Dear Lewm: And not only that even with that force vector we can't be sure how it will perform on true playback. So this is faint statement too: theory with out any test that prove that " all " is solve through that plinth.
We have to take in count that we are " playing " with induced resonances/distortions at " microscopic " level not in the " macro " domain.
We need to know which kind of resonance/vibrations, at which intensity, at which frequency range are pick-up by the cartridge and how we perceive it through playback in our system.
Not an easy task and certainly can't be solve because of that " force vector diagram ". Complex because we need to separate ( totally ) those resonances/vibrations coming from the TT body and if we are using a plinth we have to separate the plinth ( stand alone ) either as the ones coming between the TT body and the plinth.
We need to separate from the other focus of TT/tonearm/cartridge system own resonances/vibrations, we need to identified and determine each one specific influence in the cartridge overall quality performance level and then decide how to " cure " if need it.
Is this faint?, certainly is and with out a serious scientific " process " the best we have is to try the non-plinth ( naked. ) alternative and judge about against the plinthed one: easy!, other " theories " as Lewm posted somewhere are only useless speculations, facts is what we need and the naked project is a non-scientific fact that at least put some light on the subject where each one " theories " can't do it.
Regards and enjoy the music,
Raul.