VTF measurments with a unipivot

I understand the VTF changes depending on the vertical position of a unipivot tonearm, and which way (up or down) in turn depends on the type of the unipivot (center of gravity in relation to the pivot point).

Can anyone clarify how the VTF changes in the VPI tonearms, specifically the Classic tonearm, i.e., whether the VTF increases or decreases as the tonearm is lowered? Thank you.
Relative to perfectly horizontal, either up or down will decrease the VTF from that setting.
On the VPI JWM unipivot arms, from your baseline setting, the VTF increases as the stylus height increases. The VTF decreases as the stylus height is lowered. This is presuming the VTA is the same.
Another way to look at it, from your baseline setting, if you have VTA tower, as you increase the height of your VTA tower you lower the VTF. If you decrease the height of the VTA tower then you increase the VTF.

Another way to look at it, if you want precisely the same VTF on 120 gram, 180 gram, and 200 gram records, then you would need to set the baseline for 120 gram, then increase the height of your VTA tower to compensate for the additional record thickness.

From memory, the differences are slight, something around 0.03 grams or so, from 120 to 180 gram.
Thanks Steve. Perhaps I should clarify a bit more. As you know, most scales do not allow taking measurements precisely at the record surface level. So if you measure the VTF higher than the surface, will it measure lower or higher than the actual VTF at the record surface? From what you said, assuming the tonearm is parallel, it'd be lower, but I've seen some posts here that seemed to suggest that the VTF is actually lower at the record surface than when measured higher and even a few millimeters can make a difference of tens of a gram.

Also, in the most recent issue of Stereophile, Fremer wrote an article discussing various unipivot designs and stated that whether the VTF will be higher or lower relative to where the measurement is taken depends on the center of gravity in relation to the pivot point. He lists three different types of unipivots, but his discussion is horribly lacking in my opinion. It is unclear to me what type the VPI arms are. Ignoring the types, I just wanted to clarify whether the VTF will increase or decrease at the record level if it's measured a few millimeters above the record surface.
Actusreus: There are probably two relevant pieces of information for you here with respect to your question.

1) Whether unipivot or more traditional pivoted tonearm, if you measure VTF above the height of the record you will end up with a reading that is higher depending on how high above record height you are taking the reading.

I've included a URL below to a review of the Canrong scale below, which is a scale that I've used for the past 4-5 years. Weigh platform on the scale is about 4-5 mm high so a couple of mm higher than a typical 140-180 gram record. I did some experimentation early on with my scale and figured that measuring with the scale would give me a reading that was about .015 gram high, which is not a lot but certainly is a discrepancy.

All things being equal, you should try and measure VTF as close to real record height as possible.

2) With respect to unipivots, though, and especially unipivots with low slung counterweights apparently, there are other issues at play. Don't ask me to explain why (I'll leave that to other physics experts here) but with a typical scale, even measuring VTF at exactly the height of a record will result in an overread of actual tracking force, possible by as much as .2 grams, which is much more significant.

I believe this is why VPI often recommends VTF which is at the higher end of the recommended range with most cartridges (generally a pretty good idea anyway but a particularly good idea with unipivots); I could be wrong but I vaguely recal reading somewhere a few times that VPI actually recommended a VTF for a specific cartridge that was slightly higher than the max VTF for the cartridge recommended by the manufacturer and this may well be the reason why.

Don't you start with the manufacturer's suggested vtf and then adjust by ear? Why the need to know the exact vtf to the nth gram?
Since the difference in vtf between a 120g ad 180g record is less than 2% do we really care? Since VPI arms lack fine and repeatable vtf adjustment the issue becomes a moot point.
Hdm and Captain Winters,
Thank you for your responses; this is exactly what I was looking for! My digital scale has the same design as the one in your link, Hdm, only is a cheaper version. It does seem to do a decent job with the exception of a negative reading as the cartridge approaches the dot. However, I get consistent readings both ways (I place the stylus on the scale and then turn it on; once I get a zero reading, I lift the tonearm).

Yes, I agree with you that the final adjustments should be done by ear, but Lyra cartridges have a very specific VTF recommendation for optimal performance. So before I start adjusting, I'd like to know what 1.75 g sounds like and take it from there. To get the best sound out of my Delos might be as simple as making sure the VTF is set up correctly.
Actusreus, you can always measure the distance from the plinth to the record surface, take the platter off the table and build a platform out of playing cards to hold your digital scale to equal the playing surface height desired.

Not sure if there is a difference at starting at 1.75g or 1.78g when you will make the final adjustment by ear.

I any event, good luck. Time spent dialing in a cartridge yields great results, although sometimes, it can be very frustrating.
Dear Actusreus : Cartridge set up always is important and critical to have a good quality performance level from the cartridge.

Now, IMHO it's almost useless to ask for the penultimate VTF set up in a cartridge/tonearm set up due that that " perfect " VTF set up never works ( not only because there are different thickness in the LPs. ) because each single LP is not perfectly flat so almost at each " groove " VTF changes " like it or not ".

Good that you are interested in " perfect " VTF but this does not works during playback and that's why cartridge manufacturers give us a VTF cartridge range. Normally I made my cartridge VTF set up at the middle of that range or a little over it.

regards and enjoy the music,
Thank you for your input, Raul. I think the "perfect" VTF in that sense is simply a VTF that optimizes the performance of the cartridge over the wide(st) range of recordings, similarly to the VTA. Jon Carr determined that for some of his cartridges, this is achieved with a VTF of 1.75 grams, and I trust his expertise. I find that the way I perceive sound varies from day to day depending on many factors so the by-ear method does not always work for me. I'm also more of a set-it-and-forget-it kind of guy. I'm not a "tweaker"; I want to spend my time listening to music, not analyzing the sound of every record.

Also, it seems to me that by your logic, you could never find a "perfect" (or perhaps "correct" would be a better word) VTF for a cartridge unless you'd be willing to adjust for every record, which is surely not true. There must be a VTF setting where any given cartridge performs best, whether it's a range or a pint point value, all else being equal. Again, if a designer of Jon Carr's caliber tells me it's 1.75 g, I listen. It may not be the way many audiophiles do it, but it has worked for me and my Delos so far.
Dear Actusreus: Normaly the cartridge manufacturer/designer as J.Carr makes the advise on VTF where the cartridge coils are centered and is a good trend try to be there. As I posted I try to make the set up at the middle of the cartridge VTF range after the other cartridge set up parameters.

As you said we can't stay changing the VTF every single LP and not only because " boring " but everytime we change VTF we have to adjust VTA/SRA and sometimes azymuth too.

We have to find out an average cartridge set up, IMHO these are the kind of trade-offs that we have to accept with an imperfect medium as analog.

regards and enjoy the music,
I agree, Raul. Perhaps worthy of noting is the fact that the new Lyra cartridges have a much narrower VTF range then most cartridges on the market. Most cartridge manufacturers seem to suggest a range of 0.4 grams and up, whereas Lyra range is 0.1 gram, at least four times smaller. So not much to experiment with, and the middle of the range, as you suggest, is 1.75 g.
Hi Actus,
Realising that I'm possibly highlighting my lack of advanced mechanical knowledge, could you try the following experiment?

Assuming the bearing is as frictionless as possible :

- Balance the arm
- Tilt the arm upwards by a couple of degrees and release.

Does the arm fall, as if due to additional VTF, then gradually normalise in the horizontal position, or does it remain where you stationed it?

- Tilt the arm downwards a couple of degrees.

Does the arm rise as if due to a deficiency in VTF or remain stationary?
Alternatively, if you don't wish to upset your current settings, can you recall how it behaved when you last set up the cartridge?

Please note every arm I've owned has followed the former damped oscillation pattern but I don't preclude design ingenuity! :) :)

To my inexperienced mind this should give a clue as to it's "loaded" behaviour but I'm probably mistaken. :)
Kind regards....