effective mass ???

The Grace 940 is an S-shaped tonearm with an effective length of 237 mm according to the specifications. If you look at the effective mass of several similar tonearms from the era (SME 3009 II, Micro-Seiki MA-505, Fidelity Research FR-64fX) they all run about 4-6 grams effective mass NOT counting the headshell. This is based on tests by Hi Fi Choice back in the 1970s and 80s. So figure that 5 gms is a good guess and close enough for the 940.

To get the total effective mass you then add the mass of the headshell (anywhere between 6 gm for the skeletal shells such as the perforated SME, to over 20 gm for the heavyweights) plus the mass of the cartridge. For example if your headshell weighs 8 grams and the cartridge 9 grams then the total effective mass is 5+8+9 = 22 gms. Note that the total effective mass is dominated by the headshell and cartridge weight, so any error in the weight of the tonearm minus the headshell is negligible.

The suitable cartridge weight range does NOT include the weight of the headshell. However, that range is likely based on the stock headshell, so if you are using a headshell that is significantly heavier, for example, you may not be able to balance a heavier cartridge even with the balance weight all the way back.

Jlin, thanks for a very informative post, I really appreciate it. I have been trying to understand this for some time now, and I guess it was an evolution of questions that brought me to an understanding. thanks again
richard

Jlin, Can you reference the article in Hi Fi Choice wherein they measured or in some way estimated tonearm effective mass? The Fidelity Research has a stainless steel armwand, or so I have read, that was quoted elsewhere to give a MUCH higher effective mass than 5 gm, in fact well over 20 gm. This is important because it catapults the FR from a low-mass category to a high-mass category. If the data in the Hi-Fi Choice article are incorrect for the FR tonearm, they may also be incorrect for the other tonearms mentioned. For the MA505, I know the mass will be very different depending upon whether one is using the optional straight arm tube (low mass) or the S-shaped arm tube (higher mass).