences: (a) The position of the sweet spot depends only on changes in the curvature of the blade. It does not depend on the position of the tip of the drag pick. (b) As the blade is repeatedly sharpened, if the original profile of the blade is maintained, the position of the sweet spot along the blade remains constant. (c) If the blade has multiple changes in curvature (such as the compound spin rocker of the Coronation Ace), the blade will have multiple sweet spots.
* From my reading of the figure skating literature, the “sweet spot” is defined in the context of a scratch spin. There are geometrical complications arising from interaction of a blade with an actual ice surface; I won’t address those here. Here is a simplified procedure that gives a close approximation of the sweet spot. Place the skate upright with the bare blade in contact with a flat surface (such as a wooden or plastic cutting board). Rock the skate forward until the tip of the drag pick just touches the flat surface. Look at the blade from the side. In the ideal case, the blade will touch the flat surface at one other point, the tangent point; this is the sweet spot on the blade [in an actual case, the blade will touch the flat surface along a region, rather than a point; but I won’t address this complication here].
* In the context of a scratch spin, if you don’t rock forward enough (drag pick not touching the ice), you will be spinning only on a single point further back along the blade. If you rock forward too much (working edges lifted clear off the ice), you will also be spinning on a single point, the tip of the drag pick. [Again, I’m using “single point” for a simplified discussion.] But if you rock forward just right (Goldilocks scenario), you will be spinning on two points, the tip of the drag pick and the sweet spot; consequently, the spin will be better controlled and more stable.
* As a consequence of this definition: (a) The position of the sweet spot depends on both the profile of the blade and the position of the tip of the drag pick. (b) As the blade is repeatedly sharpened, even if the original profile of the blade is maintained, the sweet spot moves backward (assuming you don’t trim the drag pick), because the position of the tip of the drag pick relative to the edges changes as the edges are ground down. (c) If the blade has multiple changes in curvature, there is still only a single sweet spot [here I’m talking about a normal figure skate blade profile, not something anomalous like a serpentine profile].
* Note that commercial blade maintenance gauges (such as Sid Broadbent’s Wellness Gauge and the PBHE Blade Curvature Gauge cited previously) use the definition of a sweet spot I’ve given. They therefore work independent of whether the spin rocker is round or flat, short or long. There is no error in accidentally characterizing the main rocker in the case of a short spin rocker; the tangent point is the tangent point, irrespective of the portion of the blade you
ad hoc classify it to be in. [Although the various zones characterized as “Good”, “Repair”, and “Discard” are based on Wilson/MK spin rockers.]