Direct drive vs belt vs rim vs idler arm

Halcro -
I think your test methodology would prove that only turntables with error speed correction built in will pass. We know that stylus drag exists and should always set speed with the stylus playing. The question then becomes how significant is stylus drag variation, should we speed correct, and if so how. Remember also that the timeline is only a sampling of 1 per revolution, what is happening in between that interval is unknown.
You could have Turntable A that has a much bigger variation when dropping the stylus than turntable B but has a better instantaneous speed correction. Which is better for sound reproduction ?
We also know that the natural recovery of AC motors and Brushless DC motors if the motor "slips" due to back emf and goes out of phase slightly that the recovery is different - some say the AC motor recovers sinusoidally whereas a DC motor tends to recover trapezoidally. Can you measure this ? What difference does it make to the sound ?
Lots of unknowns here.

From Dover's post, "The point I take from your Goldmund example is that you are suggesting that mass alone will not provide stability with a motor that simply doesn't have enough torque to start with."

Let's think about this for a minute. Is the platter solid? Where is the bulk mass located? If the platter is solid, most of the mass is toward the outside, correct? Is that where it should be? One would assume that it should be because almost all turntable platters are made that way. But, it it really the right way to design a platter, regardless of the drive type?

Now, let's make some analogies.

1) You have a playground merry-go-round with six kids on it. They are positioned towards the outer rim.
2) You have a playground merry-go-round with six kids on it. They are positioned, so that the merry-go-round is perfectly balanced from center to edge.
3) You have a playground merry-go-round with six kids on it. They are positioned as close to the center as possible.

All the kids and all the merry-go-rounds weigh exactly the same. Which merry-go-round is easiest to control, if you are the guy pushing it? Would a scenario exist where it be possible for a small girl push one of them, but not the others?

I submit to you all that platter design is the most seriously overlooked aspect of a turntable. Location of mass matters, and it matters a lot. I consider the platter to be more key to the sound of a given turntable than drive type, or speed accuracy for that matter.

Disclosure: I manufacture idler drive turntables with speed controllers.

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Halcro/Tony

I've just done the Halcro drop test of dropping the stylus on and off without adjusting the speed. The fall off in speed is minute. I would estimate it would take a couple of hours at least for the error to translate to 1 complete revolution. So according to my rudimentary maths for my TT
1.8seconds per revolution/7200 seconds ( time to complete 1 rotation error ) = a speed variation of 0.025%. Even an hour would only be 0.05%.
( Tony correct me if I'm wrong on the maths, my brain is disfunctional at the moment ).
With the speed adjusted for the tracking weight there is 0 drift.
To me the big question is how consistant is the impact of the stylus drag on speed and if error correction is used, is the bark worse than the bite. This will no doubt come back to the quality of the particular solution offered.

Mosin,

Location of mass matters, and it matters a lot.

Interesting.......I must admit I had never really considered this?
It makes sense?

Mosin -
Absolutely right, I used mass loosely. I read somewhere the EMT idler has a platter of 5.5kg ( same weight as the copper mat on my deck ) but due to its weight distribution ( 16 & 1/2" platter ) is the equivalent of a 50kg solid platter. Although not as radical my TT has an inverted bearing with a substantial subplatter which is an inverted truncated cone (frustum), so most of the mass of the 20kg platter is between the inner and outer grooves & the thickness increases inside out ( and below the bearing point ). So yeah the effective mass of my TT platter is well in excess of 20kg.