Question Counterweight/Tonearm Effective Mass

A question.

lostwin

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Two linked questions here. Bear in mind that my tonearm has two or more counterweights which allows flexibility on positioning when balancing the arm.

1) presumably a tonearm’s effective mass is calculated without counterweight. Once adding the counterweight, does the effective mass of the arm vary depending on position of the counterweight?

2) Accepted wisdom is to try and keep counterweight as close to the pivot as possible, do you agree and does this effectively increase the arms effective mass?

3 - and most likely - Am I confusing two completely separate things?!
 

rabski

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I've always assumed it should include the counterweight, but that the position of the weight makes no difference in the calculation.

The effective mass mechanically means the inertia of the tonearm as 'seen' by the cartridge, so as far as I can see, it will be the entire mass (weight).
 

rabski

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Though thinking about it, point 2 is interesting. Pencil, paper and remaining brain cell needed.
 

lostwin

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Though thinking about it, point 2 is interesting. Pencil, paper and remaining brain cell needed.
It’s reassuring it’s not just me!

I have two potential options to balance the arm. Use a lighter counterweight further away from the pivot or a heavier one closer to the pivot. I am also keeping an eye on my effective mass as my arm is a bit too light for the cart compliance (I also use a headshell weight). Got me wondering if the counterweight position would also make a difference.

Edited to correct lighter/ heavier positions
 
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rabski

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As I see it, mechanically the position of the counterweight will change the moment of intertia, but will not alter the effective mass. I may be wrong, but I would imagine the counterweight as close as possible to the pivot would be better, as it will mean lower effective intertia, though will not change the effective mass.

It's too bloody early for thinking :ROFLMAO:
 
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John (big)

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It’s reassuring it’s not just me!

I have two potential options to balance the arm. Use a heavier counterweight further away from the pivot or a lighter one closer to the pivot. I am also keeping an eye on my effective mass as my arm is a bit too light for the cart compliance (I also use a headshell weight). Got me wondering if the counterweight position would also make a difference.
I believe the converse is true, when I fitted an XTC counterweight to my Hadcock 242 Silver plus arm made as a one off by John Choong for my deeper Cartridgeman Musicmaker cartridge fitted with an "Isolator" the XTC was heavier enabling the counterweight to be closer to the pivot point.

The counterweight is usually moved away from the pivot point to lower tracking weight, Having spoken to Len Gregory & George Hadcock when I proposed a modification to the 242 arm ie going from a flexible counterweight stub to a rigid one they confirmed that as I had read years before regarding SME arms it was preferable to have the counterweight closer to the pivot point as there would be a lower moment of inertia seen by the stylus,
since my maths is non existent I cant demonstrate this mathematically.
 
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Nopiano

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Like Richard, it feels too early to fully fathom this, but the heavier weight nearer the pivot was always the thing to aim for.

Inertia is what I thought this reduced, but now without checking I’m less sure. But that was my instinct.
 

Nopiano

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On point 2), the arm mass is the arm forward of the pivot, nothing to do with the counterweight.

I had a very low mass SME 3009 series 3 years ago, and it had slivers of lead to put in the counterweight housing. But the arm wand was practically weightless.
 

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If I remember correctly the SME III had a selection of weights that fitted into a cradle and always stated that the heaviest weights go in first so that the higher mass is closest to the pivot point .
 
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MotherSky

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Interesting stuff - this has been on my mind recently, specifically I was wondering how one accurately measures effective mass of an arm (my arm bears very little resemblance to its vintage Hadcock origins) - also the merits or otherwise of a sliding weight on the armtube itself to apply tracking force - I see Funk have done this on one of their arms - probably the answer would be "it depends...."

One thing I am reasonably confident of is that lowering the centre of gravity of the counterweight has benefits for conventional arms, not just unipivots - in addition to keeping the mass close to the pivot as noted - MH
 

lostwin

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Interesting stuff - this has been on my mind recently, specifically I was wondering how one accurately measures effective mass of an arm (my arm bears very little resemblance to its vintage Hadcock origins) - also the merits or otherwise of a sliding weight on the armtube itself to apply tracking force - I see Funk have done this on one of their arms - probably the answer would be "it depends...."
One thing I am reasonably confident of is that lowering the centre of gravity of the counterweight has benefits for conventional arms, not just unipivots - in addition to keeping the mass close to the pivot as noted - MH
as I understand, the sliding weight on the arm tube would adjust the effective mass. Obviously, it would also impact the VTF , but this is not the primary purpose for the adjustment.
 

MotherSky

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Interesting stuff - this has been on my mind recently, specifically I was wondering how one accurately measures effective mass of an arm (my arm bears very little resemblance to its vintage Hadcock origins) - also the merits or otherwise of a sliding weight on the armtube itself to apply tracking force - I see Funk have done this on one of their arms - probably the answer would be "it depends...."

as I understand, the sliding weight on the arm tube would adjust the effective mass. Obviously, it would also impact the VTF , but this is not the primary purpose for the adjustment.
https://thefunkfirm.co.uk/tonearms-1/fx3-9042907-9-5f9432b53867be31021eb1e0-phonos-fx3-9in-pho

If you read the blurb on this link, it does seem that they are expecting you to use the sliding weight to adjust VTF - it does seem odd as clearly it has an impact on effective mass - I am thinking of trying this approach, but would really like to understand the variables rather better
 

rabski

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On point 2), the arm mass is the arm forward of the pivot, nothing to do with the counterweight.

I had a very low mass SME 3009 series 3 years ago, and it had slivers of lead to put in the counterweight housing. But the arm wand was practically weightless.
Sorry Nick, but I disagree with this first bit. The 'mass', which will have an effect in terms of resonant frequency, will be that of the entire arm and counterweight(s) on both sides of the pivot. The placement of the mass will influence the intertia, so I would assume you want (as suggested above) the weight as close to the pivot as possible to keep the moment of inertia low.

Mind you, once we get into fully damped arms, it all gets horribly complicated.
 

Nopiano

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Sorry Nick, but I disagree with this first bit. The 'mass', which will have an effect in terms of resonant frequency, will be that of the entire arm and counterweight(s) on both sides of the pivot. The placement of the mass will influence the intertia, so I would assume you want (as suggested above) the weight as close to the pivot as possible to keep the moment of inertia low.

Mind you, once we get into fully damped arms, it all gets horribly complicated.
I beg your pardon, yes you must be correct.

This aspect was what was ‘phasing’ me earlier. Then re-reading it, I convinced myself the counterweight couldn’t be a factor while trying to remember that old SME. That was of course damped with a trough, but let’s not go there!
 

Nopiano

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If I remember correctly the SME III had a selection of weights that fitted into a cradle and always stated that the heaviest weights go in first so that the higher mass is closest to the pivot point .
That’s exactly it. As it was such low mass I mistakenly convinced myself only the wand part was relevant.
 

lostwin

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Been experimenting…

This was my existing setup, I had deliberately positioned the heaviest counterweight furthest away from the pivot- I like to challenge the orthodoxy I guess.
B46799EA-8398-4183-98E0-748FE0854CB4.jpeg
Using the hifi news test record I get quite an extended vertical resonance starting at a smidge over 12hz and stopping around 9hz.

Switching this around as best I can - bearing in mind my arm wand is light and I have about 9g of additional weight plus the cart at the headshell end. As such I have to use all 4 counterweights and can’t pack them all close to the pivot end. VTF carefully reset to the same 1.95g.
F3E01160-D385-469B-ACAD-A99DD969F80A.jpeg
The resonance on the test record seems narrower , starting a bit below 12hz and not dropping below 10hz. Interestingly, I also got some output from the speaker at 20 to 18hz that I am sure wasn’t there before.

Listened to the same Grace Jones track before and after. A good 10 min gap though, which is not ideal. Bass with the new counterweight position does seem to be cleaner and tighter. Maybe the mids and treble too, treble perhaps a little brash.

Based on previous responses, the new counterweight position should have lowered the inertia a little. Do you reckon my test and listening results correlate with this?
 

rabski

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Do you reckon my test and listening results correlate with this?
Possibly...

I would expect reducing the inertia to result in the stylus responding faster in terms of quicker physical movement tracking the groove, so 'tighter' would go with that.

The downside is that difficult grooves may possibly not track as well, because there will be less physical damping of movement. Thus, possibly mistracking on 'hot' recordings
 
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rabski

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Of course, looking at the Well Tempered here, it all goes to hell. It's massively damped, the arm tube is as light as a feather and the counterweight is right at the far end. Yet it tracks like a bloodhound and is tight as a gnat's chuff.
 

toprepairman

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From a balance and inertia point of view a lighter weight further out or heavier weight nearer to the pivot comes to exactly the same thing. The only difference is the heavier weight will load the pivot bearing more. From a purely technical standpoint the best would in fact be a spring to counter the arm weight and that would give you the least inertia for the arm overall.
 

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