I have often heard that some pianists do not like the feel of the
extended keys on a Duo-Art grand and I have heard the action described
as "lumbering."  The touch of my Duo-Art grand certainly feels a
little heavier or slower than that of my Ampico.  Whether this is due
to the key length or other things in the action I don't know, but I
believe there are at least two effects that are unavoidable with the
longer keys.  How significant or noticeable these effects are I can
only guess.

There is no doubt that the depth of touch and the rise of the capstan
can be exactly the same with long or short key.  Thus the static force
needed to support the whippen can be exactly the same with either.
The two effects which I believe cannot be compensated for are:

1.  The inertia or the resistance to acceleration is larger for
longer keys.  The longer keys have a higher moment of inertia due both
to extra length and extra mass.  I used the simple approximation of a
key of constant cross-section.  The widening at the ends and the lead
weights will affect things somewhat, but the basic principal will
remain the same.  The angular acceleration (which is what we want since
the key is a rotating system) is directly proportional to the force
applied by the finger and inversely proportional to the length of the
key and the mass of the key.  (Imagine if the keys were made of lead.
Surely everyone would expect the action to feel as if it reacts
somewhat slowly.)

Since both mass and length are in the denominator of the expression
for angular acceleration, if either of these is increased (and almost
certainly both are increased for longer keys) the acceleration of
the key will be less for the same finger force on a longer key.  So,
for the same force, the longer key will take longer to reach bottom.
Or, to state it another way, to achieve the same angular acceleration,
a larger finger force is required.  The loudness of the note is
controlled by the final speed of the hammer which depends on its
acceleration while in contact with the key through the action.  Thus
the angular acceleration of the key is critical in determining the
loudness of the note.

2.  The lever effect varies along the exposed length of the key
differently for long and short keys.  Assume the depth of touch is the
same for short and long keys.  For the longer key, the key dip at the
inner end of the key (next to the fall board) will be greater than for
the shorter key.  Both the static lever balance and the force to
produce a given angular acceleration will vary less along the key of
the Duo-Art since the exposed length of the key is a smaller proportion
of the total key length.   I wonder if this is noticeable, but it would
certainly be measurable with good instruments.

My guess is that the dynamic or inertia effect #1 might be large enough
to be felt and could lead to the lumbering feel I have heard mentioned.
I suspect the lever effect #2 is not significantly important to the
feel of a piano.

I was interested to hear of the longer bass keys in a concert grand.
Perhaps pianists are accustomed to the heavier, slower response in the
bass due to larger hammers and thus the differences, if any, due to
key length are more important in the middle and upper octaves of the
keyboard.

David Sharpe