Skating force is the radial component of stylus friction. The stylus friction vector is tangential and can be resolved into orthonogal components, one running through the tonearm pivot and one running radially. The value is sintheta x stylus friction where theta is headshell angle.
One of the tenets of sliding/dynamic friction is that it is nearly totally independent of sliding speed. So friction is nearly equal whether at 45 or 33. Therefore no change needed.
Skating force has nothing to do with "centrifugal force" or momentum, or anything else other than the radial component of stylus friction. I've read many reviewers, manufacturers, and end users theorizing that some centrifugal force is at work, which is a totally different thing and has nothing to do with a tonearm tracking a record. Course, in audio everyone seems sure of everything. Beware: when someone says "it's simple physics", my experience is that they are about to spout total nonsense! |
Dear Lewm, did you note that my "centrifugal force" is in quotes, thus trying to indicate I am quoting one of the many know-nothings that spout phalse physics ad nauseum, and not using it as a real term?
Now that you'll noice my quotation marks and realize I was showing the ignorance of folks who use the term, could you not now respond and say I spelled "phalse" incorrectly? It's a little pun thing.
Smiling.....not indicating you are pedantic....but you did use the descriptor first! |
Let's put some physics and math to it for Ralph.
1. Skating force is always in the same direction, never changing from + to -. If we denote the inward radial force (skating force) as positive then it remains positive.
2. The magnitude of the vector changes in relation to the error in tangency of the stylus to the groove. Since the angle relative to tangency of a pivoted arm changes then the magnitude of the skating force changes.
3. Though skating force always remains positive (in my convention of positive meaning toward center) the DEVIATION in skating force goes from positive to negative to positive across the record as it is played from the outer groove.
However, the deviation in his force is pretty small, given that skating force can vary from record to record a bit anyway, and given that the antiskating mechanisms of arms are not perfectly linear.
4. The magnitude of change in skating force for an arm with a 23 degree headshell angle would be +7%, -4%, if the arm uses Baerwald geometry. The percentages are deviations from when the arm is at true tangency (null points).
5. Let's look at the magnitude of those percentage changes. Based on my detailed and repeatable experiments to determine actual skating forces I am confident we are looking at deviations of approximately two HUNDREDTHS of a gram! A small Post-It note weighs TWENTY times that!
Now, this is where experience can be helpful. Try as I might, I cannot reliably detect a sonic difference by changing the antiskate force by those amounts. I cannot detect a tracking ability change by changing antiskate by those amounts.
4. With poor tracking pivoted arms, yes, tracking is worse at the center of the record. But, since the best arm I can find is free of this, I have to conclude that there are many problems at work with those arms, including geometry (one low cost, ubiquitous brand uses their own geometry that nobody can figure out), resonances, and other matters that I'm not about to spend time figuring out.
Based on all of the above, I have concluded that variations in skating force across the record due to geometrical considerations are not a material concern in the sound reproduction of a pivoted tonearm. |
@ Ralph: You ask "Also, if the platter is at rest there will be no skating force at all. That was the basis for my response above; are you saying there is zero(0) change regardless of rotational speed?"
Answer: The difference in the rest condition and the dynamic condition are not relative to each other. Skating force is the result of dynamic friction, which requires movement. So the rest condition is irrelevant. One cannot extrapolate changes due to increases in relative velocity based on the change from no velocity to some velocity. The comparison between the no velocity condition and some specific positive velocity condition is a non-sequitur. That would be the case even if dynamic friction magnitude were dependent upon velocity. Fact is dynamic friction is nearly independent upon velocity.
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Going to try to be bowing out from the forums now guys. I see that they are addictive to me! Wow, it takes a lot of time to try to be clear, and I have my real job, research and design to do.
Try to remember, when you think you know it all start looking for your mistakes! You'll find them. |
Plain wrong statement: "45 presenting the cartridge with the problem of tracking a groove which makes the stylus accelerate at a greater rate"
This greater acceleration does not exist. And more energy is NOT being put into the system or the volume would be louder on a 45!
This poster is confusion lateral and vertical groove velocity with linear groove velocity.
In reality, for the same lateral or vertical velocity (the signal-generating directions) the angles on the groove are more gentle, as the same displacement of the stylus in the vertical and lateral directions happen over a greater (linear) length of the record groove.
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Just read the antiskate blog. There is no centripetal force in any part of the system. The guy is taking the facts that the record rotates, that an arm is on the record, and falsely extrapolating the conclusion that there is centripetal force. Does not exist in this system.
Could not find the Gilson papers, if someone wants to give a link I'll read them. |
Misunderstanding: it was the blog about anti-skate that an above poster referenced which mentioned centripetal force, which makes the science credentials of the blogger suspect. |
1. The difference in sound between 33 and 45 has nothing to do with the amount of energy being put into the stylus or the energy in the system. It has to do with less information density on the groove. There is greater length groove per second of musical signal. Lower information density, which is storing the same information in a greater storage area/length/volume (depending on the technology) gives greater fidelity. This is true in records, tape (30ips instead of 15ips, for instance), photography (a 4" x 5" negative compared to 35mm, which is 1" x 1.5", movie film....any analog medium.
This is a huge thing, and it is why more tape run by the head, more groove length under the stylus, and greater info on a negative enlarged to the same size all yield great improvements. The record does not play louder with a 45, which it would with higher modulation, the tape does not play louder.
2. As for energy, you are mixing energies. Energy put into moving the cantiliver is the same or volume would be louder. Kinetic energy of the platter is higher, yes, but that has nothing to do with playback and has nothing to do with more energy put into the cartridge, which is not done.
But, there is more kinetic energy in the platter. You are wondering about it. Well, where it came from is the energy put into it by the motor. That kinetic energy came from the torque required to get it up to speed over the period of time of acceleration. That energy is = i(w x w) where w (omega) = rotational velocity and i is the moment of the inertia of the platter. That energy stays constant, and when the switch is turned off the energy gets bled off, turning into heat, in the bearings and motor "off torque". That amount of energy during deceleration = the amount of acceleration = the kinetic energy while the platter was turning. Thus, energy is neither created nor destroyed, but is conserved, only changing forms. Newton is satisfied. Or at least his laws are, I trust he is dis-satisfied about how much physics is twisted and mutilated in this industry!
And finally, here is the bit of wrong information on the forum John sent us to, written by a "John" on that forum.
There is no centripetal force involved, as the arm is not rotating with the record. Now, if we put the whole turntable and tonearm mechanism on a rotating sub table and rotated the subtable then the cartridge would indeed try to fly outward.
But I know of nobody spinning their ENTIRE turntable setup while playing a record, so there is not such force.
It is a great optical illusion: stylus on a rotating record, so it must fly outward! False illusion. Spin the platter faster and faster and faster and there still is no centripetal force: only the platter is spinning! There can be no such force as the velocity of the stylus/cartridge/headshell in the tangential direction is essentially zero.
Don't worry about getting this wrong, most first year physics students would. A lifetime studying and working in physics and engineering: there's no substitute for that plus the education beforehand. Nobody should feel bad about that, I do not feel bad about not knowing as much about their respective fields as my doctor and lawyer do! |
Timetel, please read my earlier entries on this thread where I explain the formula for skating force. Skating force remains the same since it is based solely on the magnitude of friction and the offset angle of the tonearm. Same tonearm, same friction, same skating force, so anti-skate should remain the same.
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Here we go again: VTF cannot be different for each channel. Left wall and right wall are identical - how could they not be given that they are cut by a single cutter?! Each nudge, each bump, each movement of the inside groove wall is duplicated on the outside groove wall. This is the Westerex system.
Horizontal movement is L + R channels. Vertical movement is L - R channel. 45 degree orientation of coils give the sum and difference results of L and R. |
45s are NOT cut with greater modulation (if they were the sound would be louder through the speakers), the goal of the mastering guys is same volume, so the idea that different anti-skate is required is far fetched. Same anti-skate. |
Tracking angle error is negligible in the calculation of anti-skate, the magnitude of changing force being very small, tracking error only being +/- about 2 degrees even for a 9.5" tonearm. No arm's anti skate mechanism is within that range of accuracy, you can ignore that amount of change (that due to tracking error).
I have designed several arms, tested nearly any arm you can name, have dynamometers (force gauges) that can pick up the actual numbers. The theory coincides with actual measured results quite well.
We have some folks who are obviously not science people here (meaning not physicists, not engineers) "correcting" good information with misinformation. Overhang is not part of skating forces, RPM is not, 45's are not cut at a higher groove velocity (groove velocity that creates sound is vertical and horizontal movement), it is just the linear velocity past the stylus that is increased.
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Rtilden, you surmised correctly that I did not want to respond, it's not worth it. I have graded students that have been sure they have been right until I drew a free body diagram showing all the forces balance, in front of them, and verbally went through it. Then in real time when they offer up their view, I can in real time show the mistakes. This is not feasible in a thread on a forum where you cannot include drawings such as a free body diagram with a vector analysis.
I also realize that I was wrong in thinking everyone wants to learn. Some would just as soon remain ignorant, as long as they can convince some others they are right.
Thanks for your note!
So, over and out for me. Thanks much for your note though. |
