VPI TNT Turntables

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.

Fellows,

I am no engineer (though I did stay at a Holiday Inn Express) but I have experienced how greater mass/weight has more inertia and would be much harder to change the speed than would one with less mass/weight.  For example, picture changing the speed on a playground merry-go-round that is empty, vs. one that is totally weighted down by a full load of equally distributed passengers.  It will take much more effort to slow down or speed up the heavier one.  The flywheel effect in cars and trucks is another example of the heavier, more massive one continuing to spin at a more stable rate, while the lightweight one will lose rpm more quickly.  I have noticed this too when sharpening tools on a blade sharpening wheel.  It takes much more force to affect the speed of the heavier wheel.  It seems to me that the heavier more massive turntable motor flywheel would be more stable and would be less susceptible to external forces, like fluctuations in power supply, intermittent friction, etc.  I am sure I'm wrong, please explain why. 

Well, from the formula, there are only 2 parameters that effect inertia, mass and diameter.  Diameter will have a greater impact because inertia increases with the square of the radius.  If your flywheel is 5" instead of 3", it changes the additional inertia from <4% to ~20% of the platter.  Maybe that's enough to make a difference?  The periphery ring is probably the better of the two as all of its mass is concentrated at the largest radius.

As far as the flywheel and stylus drag goes, it won't isolate the platter at all;  if anything it will very slightly isolate the motor from the incredibly small amount of additional torque, the platter will still slow because of the stylus drag.  With 2 belts, I would suspect that belt creep is higher than with a single belt, so speed instability caused by stylus drag would potentially be worse.

phoenixengr,

I am going to interpret that as an agreement to what I said.  All of my examples are increasing the mass and the inertia, right?  So the vpi flywheel would be more stable, due to its increased mass?  The more massive flywheel in motion would be more likely to stay in motion, compared to just a simple motor.  We are talking about the speed stability of the motor/drive system here.  You seem to be agreeing, but believe that the two belt implementation is faulty?