It is so nice to be popular. I draw the English majors like a magnet. Must be my crappy punctuation. If it were not for spell check I would be the laughing stock of this site.
- 101 posts total
- 101 posts total
Air bearing linear tracking arms can be made to have reasonably low moving mass, but, they lack the mechanical advantage of a fulcrum and pivot of a conventional arm, meaning that for any given effective mass, they do impose a lot more force on the cantilever to drag the arm to a new position than a conventional arm imposes in order to swing the arm around the pivot point. This is an issue even if friction is zero.
This is not the case with linear arms that employ a conventional pivot and a sensor that detects when the arm is out of linear position and then turns on a motor to move the entire arm assembly. But, as with every design, the motorized arm version has its own shortfalls, such as, vibration from the mechanism getting into the arm, lack of overall rigidity and mechanical grounding of tonearm vibrations).
The very short arm on some linear trackers (e.g., the ClearAudio arm), may give rise to another problem--a change in record thickness would mean a bigger change in VTA with a short arm than a longer arm.
The Schroeder LT is not a tonearm on a string design (I've helped set up and listened to a Schroeder arm-on-a-string and it is a good arm). It has conventional pivots, but also an innovative mechanism that moves the arm, including the pivot point, in a semi-circle to greatly reduce deviation from perfect tangency while not causing skating forces. Because it is using the drag of the arm tracking the groove to move the pivot, I don't know if it increases friction seen by the arm. The Reed T-5 uses a sensor to operate a motor to move the base of the arm to achieve the same kind of result as the Schroeder.
The bottom line is every type of arm has its pluses and minuses, and I haven't heard any type that I thought was obviously superior to another.
@millercarbon could you post a link to any article that supports or further explains your skating force theory.
It's not a theory. It is physics. It is so obvious that Michael Fremer throws it out there as an off hand comment. I can explain it faster than I can find his 2 second sound bite in his 90 min video. So you go find it yourself if it is so important to you.
Here's the physics:
Draw a circle. Draw it nice and big, this will help for later. Put a dot in the center. That's your spindle. Now draw another point anywhere outside the circle. That's your tone arm pivot point. Now take a compass, or a stick, ruler- anything nice and straight- and set it up to go from the pivot point to anywhere, but let's stick with roughly an inch, beyond the spindle. This is your overhang.
Now keeping the compass on the pivot point, swing it around across the platter until you get to the outside edge of the circle. Are you with me? Okay.
Now go to the point where the compass is on the outer edge of the circle and very carefully draw a line parallel, that is tangential, to that point on the circle. Got it?
Okay. That was all geometry. Now here comes the physics. The circle/platter is rotating. Rotational motion breaks down into vectors. The motion of each point on the circle breaks down into a vector that is pointed straight ahead, ie tangentially, and straight towards the center. Each and every point on a circle is the same distance from the center. Therefore the vector pointing towards the center is zero. The motion is entirely tangential.
There are other forces involved but this right here explains why it is that if you spin a ball on a string and let go the string, the ball does not spiral off it goes in a straight line. So your straight line tangential to the circle is the only vector, and this means at this precise point where the stylus is the groove is moving in a perfectly straight line. A line that is infinitely short, to be sure, but straight nonetheless. (And this is why Newton invented the calculus, but never mind.)
So now look at your drawing. Notice anything? You did draw it I hope. No cheating! You asked me to explain, I'm explaining. Draw the damn thing!!!
What do you see? What I see is a straight line coming from the pivot point to the stylus, and another straight line tangential to the circle, and they cross at the point of tangency. They cross. They are not parallel. Are they? No. They are not. If they were parallel there would be no skating force. They are not. Which way is the tangent line headed? Slightly away from the spindle? No. Slightly towards the spindle? Yes. Draw an arrow on it. There is your skating force.
What happens with overhang is the spinning platter exerts a large force pulling straight away from the pivot point, and also another smaller force pulling the stylus ever so slightly to the left towards the spindle. This is your skating force.
You can change the shape of the arm. You can change the offset. You can align the cartridge any old way you want. As long as there is overhang the inward vector will be there and that is your skating force.
Cognitive Dissonance... to say it again.
No logical, rational argument will overcome it, as it locks one into ones belief system.
MC, what we have here is just that now.
So how to overcome Cognitive Dissonance?
By telling the opposing party(ies) a good joke or tell them you love them! 🥰... i.e. to stop make those opposing feel defensive!
This, despite their stubborn opposition to logic, rationality, physics, geometry, newtons law, et al. Die using diverting arguments to obfuscate the issue at hand.
Then maybe, just maybe, the opposing positions will be actually prepared to hear you, and stop 'sand-bagging'.
Psychology is needed to let rationality come through!
It's the human condition - not taught during 101 Physics, Geometry and Maths. 😏
BTW, it also seems so many folk were *seriously* at odds with Newton at his time! (Must have known too few good jokes?) 😝
MC, It's a matter of vector algebra, adding the various force vectors results in a net side force that can only pull the stylus toward the spindle (in the case of an overhung tonearm), because the stiffness of the arm wand prevents movement in the actual direction of the major net force, which is toward an ever-moving point that is always pointed to the rear but to the inside of the pivot (with a pivoted, overhung tonearm). With an underhung tonearm, the direction of the side force actually changes from pulling the tonearm inward to pushing it outward, after the stylus passes through its single null point, where there momentarily is zero skating force.
"Each and every point on a circle is the same distance from the center. Therefore the vector pointing towards the center is zero." It's not that the two statements are wrong. It is that the two statements have nothing to do with each other. Moreover, an LP groove is actually spiraling toward the spindle or the label, so each and every point is NOT the same distance from the center. And there is a net vector force toward the spindle; we call it the skating force. (I know we agree on that, but you seem to lose sight of it once in a while.)
The ball on a string goes off into space on a straight line tangent to its circular orbit, when you let go, because you were applying a force that kept it circling, until you let go of the string. That is called a centripetal force. Because as Newton tells us, "every object persists in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it".
You wrote, "The motion of each point on the circle breaks down into a vector that is pointed straight ahead, ie tangentially, and straight towards the center. Each and every point on a circle is the same distance from the center. Therefore the vector pointing towards the center is zero." What?
The reason why overhung tonearms can never have zero skating force can be shown by the Pythagorean Theorem. As you say, tangency to the groove is what we are talking about, but we need tangency to the groove where the friction force generated at the stylus tip has a vector that passes back through the pivot point. Then and only then do we have zero skating force. Consider an underhung tonearm with no headshell offset angle that can achieve zero skating force at its single null point. In that one moment, the distance from the pivot through the tonearm/cartridge is one side of a right angle triangle (side a). The distance from the stylus tip to the spindle is another side of a right angle triangle (side b). And the pivot to spindle distance would be the hypotenuse of the right angle triangle, side c. Pythagorus told us that for any right angle triangle, c-squared = a-squared + b-squared. But if you have an overhung stylus, side a (tonearm effective length) is always larger than side c (P2S). So you can never achieve even a null point, let alone zero skating force, with an overhung tonearm, UNLESS you invoke a headshell offset angle. The founding fathers of cartridge alignment handed down to us a headshell offset angle, so as to achieve two null points across the surface of an LP. But they didn't give us any condition that satisfies what we need for zero skating force, because headshell offset per se causes a skating force.