Why not use an underslung counterweight on the Rega tonearm to lower the centre of gravity?

There are a number of underslung counterweight modifications for Rega tonearms. The supposed benefit is that this lowers centre of gravity and increases stability. All sounds logical and appeals to intuition - right?

We started supplying modified Rega's using underslung counterweights long before anyone else climbed on the bandwagon and produced inferior variants. There is so much more to a good counterweight mod than lowering the centre of gravity.

We also realized subsequently that the reason this Rega modification worked, had nothing to do with being underslung but much rather the way the counterweight is decoupled from the rear stub.

Having experimented on occasion for over 10 years with low slung weights (all other things being equal) we have proved conclusively in our listening tests that an underslung counterweight does not deliver any significant performance boost - not only this but the whole argument is based on ideas that don't bear scrutiny from a purely theoretical argument. Unless one is familiar with the laws of rotational inertia it is easy to jump to entirely the wrong conclusions.

The problem with the underslung counterweight idea is that it confuses uni-pivots with gimball tonearms - In unipivots it is utterly essential to have low centre of gravity to maintain stability - However a gimbal arm derives it's stability from 2 spaced gimball bearings. This is not similar to a high performance car benefitting from low centre of gravity because the physics are entirely different - a car is essentialy pivoting on the road surface and you want to reduce inertia above this point, a gimball tonearm on the other hand is normaly pivoting on it's bearing centres and the higher the radial distance of weight is away from the centre, the higher the inertia.

Inertia in tonearms is a subject that I cannot dwell as it appears deceptively simple but is in fact highly complex. The above is merely written to prevent you being led along an argument based on flawed logic.

Another way of looking at this is to ask why the Rega RB300 uses a
Tungsten counterweight which is considered superior to the identical
mass of the Stainless steel version? Because it is DENSER - this has the
effect of lowering the moment of inertia (rotational momentum). The effect of spacing a
weight away from the axis of rotation (which is what happens when you
"lower" the centre of gravity) is the exact opposite - the rotational
inertia increases which is what arm designers are generaly trying to

I used to design ships and spent a lot of time in lectures
with aeronautical engineers - issues of stability and rolling inertia
are matters of life and death in these situations.


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