The skating force phenomenon

At the beginning of last month's As We See It, I wrote that I've lately been focused on "analog things." I proceeded to write about refurbishing and modding my old McIntosh tuner. That's "analog thing" #1.

- BY JIM AUSTIN

So, what other analog things have I been focused on? Here's #2: I looked around and found no truly satisfying explanation of the skating force. In the wild—in hi-fi discussion forums, for example—you should expect most of what you read on this topic to be wrong, but it's not just that: Even tonearm manufacturers are often vague on this point.

The skating force cannot be understood without some analysis and thought, but my goal here is to keep things as conceptual as possible: no calculations, no fancy diagrams, just correct ideas that you can easily hold in your head.

Ultimately, the source of the force is the tonearm itself, though it starts with the frictional force between the groove and the stylus. That frictional force acts in the direction of the record's rotation, which is tangential (parallel) to the groove, with no component toward the center of the record, which is the direction the skating force is known to pull. That's the mystery: How do we get from a purely tangential force to a force that pulls inward? First consider another question:

If a frictional force is acting on the stylus, then why doesn't the stylus move? The question may seem dumb because the answer is so obvious, but it gets us closer to understanding the skating force. The stylus doesn't move because it's connected to the cartridge, which is connected to the tonearm, which is connected to the turntable at the pivot point. All those mechanical connections hold the stylus in place. The usual Physics 101 framing would be something like this: The tonearm applies whatever force is needed to offset that tangential frictional force.