I watched your movie. Looks like it proves your point but I still have some doubts. In your experiment you have used flat record, so here is no grove that holds tip in place. So effective vector of the force on needle tip in this experiment can be completely different then one on real record.
Overhang arm with no offset angle and at least one null point doesn't exist. This kind of arm would have no null points at all. Aligning cartridge (other then keeping cantilever aligned with tip to arm pivot axis) creates offset angle.
Straight arm is not no offset angle arm if cartridge is...
Are there any tonearms with overhang and whithout offset angle? I don't think so, Otherways they would have no null points at all and massive tracking error (for 9 inch arm close to 20 degres, so close to offset angle of 9 inch arms).
That is why (offset angle) for overhang arms, skating force...
I completly don't agree with that.
Underhang arms basicaly don't have any offset angle. So on one hand skating force is much lower for them and changes in direction so compensation would be too complicated.
But I will ask, how? What is the mechanics of overhand influenced skating force?
If you...
On one hand you are right. But on the other skatikg force is in function of offset angle + tracking angle error. On tonearm with offset angle it's around 20 degrees +/- error tracking angle (lets assume maximum 3).
On tonearms with no offset angle this is much lower cause it will be as you wrote...