Dear Wally, From your excellent videos (excellent in terms of clarity and presentation), I get that when the line from the pivot to the stylus is perpendicular to a line drawn on the radius of the LP, there will be no tracking force. That is exactly what happens at the single null point one can achieve with an underhung tonearm. I also get your evidence that headshell offset angle has nothing to do with the skating force, but that is shown for a spherical stylus on a groove-less LP. (By the way, where did you get what appears to be a Columbia 6-eye LP with no grooves?)
Have you repeated the experiments with either a non-spherical stylus or an LP with grooves, say a 1kHz steady signal? I realize that with grooves one would not so easily be able to visualize the skating force. As to the video in Russian, the set-up is cool but the dialog is unintelligible to me.
Where did I miss your definition of "Effective Moment Arm"? I haven’t a clue.
From the point of view of pure physics, I still don’t see why an overhung tonearm with an offset headshell, aligned according to any of the popular algorithms, would not exhibit a skating force at either of its two null points that is for that moment due only to headshell offset angle. Maybe you need grooves and/or a non-spherical stylus tip to show that. I do agree with you that a lot of so-called authorities have run to far with that ball, saying that headshell offset is THE cause of skating, which I agree it is not.