Belt stretch

Because we track our cartridges at anywhere from 1 gram to 3 grams or so, we tend to think that the braking action of the stylus in the groove is minimal and that a belt-drive is then able to overcome this negative force with ease. This is the first false assumption.

Stylus force drag is very serious indeed. I took the following from the website "www.Micrographia.com", (here's a link to Pressure) on the issue of the pressures involved: "Neglecting factors such as the elastic deformation of vinyl, the distribution of forces in a V-shaped groove and the accelerations at the stylus tip during tracking, simple calculation based on these figures gives a stylus pressure of 240 grams per square mm, or 340 pounds per square inch.

Johnnantais,
In a thread about belt stretch due to stylus drag you added the information above to the discussion, but either I or you are confused. You seem to think that the force seen by the belt and/or motor is something great, like 340lb/sq. inch. That is not the case.

A simple experiment (that you should not do!) is to put a 1"x1" block of wood on your TT, place a 340lb weight on top of it, hold it in place, and turn on the TT. Of course it's not going to turn at all. The pressures you noted above (if the calculations are correct) are only seen by the vinyl and stylus but never by the belt or motor. The forces needed to overcome the friction of stylus drag are minimal.

Here's an experiment that will be hard to implement but easy to think about. Imagine a cartridge attached to your finger with a string, tracking an LP that's turning on a TT. The cartridge would have to stay up somehow, of course. Use your imagination. Because there is no counterweight, also imagine that the 8 gram cartridge is not going to destroy its suspension and cantilever. How much force do you think you are going to have to use to keep your hand in one spot? You probably aren't even going to feel a pull on your finger... and that's with 8 grams of tracking force. Belts and motors don't see as much drag as you think.

FWIW, I don't disbelieve that belt stretch and rebound occur, I just think this is good discussion :) I do think that it's not as serious as some believe, though.

I find this thread very interesting. I thought I would throw a monkey in the wrench so to speak. We know for a fact that for a certain amount of mass, a belt that has a specific amount of elasticity and proper torque to spin such mass, we can produce a specified speed according to the accuracy of the drive source, albeit; digital control speed controllers or DD quartz reference. The monkey is how much more accurate does a belt drive system become when you multiply the amount of motors with a specific torque factor to spin the platter? We must also account for the amount of belt elasticity and compute that into any formulas for an accurate answer.

I would like to use this example, my Transrotor has three motors with three belts to spin a fairly heavy 80MM platter. With one motor, it could be done, however, I did notice a lot slower start up and some belt slippage at start up. With 3 motors and 3 belts, the belt slippage at start up is minimized and the platter is spun up to speed fairly quick. The speed maintained fairly accurate readinds, but the belt wore out faster, Note, I do use a quartz controlled speed controller motor assembly that I check with the Clearaudio 300MHZ stroboscope for accuracy. From my experience, the belts over a period of time will wear out and the diminished amount of belt elasticity will cause the turntable to flucuate out of speed tolerance and will eventually need to be adjusted via the speed controller. The formula for torque is: T = r x F. The other formulas needed to find the rest of the story can be easily looked up. Basically, if we have three seperate effects of torque acting on a stationalry mass that becomes rotational, it is easier to transfer said torque or power if you want to convert to power and cause the movement of the mass than with one moment of torque. The caveat is if the initial amount of torque with one motor is the same as the same with 3 motors, most likely not for a turntable. If manufacturers published their torque specs for the seperate motor assemblies in either newton meter or torque specs we could calculate the amount of torque that is applied to the mass and see how that would effect our perfect 33.3 rotational speed.

The talk about the belt stretch vs. DD is going down the tubes so to speak, much good information about both types of drive systems, though, many hard core audiophiles. I would state that whatever you like, just like it, and not worry about anything else. Just listen to the music.

We can get down into the weeds of this, I have not seen anyone speak about the effect of rotational torque with additional motors vs one on a platter and how that would help speed consistency with a turntable. Their are so many ways and methods manufacturers are using to get to the means to the end, it is making my head spin.

V/r
Audioquest4life

John, you have offered no measurable explanation and per Ketchups post misinterpreted the data. If thats your version of logic I doubt we'd gree on anything. My mind is open to the possibility of these effects but I dont think enough testing has been done to prove the point. Ill concede Im very much a believer in blind listening and measurable performance. I do alot of comparative listening. Ill add that consumer audio is dominated by myth and conjecture as well as the magnitude of effects being grossly exaggerated. I think that kind of thinking does a disservice to the hobby at large and so when I hear performance claims being attributed to specific systems I want to know that they are on the right track. Does that sound logical?
Mosin, Thats a very good point. I think the core question is what amount of speed deviation is audible, how can it be measured and what are the effects (dynamics feq response image ect) I think its a question worth pursuing and will be experimenting with it in great depth. Thanks for your input.

In defense of Rccc

Amazing how stylus drag can make a belt stretch even when the platter mass exceeds 10, 20 or even 30 pounds, yet somehow unclamped LPs on felt mat do not slip on the platter!

So funny how the direct drive folks boast about speed stability, but when you see their rigs many (if not most) do not use clamps. They seemingly find it easier to believe a 10 pound platter and belt drive has less rotational force than a 120gram LP coupled to a felt mat by gravity only. Mmmm.

I did an experiment some time ago. (http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1181854929&openflup&74&4#74) From my own observation LP slippage (unclamped + felt mat) for 20 min of play amounted to only 9.235e-5 % - less than 1/64 of an inch after approx. 16,920 inches of grooves.

I’m not saying folks don’t hear differences between direct, rim and belt drives, but something else is at play.

Regards
Paul

Thanks Paul I was startin to get lonely.