Turn table speed variation question

Well, since someone brought up the timeline again... For sure DD turntable owners like to brag about how stable the laser line stays on the wall even when dropping a tonearm or two on the record. DD tables have closed loop speed control systems. Short of dropping a brick on the platter, the speed will stay constant under varying loads. The thing is, what happens between those laser flashes is just as important. Closed loop speed control systems have their own issues, like cogging or low frequency hunting that will affect the music playback. I'm sure some of the better DD tables minimize this; but the only way to know how well it does requires a scope.
Belt drive turntables sound more musical to me. Sure the speed stability is not as good as a DD under varying loads. And I know too that my belt drive has to run 5-10 minutes to warm up in order for the speed to settle in. That is because a belt drive turntable is typically an open loop speed control. If the load changes slightly, the speed changes. So it can be expected to have to adjust speed while playing a record if you want speed to be dead on. Once the bearing and motor warm up the speed is good and stable. The belt drive turntable sounds sweeter because the motor isn't hunting around a set point. The belt drive turntable also has many more options for isolation and vibration dampening over a DD turntable. As far as torque variation while playing a record with loud passages, I showed those calculations before. With the massive platters on most belt drives, the minute torque changes have little affect on speed. As others have already noted; the center hole is the big issue with vinyl playback. Speed control on any good turntable is easily an order of magnitude better than the affects of the center hole tolerance.

And on the subject of pianos- I agree with some of the insightful comments here. I have heard live pianos sound like they have a Wow & Flutter problem. I wondered about that and I think some of the comments here answered that.

Small correction: 5.2 minutes of playing time is 173 revolutions. You dropped the "1". :)

Keep in mind that closed loop speed control systems like used in DD turntables will compensate for changes in load (such as dropping a needle on a record or raising it) and depending on the amount of torque that the motor has- the platter may hold speed even with light pressure of your finger applied to the edge. But closed loop speed control has its own design challenges.
I suspect belt drive became popular because open loop motors w/o controllers is much cheaper to execute and high mass platters do a lot to smooth out the speed variations; not to mention easier to apply isolation and dampening from vibrations. In other words- belt drive offered more bang for the buck. In the end, given the resources and proper design execution, the type of drive system becomes a moot point. I think Peter's website, if successfully populated with data from various types of turntables will reach that conclusion.