What to listen for when setting VTF?

I've read that folks fine tune VTF by ear, so was curious what you listen for. How does more or less VTF affect sound? Thanks.
Usually the lower the arm in the rear the thicker the sound and vice versa as you go up. A good starting point is arm just slightly down from parallel in the back.
If the arm is to high in the back, the treble will be bright and could have "sibilance" if to high. Also, bass will be very thin and lightweight.

If the arm is to low, the treble will be recessed and light in detail. Bass will be "boomy" and bloated.

I agree with the starting point a little lower than level.
The responses above reflect VTA (angle) changes but Gbb is asking about VTF (force) changes.
Sorry, as usual I can't read and/or comprehend. Below is a great link with info:

VTF and VTA are interrelated variables. As you change one, it affects the other. Once dialed in, minor changes in VTA won't require readjusting VTF (thank goodness for those of us who change VTA for every LP). But changes in VTF always require a re-listen for correct VTA.

To the OP's specific question on VTF, I highly recommend reading the "what to listen for" process posted by Lloyd Walker on his web site: Fine Tuning Your Turntable. I've been using this process for years with great results.
I don't understand how people can change VTA for different Lps. Wouldn't that change overhang?
My experience with the UNIverse cartridge is that a lighter VTF works best. I used a method similar to one that Dougdeacon reviewed in an earlier string. It basically is to reduce VTF until mistracking occurs then increase it slightly. for the UNIverse, it resulted in a tremendous sound stage (resolution between the tracks or instruments/vocals) and clean/crisp notes versus some smearing of the musical notes. I found that increasing vtf did smear the sound and collapse the sound stage. As stated by rushton, VTF does require changes to VTA so my intial method (when i install a cartridge) is the get the vtf close, then optimize the VTA, go back and retune the VTF and finally reoptimize the VTA. I do this once then adjust vta only for various record thickness or when my ears tells me something has changed. I have found that sometimes ambient conditions (colder/warmer in my listening room) can impact my vta settings. As for other cartridges, (my experience with the airy3 or frog) was not nearly as dramatic as the UNIverse. The frog liked a slightly heavy VTF whereas the Airy3 had a wide spot concerning vtf. The Airy3 is very sensitive to VTA but fortunately has a pretty large sweet spot. In all cases, the ear is what i use to tell me when i am there. It also takes many attempts over several days to get it right as I have several times said "its perfect" only to come back a day later at tweak and find a better setting. My experience is heavy on VTF tends to augment bass and mids and dull the highs whereas too light on vtf results in a loss of bass, smears the mids and makes the highs shrill or tinny. THe other comment about overhang is true particularly when you move vta a lot. For very small changes in VTA, i would not even check overhang. As always, my experience may not be the same as others
Nice detailed post. Thanks for answering my question about overhang.
My system, VPI Scoutmaster with Zu Denon 103 sounds the best when the VTF is 4.0g (violin without distortion)???? What am I doing wrong? Would it be wrong to leave it that high?
"My system, VPI Scoutmaster with Zu Denon 103 sounds the best when the VTF is 4.0g (violin without distortion)???? What am I doing wrong? Would it be wrong to leave it that high?"

Picture your cartridge as a plow and your vinyl record is the snow.

That is more than double the rec tracking force, I can't imagine that is very good for your records. I would think that something else is off requiring you to over compensate with the extra weight.

If it were me, I would start by leveling my platter and start set-up from scratch. That's way heavy.

Are you certain you're actually playing with VTF at a true 4.0g? How are you measuring? The dials on some tonearms are notoriously inaccurate. Some Shure balances were (idiotically) manufactured of magnetic steel, which exaggerates the measured downforce. Etc....

Unless you're weighing with a reasonably accurate scale, you can't be sure you're actually playing at that seemingly outlandish tracking force.

But if you are, I agree with Zenblaster. Something is very wrong if you have to play a Denon 103 at 4.0g to sound good. What happens if you play in the normally recommended range? What sounds "bad" about it?

Stop! In the name of love - for your LP's, not to mention your 103.
I got a digital gauge to measure the VTF, I am pretty sure it's at least in the right ballpark. It still sounds good at 2.0, but for some classical recordings, the strings have some silbius or distortion, that are corrected by going to 4.0. I am still running it at around 2.2, but was curious why it is that way. By the way, I have messed around with the VTA as well, but the problem went away only after I set the VTF. Thanks!
...for some classical recordings, the strings have some silbius or distortion, that are corrected by going to 4.0. I am still running it at around 2.2, but was curious why it is that way
There are several possible causes, including various setup parameters and phono stage slewing distortion, but here's one cause I'm certain is involved to some degree in your system.

Every stylus has a contact radius, which determines the minimum size of the groove modulations it can trace. When a stylus encounters modulations smaller than it is, all it can do is scoot past the tops, playing part but not all of the waveform at that frequency. This necessarily results in clipping of the waveforms at these higher frequencies and perhaps in the momentary loss and regaining of stylus-groovewall contact, all of which we hear as "sibilance". The larger the stylus, the lower the frequency where this will begin to occur.

The contact radius of the 103's conical stylus is much larger than the contact radius of styli in some more modern cartridges. Finer stylus profiles trace smaller modulations (ie, higher frequencies) cleanly, and do not start "scooting" past small groove modulations until higher frequencies.

Increasing your VTF to excessive levels will not make the stylus fit into modulations smaller than it is, but it may mitigate the resulting distortions in a couple of ways:

1. Increased pressure between stylus and LP assures that the stylus will not lose contact, which makes the (unavoidable) clipping of too-small modulations as smooth as possible.

2. Increased pressure between cantilever and suspension is smothering higher frequencies, much as soft earplugs block higher frequencies while letting lower ones through. In effect, you're using excessive VTF as a HF rolloff filter.

If you listen to a lot of this kind of music, and the distortion bothers you, I'd suggest that your next cartridge be one with a line contact or micro-ridge stylus. All else being equal, they reproduce HF's more cleanly than conical or elliptical styli.
Dear Toufu: I can't say if the Zu is a good match for the tonearm you own ( ask directly to the manufactirer about ) but if it is then the problem is in other " place ".

The Zu specs " talk " that the recommended VTF is 2.5grs but this manufacturer does not point out a VTF cartridge limit ranges ( up-down ) like almost any other cartridge builder so maybe the Zu can run " safe " up to 3.0grs we don't know about and you have to ask, here too, directly to the manufacturer.

The " right " VTF is not only for the cartridge track fine with out tracking distortion and to protect the LPs from fast wearing but along the suspension design to center the coils to achieve the quality performance design targets.
If your cartridge is out of that " right " VTF then all those factors name it could suffer a " degradation ".

+++++ " but for some classical recordings, the strings have some silbius or distortion.... " +++++

Toufu your cartridge ( like any other one ) is not a perfect one, it is far from there, the ZU performs according its price range or something a little better: nothing more nothing less.
In the other side all the recordings are far from be perfect too so you must accept " distortions " all over the analog music sound reproduction.

Maybe you are " waiting " that your analog rig performs near perfection. Well you can't wait or look for in your today audio system, you have to accept its limits.

Anyway, IMHO it will be worth to contact directly to the Zu manufacturer and explain to them what you are experienced and see which advices and support they can give you, maybe your cartridge is out of specs somewhere.

Regards and enjoy the music.
Raul makes another good point. I'm not familiar with the Zu version of the Denon 103, but the normal versions of that cartridge are not an ideal match for lighter weight unipivots like the JMW 9. Compliance, while not actually as low as Denon's quoted 5cu, is still lower than optimal for this tonearm. That can affect tracking.
Thanks guys, I will look into these things...
I added a 5g headshell weight to my VPI and the problem went away... Now I get no distortion at 2.2g for those records. Thanks again for the help.