Dear Mijo, I never said the skating force was not affected by groove modulation. In fact, I agreed that it IS affected by groove modulation, which I described as “tortuosity”. My objection was to your repeated insistence that velocity or speed affects the friction force. It does not. Not because I say so, but because the equation for friction force says so. Thus we have to find another mechanism for the effect of groove modulation on the skating force that does work with accepted theory. I described two possibilities: (1) groove tortuosity causes momentary acceleration of the cartridge moving mass that also pulls the arm inward, adding to the skating force, and (2) Groove tortuosity causes mistracking. Mistracking causes momentary variation in VTF. VTF does affect friction and the skating force.
Wally, It is my impression that underhung tonearms elicit a skating force in BOTH directions. The force changes direction (where by "direction" I mean toward the spindle vs away from the spindle) at the single null point, where for one magical moment, there is zero skating force. So if you made a graph of the the skating force across the surface of an LP, for an underhung tonearm, you would get a straight line, give or take, that starts on the positive side of the X-axis, passes through zero at the X-axis, representing the null point, and ends up on the negative side of the X-axis.
Also, is it not the case that headshell offset angle does produce a skating force all by itself, at each of the two null points for an overhung tonearm, where the stylus is tangent to the groove but headshell offset still produces skating force?