Tonearm effective mass

Correction. If ALL you change is the counterweight (same cartridge, same mounting screws, etc.):
- a heavier c/w closer to the pivot REDUCES effective mass
- a lighter c/w farther from the pivot INCREASES effective mass

Effective mass is a function of mass X the SQUARE of the distance from the pivot, so changing the distance has a larger effect than a change in mass.

Aside from that, for practical purposes I agree with the rest of Raul's post.

Excellent clarification by Quiddity, thanks. I should have included the proviso that the two c/w's be of the same shape.

With apologies to my friend Raul, this question is not as simple as you say. You're correct that changing to a heavier c/w would increase effective mass (moment of inertia) IF you mounted the heavier c/w the SAME distance from the pivot. But that would unbalance the cartridge, so this fact has no real world application.

For the *same* VTF...
with the *same* cartridge...
for two c/w's of the *same* shape but *different* masses...
the heavier c/w mounted closer to the pivot will result in a lower moment of inertia than the lighter c/w mounted farther from the pivot.

Hdm,
You left out a key factor. The heavier headshell will be mounted at (virtually) the same distance from the pivot as the lighter headshell. Therefore, its greater mass will increase the system moment of inertia, quite substantially given how much heavier it is.

Read Quiddity's excellent post. A headshell is mounted farther from the pivot than a c/w, so a change in mass at the headshell has a greater effect on moment of inertia than does the corresponding adjustment of c/w position. Adding mass at the headshell and adjusting the c/w to maintain the same downforce at the stylus increases moment of inertia (as Raul correctly stated above).

This will be true whether you use the existing c/w or go to a heavier one, although in the latter case the increase in moment of inertia will be less, due to the effect that was the original topic of this thread.

Raul,

I agree that for practical purposes moment of inertia changes must be made at the headshell end. (Of course as you know, raising headshell mass can impair tracking performance over warps, there's no free lunch.)

Hdm,

The answer to your last question will be specific to the tonearm and cartridge. The difference in moment of inertia caused by changing counterweights (very small difference, as Raul points out) is unlikely to make as much sonic difference as the change in the system's INTERNAL resonance behavior.

I'm not talking about the change in resonance frequency of the spring-loaded cartridge/tonearm system that we've all seen the math for, and can demonstrate with the HFN record. Moment of inertia is of little sonic significance unless you make a BIG change. I'm talking about how the cart/arm reacts to stray energies in the audio band being fed from the cartridge into the tonearm. That will vary depending on the equipment.

The only way to know is to try, unless someone has tried the same combos before and can report.