Richardkrebs,
A simple dynamic load test you might want to try if you havent already done so, is to hold your finger against the rotating platter while playing music.
I'm not sure how relevent this is - the force reative to the incremental change in stylus drag has not been quantified.
In my earlier post I quantified the impact on speed due to the stylus drag differential between stylus not playing versus playing, with my high mass thread drive. The Final TT has a 20kg platter with offset weight distribution, it is around 3920kg/cm vs your 1100kg/cm of the SP10mk3 (10kg platter). Most DD's would have substantially less than your SP10mk3.
The speed change with and without the stylus playing was approx 0.008%, so presumably if there were variations in the drag due to musical peaks and troughs then the variation it would be a very small small proportion of this.
You would have to agree this is tiny.
So the issue becomes what is happening at a micro level. The thumb test is not measuring this micro behaviour.
To me the nub of the issue is what is happening momentarily. If the direct drive has less inertia than say a high mass thread drive, then it may have a bigger momentary drop in speed, but its recovery might be quicker if properly designed.
In this scenario we clearly have big speed drop/quick recovery versus small speed drop/slow recovery.
Now an interesting comparison here is your old Goldmund Studio versus the Kenwood L07D - both DD with speed correction, but with different feedback behaviour.
Did your old Goldmund Studio pass the thumb test. I doubt it very much.
You could hear the poles ( 8 pole motor from memory ) as they slewed past the start line. The Kenwood L07D blitzed it in timing and microdynamics.
Ironically the Goldmund would probably pass the Timeline test as would the Kenwood due to the speed correction. As far as music reproduction goes they were on different planets. Interestingly if I recall correctly the Kenwood trumpeted "Transient Load Fluctuation" in their manual as opposed to "wow and flutter".
Again the method of drive/ control comes into play here and we each need to decide which does the least harm.
Here you seem to have shifted your stance from an earlier post of championing good design to one of "choosing the method of drive that does the least harm".
I would argue that it is the quality of the design irrespective of drive as in the Goldmund/Kenwood example above.
There are other factors in speed. I tend to judge a system by its speed and coherency. Many systems are not that quick or coherent and the speed and coherency of the front end is masked somewhat.
As Tonywinsc has quantified eccentric records can have a significant impact on speed and timing. Cantilever design now comes into play. I have a new Koetsu Black Goldline ( shelved ) and a friend has a new Koetsu Rosewood - both of them slew around on eccentric records. Having had many Koetsu's over 30 years I suspect they have screwed up the cantilever design integrity somewhere along the line.
Mismatched arms/cartridges in terms of compliance mismatches may well skew speed and timing. Interestingly Bruce Thigpen argues that his Eminent Technology produces less distortion on eccentric records than a conventional pivoted arm.
Stylus shape may well have an impact on speed and coherence. There is an argument that a spherical stylus will have less phase errors than a fine profile tip. Phase errors will skew timing.
Tangential tracking versus pivoted arms - again the tracking error of pivoted arms may skew phase & timing.
An example of this was my old Sota Star/ET2/Denon 103 Garrott - very very good speed and coherence despite the rubber band drive. Yes the motor regulation had been worked over to reduce cogging somewhat. But I am resonably sure that that front end had much better timing than say any Technics SP10mk2 or Goldmund Studio I've heard and I believe that the sum of the parts including TT, tonearm and cartridge all come into play if we want to talk about musical timing and coherence.
Let's face it no one wants DD done badly, Belt Drive done badly or Idler done badly and unfortunately most of them are, irrespective of cost. To use a hackneyed phrase - there are no free lunches when it comes to good turntable design.
