Angular Acceleration in a Piano Key

[Bill Spurlock, RPT]

Some random thoughts on the questions of inertia, touchweight, and how different
actions feel:

The hammers used on early 1900's Steinways and such were extremely light, due to
soft (less dense) hammer felt, real mahogany molding wood (lighter than anything
used today), and extensive tapering and shaping of tails. To match those
original weights, one has to start with a light weight hammer and go all out on
shaping. Many don't do that, resorting instead to adding lead to the keys for
"proper" static touchweight measurements, resulting in some real doggy actions.

Different actions designs vary considerably in geometry, inertia, and key
weighting, yet varying designs can often perform equally well. The point is that
action "feel" is a result of many factors, and sometimes we have a tendency to
focus on only one or two as we search for that magic formula we can apply to all
pianos. In particular, although inertia is important, I feel there is quite a
wide range that is acceptable. Consider two keys on the same action: A13 with a
9 gm hammer and 4 key leads, and A73 with a 4gm hammer and no leads. The
difference in inertia between these two keys is tremendous, much greater than
the change in inertia we might make when customizing an action, yet both keys
are capable of nearly equal repetition speed. So it is action mass, geometry,
voicing, power of the piano structure, and of course regulation that combine to
give an action a certain feel. I doubt a single formula for ideal hammer mass
would apply to all actions given all the other variables. At the same time, the
factors of parts acceleration, action leverages, etc. are at least parts of the
puzzle and research here can help in understanding the big picture.

One huge factor often overlooked in certain actions is excessive flexing of
action parts, especially keys. The symptom is similar to that caused by
excessive inertia/poor geometry: plays fine softly, but peaks early and feels
very stiff and unwilling on a hard blow. This can happen when the keys are long
and not very thick (top-to-bottom), with short key buttons and no shoe. Other
factors are soft balance rail punchings, poorly bedded key frame/inadequate
glides, soft wippen cushions, and big soft knuckles. All these parts can flex or
compress, so that the key bottoms out in front before the hammer gets half way
to the string. This condition is obvious when the backchecks drag on the hammer
tails, but increasing checking distance still leaves that gutless sensation on a
hard blow. The dragging backchecks are a clue that the keys etc. are too flexy.
Modify the keys, use thin firm balance rail punchings, etc. to correct.

In general, I feel the biggest and easiest gains in piano performance come not
from hours of work on special modifications, but rather on getting the basics
right: choose appropriate hammers, shape them properly, attend to all pinning
and alignments, bed the key frame and everything else as solidly as possible,
and regulate well. Then, nit-pick the regulation and voicing some more. Not that
there aren't pianos that came out of the factory needing modification, but very
often it's the dull stuff that really pays off, and often gets overlooked in the
rush for the magic formula.