why is a 13" tonearm design really superior?

Tracking error is reduced with increasing length, but rigidity is reduced and weight increases. I use a 12" Shindo arm and love it.

Dear Thuchan: I agree with Plato, there are several factors that make the " quality performance " of a tonearm/cartridge combination.

I know very well the 8000 ST tonearm and one of its signature characteristics is that is " alive " against other tonearm designs.
It is a very well made and with a quality control second to none. Something that help to lower distortion is to put some kind of damping material all over the arm lift mechanism, I use sorbothane at both sides on the arm lift base.
I like this tonearm that miss an azymuth mechanism and VT on the fly to be " near perfect " device.

I run it with several different cartrdges and one of them that makes a very good match is the XV-1 by Dyna ( not easy to mount in the SAEC headshell. ).
With which cartridges are you running it?

Regards and enjoy the music,
Raul.

Dear Raul, we all know that there are several factors which make a good arm. this was not my question aiming at. But what you mention below is very interesting and I do agree with you on the missing features. I run it with a Titan i (see pic under my system) and an Eminent Ultra Bc. I am looking for another headshell to mount my XV-1s. Could you send me a pic of your sorbotane installation - thanks

"Why are todays tonearm developers not anymore going for a 13" or 14" design?"

I think this has to with cost, most people cannot afford the big tonearms. The 12" are more often then not well in the 4 figures.

The main reason for the extinction of long and super long tonearms our days is because there are a lot (expensive...) turntables out there which can not mount 13" and 14" tonearms at all.
There are already more than a few which have difficulties accommodating 10" and 12" tonearms (Basis Debut Gold, Linn Sondek, most plinth-type TTs - for instance).
Furthermore - of course - increased length is always a trade-off between rigidity and effective moving mass on one side and minimized tangential error and minimized VTA change during warps and uneven areas on the records surface.