Geoch
As Dertonarm has avoided the issue yet again, (he only seems to respond to posts on which he feels he can appear to be right) and (if your question was genuine and not to provoke) perhaps I can help.
You said
I have a question for you (and it is a genuine one, due to my ignorance) :
The Talea, Schroeder, Clearaudio Satisfy are designed with adjustable arc on their rudimentary headshell. The Simon Yorke has circular headshell which does not shows any preference in cartridge angle. Moreover, there are some circular cartridge bodies also! How does the Newton's law applies there?
Newton (the famous hifi enthusiast), said that to all forces there is an equal and opposite reaction.Bear with me if you are familiar with the following, but I will say it for those that are not.
The friction of the stylus against the moving groove causes a force which is aligned along the groove and reacts against the arm mount. The resultant would pull the arm inwards and out of the groove, were it not for a force reacting against it, so a counter force needs to be applied in some fashion. VTF supplies some of this force and the remainder usually as an antiskate device of some sort near the pivot.
As you correctly state, Geoch, this force has to increase as the arm approaches the centre, because, while the overhang is constant, the radius decreases, therefore the inward force varies in proportion.
All good arms have antiskate/bias, adjustable in amount and in degree. Longer arms need less antiskate, as they have less overhang. (There are arms with zero or negative overhang (underhang). These have very small skating forces, but, unless fitted with a mechanism to maintain the cartridge parallel to the groove, will have large tracking errors.)
Whether a cartridge is cylindrical, cuboid, flower-shaped or whatever, is neither here nor there. It does not affect antiskate if correctly mounted.
The bottom line is that there is only a vector towards the pivot from the stylus, and a vector from the stylus along the groove, These result in an inward rotation of the arm.
It has nothing to do with cartridge offset angles, except inasmuch as the stylus contributes more or less to the frictional force along the groove, (eg elliptical versus conical) and certainly nothing to do with the cartridge body shape. I could explain further, but for now that is enough.
So, the question of round headshells, rotating headshells is all baloney in terms of antiskate. The arms DT mentions may well not have a specified offset and therefore provide a facility for adjusting whatever offset is desired, which is a good thing (although the way they do it doesn't look as smart to me as he seems to see it, as none of them apply the rotation at or around the stylus where it would be most useful (and where, should I ever (no! no!) design another tonearm, put it - (there's a hint for DT if he's designing an arm...god forbid)
Other stuff:
About weird shaped cartridges (in my opinion the result of designers /stylists who have more interest in being different, than in good, practical functional design)
Dert says
they avoid a pre-determined offset angle and thus are much easier to adapt to different alignments (calculations) without trade-offs due to the alternation of a "pre-determined" offset angle by a cartridge's body aligned in a different angle.
Well, he would say that, given his commitment to arc protractors: the more weird shapes there are, the harder it is to align using anything else. Cartridge manufacturers must be rubbing their hands at the prospect of being able to mount generators and cantilevers any old way...
Dertonarm says
Circular bodies further reduce the problem,
which problem? he doesn't say, for it isn't antiskate.
but - unless they follow the Ikeda or DECCA/London cantilever-less principle - there is still the line of the cantilever which should be in line with the offset angle.
I hope every reader out there (with a Decca (or otherwise)) sees this error for what it is.
The Decca does not have a cantilever in the accepted sense, but has the equivalent nevertheless - the armature - it is just not visible. So it must be aligned like any other cartridge. If it wasn't cantilevered one could mount it reversed - try that with your client's Decca, Dertonarm, and you'll probably get a slap round the ear for being so stupid...
Also when we talk about cartridge offset angle, that term is usually accepted as encompassing the generator, cantilever and stylus, which ideally should all be in line. Errors here are an issue for cartridge designers and manufacturers. By all means line up to a cantilever, but that doesn't mean either the generator or stylus are lined up with the cantilever.
Geoch, Dertonarm shows his ignorance and arrogance by his response to you. Instead of explaining something he asks you for your ideas, perhaps because he is not truly clear in his thinking, as in the case of SME arms.
I realise your wisdom in avoiding getting too involved, but avoidance of bullying, for that is what it is, can lead to unwanted and unpleasant consequences, both here, in a small way, and, in the greater world, in a big way.
