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.
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