Anti skate and tonearm damping query

Oops

The Xs and Ys are reversed. Consider the arm lying along the X axis with the Y axis pointing down and it comes out right.

Sorry about that

Mark Kelly

Axelwahl

Go back and read my post 15 above this one.

Mark Kelly

Stringreen

That is not correct, stylus friction follows the classic Amonton law so the friction is independent of velocity. This can be confirmed by a modification of the test Doug Deacon used to demonstrate that Dertonarm's idea was completely false.

Using your blank disc and having satisfied yourself that skating has nothing to do with null points, set the antiskate on the arm so that the arm remains stationary at 33 RPM. Now change the speed to 45 RPM and observe the result.

Axelwahl antiskate is properly a function of offset not overhang.

Mark Kelly

Axelwahl

You are assuming attitude is important but it isn’t.

Consider the frame of reference of the tonearm with its pivot point as the origin and the arm lying along the y axis. The stylus frictional reaction force vector runs directly from the stylus tip in the direction of groove motion. The restraining force vector runs directly from the stylus tip to the tonearm pivot. The sum of these two vectors is the net force on the stylus.

The angle between the groove tangent and the x axis is the true offset angle. The stylus reaction force vector can be resolved into x and y components equal to its magnitude multiplied by the cosine and sine of this angle respectively. Since the x translational degree of freedom is constrained, the sum of forces in the x direction must be zero so the restraining force must be equal to the x component of the stylus reaction force vector.

For the y translational degree of freedom also to be constrained (eg the stylus not skip out of the groove) there must be a force which balances the y component, this force is either supplied by a reaction force on the groove wall or by antiskate.

IFF the force is supplied by reaction against the groove wall then the fact that that reaction force is not purely in the Y direction creates complexities but we can assume that the arm has been designed by someone who knew what he was doing so it has antiskate therefore we can ignore this: designing a pivoted arm with no antiskate is prima facie evidence of incompetence. The antiskate can be applied as a torque to the arm pivot or a force to a point somewhere on the arm, it’s all the same as long as the vectors resolve.

The argument from attitude rests on a falsehood which is that the stylus frictional reaction force somehow depends on the attitude of the stylus to the groove wall. A misalignment of say 10 degrees would result in a displacement betwen contact patches of about 25 microns for an elliptical stylus 7mil across, for a spherical sylus there would be no displacement at all since the contact areas between a sphere and a plane are always normal to the radius of the sphere.

This torque arm would result in a torque of about 0,5 uNm at 20 mN VTF where the actual skating torque is around 1mNmm, a difference of 2000 to one.

The issue which I’d like to resolve is that of the influence of stylus shape. Amonton’s law for rigid bodies breaks down if one body is much softer than the other, which is definitely the case with vinyl and diamond. The breakdown takes the form of a pressure dependent coefficient of friction; the coefficient decreases with increasing pressure – this seems counterintuitive but think about a car tyre: wider tyre, lower pressure, better grip. Since elliptical styli have greater contact pressure than spherical, the friction will be less for them and this was reflected in many tonearms having separate scales for elliptical and spherical styli “back in the day”. Check the owners manual for the Torens TD160 mk2 for an example. Since a line contact has, by design, a larger contact area the skating force should increase not decrease as our befuddled friend supposed.

Mark Kelly

Doug

I find your argument a little confusing. If one did as you say and merely twisted the cartridge in the arm while keeping the alignement of the arm WRT the groove tangent the same, you would create very little force, as my post above explains.

The force you do create would be because the cantilever was bent out of alignment and the suspension is trying to force it back. This force is very small compared top the true skating force and with good alignment it is even smaller.

Mark Kelly

HDM

The reason tonearm manufacturers put a scale on the antiskate corresponding to VTF is that the skate force is directly proportional to VTF, not that it equals VTF.

The constant of proportionality has already been factored in and the manufacturer based this on "average" vinyl composition and a typical contact area.

The difference between your old stylus and your new stylus will depend on the degree of polish on the diamond and the actual area of the contact patch. Note that with line contact styli the actual area changes with VTA so VTA and antiskate interact. BTW This is the only rational explanation of the effects of VTA I know of.

This also shows that even if one confines onesself to the realms of actual verifiable physics the situation "on the vinyl" is quite complex. There is nothing in audio which can't be explained with physics, its just that we haven't worked out all the physics yet. One thing is certain, if an explanation for an effect defies the known laws of physics than that explanation is wrong.

Mark Kelly

Perrew

Unfortunately I can't do that without revealing details of my tonearm's geometry which are not yet ready for publication.

Sorry.

Axelwahl

You have changed the offset angle (because the SME overhang adjustment works by moving the pivot not the cart)

Mark Kelly

Axelwahl

I have a simple test for you.

If possible, adjust your arm to zero overhang and check the skating. It will have changed very slightly (due to the change in offset angle) whereas if I read your theory correctly you would expect no skating at zero overhang.

Mark Kelly

Axelwahl

I think I have confused you by trying to simplify things. The angle I referred to as the true offset is the angle between the groove tangent and the line between the stylus and pivot. The frictional force acts along the tangent so this is the angle between the frictional force vector and the restraining force vector. The true offset angle varies with groove radius for ordinary pivoted arms and the offset angle of the arm is an approximate average of the true offset angles.

I can see how this is confusing and I should probably have used a different term.

Perrew

You are confusing force and pressure. The force is set by the VTF and the groove angle, so the sum of the forces on the sidewall is SQRT 2 times the VTF (for 90 degree groove). The pressure is this divided by the contact area. Larger contact area = lower pressure for a given VTF.

Mark Kelly

Perrew

I cannot see how you can get to that conclusion.

Axelwahl

I love how you can take a simplification and twist it until it becomes a complexity. Your jump from "no groove" to "no tangent" is unjustified.

The frictional force vector is in the direction of relative motion between the stylus and the vinyl. When the stylus is following a groove, this direction is tangent to the groove curvature. When the stylus is not following a groove, this direction is tangent to the scribed arc of the stylus on the vinyl.

The rest of your post rests on this false dichotomy so it also falls (except for the bit about our befuddled friend DT not knowing what he's talking about. That I agree with)

Mark Kelly

Ok I can see it now.

The coefficient of friction does indeed reduce with pressure but what we haven't established is that the pressure is a linear function of VTA. For that to be the case contact area would have to be constant and it isn't.

I'm not even going to try to explain why not, I have a reasonable grasp on Hertzian contact theory but not good enough to explain it to you. Google Hertzian contact theory and you might see why.

Mark Kelly

Axelwahl

You said no groove = no offset. That's not correct.

The tangent to the arc of the stylus traces on the vinyl does not pass through the tonearm pivot. Therefore there is a separate vector pasing through the same point (the stylus contact point) which does pass though the tonearm pivot. The first vector is the stylus frictional reaction force vector, the second is the tonearm restraining force vector.

The angle between these two vectors is the angle I described as the "true offset" and which we are now going to call something else, hopefully slightly less cumbersome than "the angle between the stylus frictional reaction force vector and the tonearm restraining force vector". As explained above, the fact that the stylus frictional reaction force vector doesn't pass through the pivot is what causes skating. That's all there is. Nothing more, nothing less.

Mark Kelly

Axel

I've just read your post more carefully and my first response was too hasty.

I'm not familiar with the term "tangent trace angle" but by your description it is what I described as "true offset angle".

Indeed we may have been suffereing from terminological incommensurability.

Mark Kelly

Perrew

No that's not what I'm saying.

I said I wan't going to try to explain Hertzian contact theory to you and I'm sticking to that.

Mark Kelly