Just one thing to add to Don's...
When you change hammer mass, you also change BW (balance weight) which is
part of the torque.
In the equation of motion:
alpha = T/I
torque is key radius * (finger_force - BW - friction).
If BW decreases, then torque will increase. This can be compensated for by
readjusting wippen assist springs to bring BW back to its prior value, in
which case Don's assumptions hold, or by removing lead from keys, which
further reduces the moment of inertia for that note, in which case more
thought is required :).
-Mark
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