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The Korf Blog

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10 March 2022
Antiskating, Part II
As we are waiting for the CNC tonearm parts to arrive, we have some time for another practical study. We've done Azimuth Adjustment, Tonearm and Cartridge Matching, Wow and Flutter. Now we're exploring antiskating, and this is the second part.

How Large is the Skating Force?
Let's take a typical tonearm (say, our own one) and see how much of a skating force (or torque) is being developed.

To simplify the calculations, we'll first do them for one of the null points, where the stylus is perpendicular to the groove.

In this case, the tonearm offset angle α is exactly the angle between the arm's effective length line SA and the projection of the drag force D.

If SA = 229 mm and α is 22°, the lever L (AA') is equal to 86 (85.78) mm.

But how can we calculate the drag force D?

AES e-library to the rescue! People have certainly asked this question before. And, of course, there's a very thorough December 1981 article titled "Determination of Sliding Friction Between Stylus and Record Groove". It shows that the friction coefficient μ for most records is close to 0.25-0.3. The measurements were done with a relatively "draggy" Shibata stylus, so I think we can apply them to today's cartridges.
Note: there's another excellent AES document with μ values for a wide range of cartridges, downforces and recording levels.
μ is nothing but the relationship between the downforce F and the drag force. So for a typical 2 gf downforce, drag would be 0.6 gf.

Sorry for doing all the calculations in decidedly non-SI gram-force [gf]. In this case, converting to Newtons and then back would just confuse everyone.
Now we have everything to compute the skating torque T and its projection to the stylus Fs!

$$T={\mu \cdot F \cdot L} \qquad F_s={T \over SA}\qquad \text{so} \;\; F_s={\mu \cdot F {L \over SA}}$$

And with our actual numbers:

$$F_s={0.3 \cdot 2 \cdot {86 \over 229}} \qquad F_s = 0.23\;gf$$

The side force acting on a stylus is approximately 0.23 gram-force when the downforce is set to 2 gram-force. If we look beyond the null points and account for a worst case ±1° tracking error, the skating force would change about ±0.01 gf — not significant at all. Remember, the starting torque ("stickiness") of good tonearm bearings is about 40 mg at the stylus. That's 0.04 gf, four times more than the change in the skating force over the whole arc.

The antiskating force should be exactly the same, just applied in an opposite direction. I wonder how much force actual tonearms apply with antiskating set to "2"?

Real World Antiskating Forces

There are a few curious Rube Goldberg contraptions that can measure the antiskating force. Dual Skate-O-Meter scale and Orsonic (Namiki) SG series of cute cartridge-like devices are the best known (but still very obscure). They are so rare that I have seen neither in the wild.

Fortunately, there are much easier ways to measure antiskating force, afforded by today's abundance and cheapness of tensometric (strain gauge) sensors. We've had one for decades, and measured the antiskating forces of (almost) every tonearm and integrated turntable that came our way.

Below is a short extract from our database, with names changed to protect the guilty.

Image source: https://www.hifido.co.jp/, Dual manual
The numbers "1", "2" and "3" correspond to the tonearm's scale. The measured forces are in gram-force.
You can clearly see that, while the average for "2" does seem to converge on the number we have just calculated, the variation between individual tonearms is immense.

What's more, the situation today is scarcely better. Modern tonearms are just as likely as something from the 1960s to get the antiskating scale completely wrong.

This means we can't fully rely on the tonearm's scale to set the antiskating right. What shall we do?

Come On, Tell Us Already!
I certainly will! The next post will have the full set of instructions on setting the antiskating.

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