Theoretical Best Unipivot Bearing Height

Theoretical Best Unipivot Bearing Height

Postby Redsfan2144 » Thu Apr 19, 2018 9:28 am

I own the Prime Signature and Sound-smith Nautilus. Going through the final setup of my cartridge, I've played with the female bearing cup adjustment a bit, and I'm curious on what theoretically would be the best position of the female bearing cup in relation to the stylus in the groove. I understand the 1/8-1/4" clearance between the vta tower and tonearm bell housing. But being that the pivot spike height and the female bearing are both adjustable, there has to be a sweet spot and mechanical reason to where the bearing and spike make contact.

Most tonearms strive for the bearing height to be parallel to record height, but due to the shape of this tonearm, that would be almost impossible. I feel that having the female bearing as low as possible in the bell housing would get the arm's bearing as absolutely close as possible to record height. Is my thinking correct here? Or would it be more beneficial to have the female bearing deeper into the tonearm bell housing to make the tonearm's center of gravity as absolutely low as possible? Even with the female bearing as low as possible, there still is a substantial weight below the pivot point, stabilizing the arm. And I also prefer this setup without the dual pivot mod. I own it, and I prefer the sound without it.

I'm just trying to get this cartridge dialed in once as perfect as I can, so I can set it and forget it. Thanks!
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Re: Theoretical Best Unipivot Bearing Height

Postby Peer Gynt » Thu Apr 19, 2018 10:22 am

Following the same logic that is used to defend the inverted platter bearing concept, the tonearm should Be more stable the higher that conical thrust bearing is in its housing. By specifying a relatively modest clearance at the bottom of the housing, it would seem that VPI recognizes this too.

Of course the proof is in the listening. As usual, YMMV.
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Re: Theoretical Best Unipivot Bearing Height

Postby Brf » Thu Apr 19, 2018 10:43 am

With a uni-pivot it’s a balancing act. You must have free rotation around the pivot axis. Stability is defined by the amount of mass and the position of the mass below the pivot axis. The perfect amount of stability would allow non-constrained movement of the tonearm, but resist rotational torque caused by the stylus in a modulated groove.

One of the real benefits of installing a dual pivot is that the free rotation is now restricted (constrained) along the longitudinal axis (azimuth) but retains free rotation in both the perpendicular and lateral axis.

I would go the dual pivot route, as it will yield a greater sonic benefit than a highly tuned non-constrained uni-pivot.
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Re: Theoretical Best Unipivot Bearing Height

Postby Redsfan2144 » Thu Apr 19, 2018 10:56 am

I think the dual pivot does well at constraining those movements. But I don’t know if the steel on steel interface is best. Loading the dual pivot is too imperfect to get right. It chatters on highly modulated grooves, to remove chatter, you have to load it with excessive forces resulting in increased friction. And it flat out doesn’t sound as well with that drag.

I’m ok with the unipivot wobble, it’s been ok for years and years with many happy customers. I am wanting to optimize using the arm as that. Just curious what is best pivot height to minimize that wobble. There has to be a trade off in adjusting pivot height in that you can slightly change the arm’s center of gravity by adjusting the bearing. But raise the bearing too high within the housing, it may not track as well vertically because of changing vtf over warps because the pivot is much higher the the stylus in the groove.
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Re: Theoretical Best Unipivot Bearing Height

Postby Peer Gynt » Thu Apr 19, 2018 11:02 am

I think the ears would be the best guide. It probably varies with cartridge mass and compliance.

It’s nice that experimenting is relatively easy on the VPI.
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Re: Theoretical Best Unipivot Bearing Height

Postby Brf » Thu Apr 19, 2018 11:59 am

Redsfan2144 wrote:I think the dual pivot does well at constraining those movements. But I don’t know if the steel on steel interface is best. Loading the dual pivot is too imperfect to get right. It chatters on highly modulated grooves, to remove chatter, you have to load it with excessive forces resulting in increased friction. And it flat out doesn’t sound as well with that drag.


I would agree that the "process" for loading the dual pivot is imperfect, but a perfect load can be achieved. IME, a dual pivot point load/force should be 60-70% of the cartridge’s vertical tracking force, which is enough to resist the rotational torque causes by the stylus. Stainless steel used as a skate plate and pivot point both have a very low coefficient of friction. Coupled with a ≈1.5g minimal load, the dual pivot resistance is a non-factor. You can test this theory by significantly lowering the dual pivot resistance with use of a dry lubricant (not recommended for long-term use) and you will not hear a difference.

Redsfan2144 wrote:I’m ok with the unipivot wobble, it’s been ok for years and years with many happy customers. I am wanting to optimize using the arm as that. Just curious what is best pivot height to minimize that wobble.


The more mass below the pivot = more stability

Redsfan2144 wrote:There has to be a trade off in adjusting pivot height in that you can slightly change the arm’s center of gravity by adjusting the bearing.


Correct, too much weigh below the pivot hinders free rotation and cause over dampening which will may result in loss of high end extension and inner detail

Redsfan2144 wrote: But raise the bearing too high within the housing, it may not track as well vertically because of changing vtf over warps because the pivot is much higher the the stylus in the groove.


Any adjustment to the bearing interface must be offset with a change in the vta tower height to maintain the same stylus rake angle. The stylus will track the same with regard to warps and change in VTF
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Re: Theoretical Best Unipivot Bearing Height

Postby Redsfan2144 » Thu Apr 19, 2018 12:37 pm

Thanks BRF, I basically have to find that balance between free rotation and stability. Since the pivot spike bearing point will be above the stylus, i’m guessing the center of gravity will be what I need to approximate to be level with the stylus. The center of the azimuth side weights might be that center of gravity i’m aiming for.
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Re: Theoretical Best Unipivot Bearing Height

Postby Peer Gynt » Fri Apr 20, 2018 9:26 pm

I set up a Classic 3 with JMW 10 3D tonearm today, and so I took special interest in where the inverted pseudo-conical thrust bearing rides in the bell housing.

It's awfully close to the level of the headshell in mine. The clearance between the bell housing and the platform is quite close... certainly no more than 3/16."

FWIW, the final result sounds exceptionally good, and I'm comparing it side by side to a Linn LP12 with Keel, Ekos SE/1, Radikal, Harban plinth, Lyra Delos, etc... in other words the deck next to it has an msrp more than 3 times that of the VPI.

People complain about the wobble of the uni-pivot tonearm. I am not getting wobble on needle drops, and the arm has not shown a tendency to become excited in normal play. It would not surprise me to find that most complaints about how the arm performs can be traced to set up issues.

The VPI Classic 3 with JMW 10 3D tonearm is an incredible bit of vinyl playback exotica. If I had it to do over again, I'd go straight to this deck, bypassing my quest for ever more complicated and expensive products from the UK. I'll be parting out at least one Linn LP12 during the coming year as a consequence.

So how is this related to the OP? Well, I said in an earlier post that the ears ought to be the guide to deciding what was best.

I'm now going to add that the eyes can also help. Mine is set up as from the factory, without changing the thrust bearing height. It tracks beautifully, it doesn't wobble. It works exactly as VPI said it would. Why mess with success?
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Re: Theoretical Best Unipivot Bearing Height

Postby Redsfan2144 » Sat Apr 21, 2018 8:01 pm

I went ahead and adjusted the female bearing cup to be flush with the inside of the bell as done at the factory. After doing plenty of research and some practical knowledge, I feel this is best and should not of been tampered with it to begin with. The tonearm achieves stability by placing the center of gravity below the pivot point. In which that center of gravity must end up fairly close to stylus height. There is very much weight below the pivot point, and it’s spread out via the side weights and rear counter weight as well. Having the mass further away from pivot may make more of of a difference in stability than how far below pivot. This leads to a higher lateral inertia which has less of an effect to the cartridge than what vertical inertia can do to a cartridge’s suspension/compliance. As brf pointed out, the further the weight is below the bearing point, the more dampening aka higher effective mass. Maybe this all makes more sense in my head than actually typing it up. But lesson learned, female bearing is best not bothered with unless it’s not flush with inside of bell housing as stock. Or if a cartridge needs additiinal stability but has low enough compliance to handle that increased vertical inertia.
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Re: Theoretical Best Unipivot Bearing Height

Postby tony22 » Mon Apr 23, 2018 8:08 am

I've wondered if there was a default "spike height" for the tip of the pivot post, measured from the mounting platform base. It kind of always bothered me that with being able to change not only the height of the bearing cup, but also the pivot spike height, there never really would be a predictable frame of reference for this sort of thing.
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