Why Do 12" Tonearms Cost So Much More Than 9"?

mijostyn

... The Reed 5T and the Schroder LT do not have fixed horizontal bearings. Both arms are free to move horizontally just like any pivoted tonearm.

That’s pretty obvious. But if the horizontal pivot itself is fixed, then the arm will generate some skating force. That’s explained in the link I provided you. Skating force is the product of both offset and the pivot itself, which is something you seem to overlook.

Skating force is dynamic. That the Shroder arm can be set so that it has zero skating force at one point in its arc of rotation does not mean it has zero skating force across the entire arc, especially because skating force is affected by other forces such as VTF, which will also vary across the record. Again, this force can be measured, so its silly to dispute it. And more information is in the link I provided.

mijostyn

The Reed 5T and the Schroder LT are very similar arms ... and if set up correctly do not generate any skating force. They have to be right on the tangent, the turntable has to be dead level and the record has to be dead flat. All of these parameters are unlikely to be met so there will always be a little skating force one way or the other but magnitudes lower than any offset arm.

I do not have firsthand experience with either of those arms and have not even seen them up close. However, any pivoted arm with a fixed horizontal pivot will generate some skating force, even though an underhung arm will show much less than any arm with an offset.

By way of real-world example, consider the lowly Garrard Zero100. Its zero-tracking-error "pantograph"- type pickup arm included an antiskate mechanism. Even then, it was understood that skating force was a byproduct of both offset and the pivoted arm itself.

I continue to be surprised at the level of confusion that exists here regarding pickup arm geometry, because all the math has already been calculated. Here’s a fair treatment on skating force that includes some excellent references.

uberwaltz

Simple math states that from a pivot point of A to a fixed point of B that the greater the length between A and B is then the greater the measured error will be at point B from a change at point A.

What "measured error" are you talking about?
If we’re talking about tracking error - deviation from tangency between stylus/cantilever and the record groove - the longer the arm, the less the error. That’s because the longer the arm, the greater the arc its pivot describes; the bigger the arc, the less the tracking error. It is simple geometry and the reason d’entre for a longer arm.
lewm

... the skating force would result from the constant fact that there is a head shell offset angle which is changing all throughout play.

Some of these pivoted arms have no offset. There’s still some skating force, by simple virtue it being a pivoted arm.

mijostyn

Schroder LT.
The Reed 5T is another example. These arms stay tangent to the record, do not skate and totally avoid the problem of very high horizontal mass which plagues the vast majority of designs.

Any pivoted arm is subject to skate, a force that is the result of both the pickup arm's pivot and its offset, if any. None of this is a mystery and an arm's skating force can be measured.

There have been a variety of "pantograph" type arms over the years, the infamous Garrard Zero100 being one especially unpleasant example. I don't have any experience with any of the newer tangential tracking efforts, which include the Nasotec Swing headshell and Klaudio arm. But in the past, a prime problem with these types of devices has been friction, so the cure was worth than the disease.