VTF measurments with a unipivot

Relative to perfectly horizontal, either up or down will decrease the VTF from that setting.

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.

http://www.tnt-audio.com/accessories/canrong_crb5_e.html

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?

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.