Adding mass to the counterweight reduces effective mass. In this case getting the weight as close to the pivot as possible will minimize moment (MOI) and give better transient response, tracking. Nice arm.
Ah yes, I had forgotten that adding mass to the counterweight will allow it to be positioned closer to the arms bearings, thereby automatically lowering the arms effective mass. Elegant, and counter-intuitive (no pun intended)! Thanks fleib.
The question remains of whether or not adding mass in the vertical plane only, rather than in the lateral as well, will make any difference? I'm thinking of the after-market counterweights for Rega arms (by Michell and Groovetracer, amongst others), which place their mass both below the arm bearings and further out from them in the lateral plane, "longhorn" style. If I did the opposite, adding the mass not laterally but vertically only, right on the bearings vertical axis (running in a line from top to bottom on the back of the stock counterweight) will help better match the low compliance of the cartridge in that plane with higher mass in the same plane, leaving the lateral mass of the arm more-or-less the same, which matches well with the cartridges higher lateral compliance.
An underhung (as it were) counterweight lowers the center of gravity and gives a mechanical advantage to a static balanced system, in the vertical plane. Adding mass to the horizontal bearings has no consequence on VTF and can provide an extra measure of bass authority, although examples cited aren't heavy enough horizontally to be of consequence.
The Moerch DP8 seems to be a good example of mass added to the horizontal plane.
The Michell and Groovetracer counterweights are underhung and wide, for the reason you state fleib. And you're right, adding mass below the bearings should provide an improvement as the Zeta is a dynamically balanced arm---hadn't thought of that, either. Thanks again!
I had a dream last night, in which I saw the counterweight with a brass bolt sticking out the back (through one of the holes in its back plate, if you remember what a Zeta cw looks like), at the bottom of the circle. A number of brass nuts were threaded onto it, for fine-tuning the mass. They probably aren't heavy enough to add much, but the lead fishing weight should. I'll attach them on the underside of the cw, and that oughta do it.
**I had a dream last night, in which I saw the counterweight with a brass bolt sticking out the back (through one of the holes in its back plate, if you remember what a Zeta cw looks like), at the bottom of the circle. A number of brass nuts were threaded onto it, for fine-tuning the mass.**
Didn't see this post before now, you had a dream about my old Zeta?
It was mounted on a Studietto. That's exactly right. Put the bolt on the bottom and add mass until you get the proper weight with the counterweight in the most forward position possible. Tracking/tracing is improved with eff mass minimized. With no azimuth adjustment you'll have to shim, if necessary.