How to measure tonearm effective mass

Lew, All I can contribute on this subject is the result of modifying a linear arm to allow micro adjustments to effective vertical mass across a wide range of adjustment. This set-up separates observations related to changing effective mass from observations related to changes in wand composition or length. The effects are remarkable. So far in limited experience with a few cartridges, small changes in effective mass have more impact than VTF on taming subtle tracking problems and improving LF performance. The granularity and wide range of adjustment necessary to optimize each cartridge, suggest that the broad categories of light, medium, and heavy arms may be too coarse to consumate perfect marriage between arm and cartridge-- unless achieved by guess and by gosh.

Mark, My tonearm has the front and rear counterweights riding on separate threaded rods going into the pivot and independent of the arm tube. Thus adjustment of counterweights does not affect the secondary resonance characteristic associated with wand composition or length. Also, its linear design allows separate manipulation of horizontal and vertical mass. Thus far I've found that the vertical mass is the more critical adjustment. Up to two-thirds of the assembly's total moving mass resides in the counterweights-- allowing a wide enough range of adjustment to characterize the tonearm as "universal."

Dan_ed, Indeed a split rear counterweight allows some adjustment. The question is whether the range of adjustment is sufficiently wide for a particular cartridge. In addition, with such an approach it is not possible to separately adjust horizontal and vertical mass; the adjustment of one affects the other.

I would add that with any long pivot arm, the rear counterweight(s) is in the final analysis the slave of the wand/cartridge lever. With a linear wand it is possible to unpack and play with each variable separately.