VPI TNT Turntables

"Melm-  Quite similar to the improvement gained by using a speed controller. More of the same. "

That's interesting.  A 12 lb 3" flywheel has less than 4% of the inertia of a 20 lb 12" platter; it's difficult to see how that would have much of an effect on speed stability.  Maybe it's the additional isolation of having 2 belts, but then you double down on belt creep.  Any thoughts as to what is causing the change in perceived sound?

Flywheel speed?

I'm not an engineer.  Why don't you buy or borrow a VPI flywheel and try it yourself?  I'm not the only one to observe this; see analogluvr above.  As all TNTs (I think) came with a speed controller of some sort, anyone who has added a flywheel has done it for a reason.  If it didn't add additional audible stability and benefit we would have heard complaints well before now.

I have always thought that the flywheel, like a heavy (and rim-weighted) platter, is the more important component as instantaneous speed accuracy is more important (to me) than average speed stability over one or a small number of revolutions.

I have what I call a 3.5 model. Heavy platter, squash balls from bladders from springs, SME-V, no flywheel and a SDS. I keep the tripod set-up as I believe it evens out bearing wear. The sound is weighty and full but not ponderous as some report. This is the only TNT I’ve heard so I can’t compare it to anything. I do have a Trans-FI air bearing linear tonearm on a different VPI and it competes easily with some of the best at a fraction of the cost. You might look into that is you feel the need to replace whatever comes on the table. It is about $1,000 last time I checked and the designer, Vic, is really great in his customer support.
http://i3.photobucket.com/albums/y57/WntrMute2/photo-11a_zpsjvdghrdy.jpg

I'm not questioning whether it makes a difference, everyone would seem to agree that it does.  I'm interested in finding out WHY it makes a difference.

The reason posited by most is the increased inertia that a flywheel provides improves stability, but the math doesn't support that assertion.  For a solid disk of even density, the formula for rotational inertia is Ir=(m*r^2)/2 where m is mass and r is the radius (note that there is no speed component in the formula).  So for a 20 lb, 12" platter the result is 360 and for a 12lb 3" flywheel it is 13.5 or ~27x smaller.  Now, adding a periphery ring to the platter would definitely help more as the mass is concentrated at the largest diameter (although the mass is smaller compared to the platter).  I just don't see where a smaller, lighter flywheel has that much of an impact at least on inertia, angular momentum or kinetic energy.

FYI, a properly designed speed controller will provide excellent instantaneous speed stability, but long term speed accuracy is improved with feedback.

Like I said, I’m no engineer, but:

Lets assume that the platter speed is affected by the stylus drag, which itself varies according to the music being played. As there is no equivalent drag on the flywheel, small as it may be, it can contribute to a more stable platter speed.

Also, my flywheel has a diameter of 5".  

"a properly designed speed controller will provide excellent instantaneous speed stability" Yes, perhaps, but not excellent enough, since the instantaneous speed stability is clearly improved by adding a flywheel or a periphery ring.