Why is the price of new tonearms so high

Well - since it is a dynamic model (or should be...) I still see excatly this dynamic aspect missing.
The down-swing and upswing of the pendulum (tonearm with cartridge at swinging end) is different in moment of inertia and force depending on the distance from the dead center of movement.
We are still talking different models.

Dertonarm

That is completely wrong (again). The moment of inertia of a rigid body does not change with movement.

Yes we are talking differnt models. Mine is a model of what is happening, yours is a fiction.

Well - to get as plain as you: I think your model is incomplete.
The resonance frequency of a given cartridge/tonearm combination can be altered by moving a fairly heavy cylinder further away or closer towards the pivot.
The total mass of the moving body stays the same - of course.
But - I guess neither of us has any problems if we do not agree about the model at all.

Axelwahl

Your suppositions are correct and the figures are reasonable.

If we model a 100 g counterweight positioned 50mm behind the pivot, its contribution to the effective mass of a 225mm arm is 4.94 g*. If this is the position for VTF of 20mN, it needs to be moved 4.6 mm to come into neutral balance. The new position will indeed make a higher contribution to effective mass, it becomes 5.89 g an increase of 0.95g.

This would increase the maximal tracking force deviation quoted above from 6.2 mN to about 6.4 mN, about a 3% increase.

* this ignores the moment of inertia of the counterwight about its own centre of mass but since this doesn't change with position it isn't important so I left it out.

Mark Kelly

Dertonarm

That is completely wrong (again). The resonant frequency changes because the moment of inertia changes. The effective mass is simply the moment of inertia divided by the square of the effective length.

The total mass is irrelevant to the argument.

As far as I can see my model is complete according to D'Alembert principle. If you can show me something I have left out and provide a reasonable basis for the claim I'm listening.

Mark Kelly