[pianotech] Touch weight dynamics and the relationship between SW and FW

[V T]

Hi David,

I once did a rough calculation regarding the contribution of the key stick
to the total inertia and came up with a figure of around 18% for the 
particular action and note I was studying.

It then occurred to me that we usually study the action as the key is being
pressed down. I think however that the effect of the key stick inertia is 
more prominent when the key is returning to it’s rest position. That’s when
the lever system of the action is decoupled or disconnected, so to speak, 
and the key stick assumes a much larger proportion of the total inertia.

This in turn is noticed by the player who may either feel that the key is 
too slow to come back up and so the action is never quite ready for the 
next note, or in the opposite case, that the key is following the fingers 
up too fast, sometimes pushing them up. Players seem to show a wide range 
of tastes in this matter, so there is no universally ideal set-up.

One other thought, regarding inertia when the key is on its way down. The 
inertia increases roughly with the square of the action ratio, but linearly
with the strike weight. Therefore, I think it will be hard to find truly 
equivalent combinations of FW and SW, even if we set aside the key dip 
question and regulation limits.

I do however often ponder the effect of the trade-off between high and low 
SW with the corresponding adjustments in the action ratio. Some would say 
that the SW must be matched to the string tension and scale, and the rest 
follows from that.

Vladan