Q. Thoughts on aftertouch affecting checking?
A. Aftertouch directly affects backchecking. If we define aftertouch as key travel after letoff, this can translate to backcheck travel after letoff, since the backcheck is attached to the key. Further key stroke equals greater backcheck reach and less equals less.
Imagine no aftertouch. The jack just clears the knuckle. As the hammer sets strings in motion and starts to bounce away, the backcheck is waiting below, stopped with the rest of the key at full dip. But think where it can be. In order to clear the tail on the way up, the backcheck can only be set so far forward and that position is the same regardless of blow distance, dip, or aftertouch. (The limit is easily found out of the piano by playing the key with the other hand resisting hammer rise, simulating the flex of a hardest blow. If the backcheck is too close, it will scrape the surface of the tail.)
Now, think about the backcheck's direction of movement when it is stopped by dip. It would have traveled vertically for a moment, if it were in line with the bottom of the key. But it sits well above the key and well started in the circle it travels back toward the player. After the hammer strikes, its tail travels down its arc (again, toward the player) to meet the backcheck: the further the backcheck has traveled, the higher backcheck-tail contact will occur (successfully catching or not), amplified by both arcs accelerating toward the player.
So, the no-aftertouch backcheck stops well short of optimum checking. Now consider too much aftertouch. The backcheck will be some degree of too far along its arc and contact the tail too soon after strike, i.e., too close to the strings (if other factors don't intrude - like jack jamming in the rep lever window). With too much aftertouch, you can back the backcheck away to mitigate, but the rep lever, though stopped at the drop screw, carries on further than desirable as its center continues to rise until the key stops.
Q. Any thoughts on the feeling of too much aftertouch? What's the right amount?
A. I think the right amount is just enough, plus a small safety margin. When letoff and drop are sensibly regulated (so a fff string's displacement doesn't intersect with a repeated note's letoff), setting an aftertouch that brings the hammer after drop back up to letoff is about right. Any more than that violates letoff’s safety margin. And too much aftertouch requires a little extra effort, reach, and time for the player. Accumulated extras in our regulating produce accumulated impediment, distraction, and lost energy for the player. Each increment in itself may be acceptable, but the overall accumulation may be unacceptable.
Too much aftertouch occurs if the dip is too deep and/or the blow distance is too short. Both of these factors erode player satisfaction. Too deep and sharps may feel buried between naturals at full dip. Raise sharps above 1/2 inch to avoid this and the player overexerts trying not to trip. Too short a blow distance robs both power and control, since there is less finger-to-hammer engagement.
By regulating a natural and a sharp through expected specifications early in the regulation, shortfalls and overextensions will force us to find solutions. Technician error (current or previous) might be a source of difficulties, but even a perfectly set up action starts to go out of spec as soon as it is played. Wear and compression change the geometry. Also, any replacement parts or materials present are likely not identical to originals. The samples provide needed information to balance all the factors, and aftertouch directs us to trouble or validates our solution.
How much aftertouch do we need? As the jack clears the knuckle, it has a fixed distance it can potentially travel determined by where it lies in the window of the repetition lever. Once the jack has contacted the felt stop at the top of the window and compressed it to its max, that's it. If the jack wants to go too far or the stop is too close, the jack jams in the window, preempting dip. So, the force of the key gets stopped (or not) by the jack in the window. This premature impediment does not feel good to the pianist. Past that point, there is no more aftertouch, even if the key hasn't reached dip. Pragmatically, there needs to be a little wiggle at the jack tender when key has fully compressed front punchings or there will be a risk of jack breakage and/or failure to play properly, including failure to check properly.
What are the ways that we can adjust aftertouch? We can adjust dip: more dip equals more aftertouch. We can adjust blow distance: shorter blow equals more aftertouch. We can also adjust letoff: more letoff equals more aftertouch. Or we can adjust geometry through changing part/rail positions, changing parts, or changing materials. Even and appropriate aftertouch, coupled with expected parameters for the rest of the setup, indicate a good setup and a good regulation. Mechanically, the piano will be enjoyable to play and tonally, we have reduced regulation as a voicing variable.