Off the original topic, but hopefully answering a question posted here:
A straight tonearm or a tonearm that is s-shaped(but is in perfect lateral balance) that relies on the counterweight to be moved for setting VTF will NOT cause a VTF change when moved up or down because of differing record thickness, as long as a line connecting the center of gravity of the arm's mass behind the vertical pivot(endstub, counterweight...) and the center of gravity of the arm's mass in front of the vertical pivot(armwand, headshell cartridge...) goes right through the vertical pivot.
But, one can't overcome inertia, so on the up a warp, VTF increases, on the way down VTF will be below its nominal value, then, finally increase above it(think ski jumper when landing) before returning to the nominal value.
If the arm has a total COG below the pivot(due to a low slung counterweight on a unipivot arm or one used on, say, a Rega arm just because it is supposedly adding "stability"(which is what lots of aftermarket counterweight ads claim), the VTF changes are more pronounced due to the resulting restoring force(think pendulum).
A so-called "dynamically balanced" tonearm will not(!), contrary to what I've read here and elsewhere, maintain the same tracking force when used with records of different thickness. It makes no differences whether a coil or a flat spring is used, VTF must increase when the arm's front is raised(spring tension is altered). This is true irrespective of the mass distribution(see above).
The advantage of using a spring is that, while the VTF will increase more strongly on the way up the warp, it will not fall (much)below its nominal value when the arm is on the way down. Mistracking as a result of too low a VTF is less likely to occur.
This was mostly an issue with very high compliance carts(70s and early 80s). Other theoretical advantages are the reduced inertia on arms with reversed tension springs(Rega: zero = - VTF), constant VTF despite a non-level tt platform(not really an argument as any turntable should be perfectly level(but Dual loved to display their cardanically suspended decks with dynamically balanced ULM arms back then...) and the more reliable VTF setting on (European)broadcast turntables(no tools, scales or gauges required for exchanging the cartridge).
Lastly, a dynamically balanced arm with an undamped spring is something I'd stay away from....
Have fun,
Frank
