I've been watching a thread on DIY Audio entitled 'Angling for 90 Degrees'.
The objective is to design a tonearm system that tracks across the record always running at 90 degrees to the spindle, or put another way always parallel to the groove.
This can of course be achieved with 'parallel trackers' that move the whole assembly across following the groove but these tend to range from primitive to extremely complicated.
This thread is exploring the possibility of using a more conventional arm with 0 offset that uses a geometrical arrangement at its mounting to swing the arm slightly forward as it moves inward.
The friction or drag of the stylus provides the only energy to achieve this.
I have rigged up a rough and ready attempt of this to see for myself this action and can report that it is possible.
However to allow the tiny energy of the stylus to provide this movement requires very low friction bearings. For this reason my experiment can be considered a failure as on my set up there is just too much friction. But it kept me entertained for a while during the lock down !
From the photos provided however you can see that as the tonnearm moves across the record the articulated arms follow different arcs which ensure the headshell and stylus are always at 90 degrees to the line on the tape and therefore are parallel with the groove.
So the conclusion is in theory this will work but needs precision low friction bearings.

Very interesting.

Most other attempts swing the headshell, rather than the whole arm, which would require less energy I imagine.
e.g Garrard Zero 100, or the Nasotec headshell.

There was also the Worden tonearm that had an elbow type arrangement part way along the arm almost at the headshell which pivoted. Got good reviews at the time ?

There have been many arms with articulated headshells but these usually still require some form of 'anti skating' bias to offset the side force being applied to the stylus.
Advocates of linear tracking turntables will tell you that not having any side force being applied to the stylus is one of the main reasons for the excellent sound they produce.
What is trying to be achieved with this arm is the elimination of any side force on the stylus by having in effect a straight line from arm mount pivot to stylus at all times by moving the arm pivot mounting position as the arm moves across the record.
With a conventional pivoted arm ,as I'm sure you know, the stylus is only aligned correctly with the record grooves at two positions. A long arm reduces this error but introduces more weight.
With a conical stylus this is not really much of a problem but with more exotic stylus profiles this is a real issue. It does seem rather silly to develop very exotic and expensive stylus profiles knowing that with a conventional pivoted arm no matter how carefully you align them they will only be in the correct alignment at these two points.
Unfortunately most attempts at designing arms that eliminate this misalignment may be technically correct but end up being too complicated, heavy, or suffer from mechanical problems. Hence the almost universal use of conventional pivoted arms.

Bravo again Graham.

There have been a few attempts in this direction.
I once got a min-bearing from a model shop for something like this.

You may have seen Frank Schröder's 'LT' tonearm, which is designed to achieve tangential tracking with a swinging pivot - similar to your implementation.

Then, there is this incredible DIY (pivotted tangential tracking) design.

And then of course there are the Thales tonearms, commercial designs.

DarkLantern blog - darklanternforowen.wordpress.com

Martin Seddon (Azurahorn) over in Perth, did similar experiments, some of Martin's pics here:

(Martin uses a simple motor drive to shift the pivot, in order to achieve linear-tangential tracking.)

DarkLantern blog - darklanternforowen.wordpress.com

This is Martin's schematic drawing for his 'sliding pivot' tonearm:
(Which I don't think he'd mind me sharing, from the Joelist group.)

DarkLantern blog - darklanternforowen.wordpress.com

Thanks for the diagram Owen.
This clearly shows that for a pivoted arm to maintain correct alignment with the record groove at all times it must move forward as it arcs across the record.
The double pivot arms on my experimental set up achieve this ( in theory anyway )
Cheers.