Kuzma 4Point vs Airline - is it worth the upgrade?

Aha! Finally, I understand it. Thank you Hiho for your pics and comments; they really helped me a lot.

In your post, you have "2points + 2points = 4points" pics. The "4points" pic shows the pivot structure with two vertical cylinders. The big cylinder is clearly for VTA adjustment.

The small cylinder is a hollow sleeve that encloses a vertical post upon which it rests. The hollow sleeve contacts the vertical post inside it at two points, as shown in the second "2points" pic, where two hands are holding an unpainted cutaway cylinder horizontally. The two points are (1) at the end of the post and (2) from the side, with a spike touching the post near the other end of the cylinder.

The first "2points" pic shows the arm upside down, revealing two small spikes (like rose thorns) sticking up. These apparently will rest in two cups, shown in the "4points" pic. The two cups are in the horizontal base attached to the smaller vertical cylinder.

Thus, the arm will rest on the two rose thorn spikes on the base attached to the cylinder. And these spikes allow the arm to pivot the head up and down. Additionally, with the base attached to the smaller cylinder, the arm can pivot horizontally because the smaller cylinder can turn about the post that is captured inside of it.

(As you point out, the partial cutout in the post, where the second "point" contact is made inside the cylinder, causes the horizontal pivot movement of the arm to be limited to only 45 degrees.)

I can also identify now the damping troughs, both of which are accompanied by screw-rods secured by knurled-wheel nuts.

Upon further thought, I perceive that the second "point" inside the cylinder must be weight loaded because the arm is resting on the base to the side of the cylinder. This would cause the cylinder to want to tip sideways, and the second "point" inside the cylinder prevents that, keeping it vertical.

In my view the use of the word unipivot is misleading - the 2 vertical bearings are like a double knife edge as in the early SME's except that they are points/cups instead of knife edge/groove. Unlike a true unipivot the whole arm tube will chatter with a low compliance cartridge..

Dover, the comparison of two points with a knife edge is flawed since the force is concentrated at two points, not over what is presumed to be a line but due to machining accuracy and tolerances cannot have the forces uniformly distributed.

In this bearing set up, the positioning of the two rose thorn spike tips must be exactly in the centers of the two cups in the base. In a normal unipivot, the spike point "finds" the center of the cup, but with two spikes, one spike will probably find its cup's center while the other spike will fall where it may.

Of course it is very important that the other cup be centered exactly where that other spike falls. If it is off by even a little bit, there will not be a properly rigid contact, and chatter could result.

In other words, the distance between the tips of the spikes must be exactly equal to the distance between the centers of the cups. They must not be off by even a little bit. I see from the first "2point" pic that one spike tip position appears adjustable since there seems to be a set screw for the spike.

Perhaps there is some step in the setup procedure that ensures both spike tips are completely centered in their cups.

Dover: "Unlike a true unipivot the whole arm tube will chatter with a low compliance cartridge.."

Conceptually similar to SME but I don't see how two needle points sitting on two cups with the entire mass of the arm preloading the bearings can chatter or chatter to the level of conventional gimbal bearings. I agree with Brian.

I did not say it is a unipivot but clearly and explicitly the designer had the concept of unipivot tonearm, which Kuzma also makes, in mind. Check specification page. (Sorry about the early post that I uploaded the wrong spec image.)

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