soft, bass trill regulation

[Greg Graham]

I'm not a concert pianist (and I don't play one on
TV), but I would like to understand the motion of a
soft trill.  

This is a learning excercise for me.  Please correct
my assumptions, if in error:

For minimum volume, the hammer must travel with
minimum velocity when it strikes the string.  For
fastest repetition, the hammer must travel from the
string to the point where the jack stops it and back
to the string as quickly as possible.  To satisfy both
minimum volume and maximum repetition, the distance
the hammer travels must be minimized, certainly much
less than full blow distance.  Half blow?  Less than
checking distance?  Obviously, the "trill blow
distance" must be more than let-off, or the jack would
never touch the knuckle.  

The jack should be waiting just under the knuckle at
this much-reduced blow distance, which leads me to
conclude that the key should be one-half, two-thirds,
or even more through its travel when the hammer
reaches this shallow "bottom".  Does that sound right?

Normally we set drop so that the hammer does not rise
above the let-off point at full key dip, but rather
ends up close to let-off.  What if we set drop a
little lower, so the jack had something to push
against in a very shallow trill, with the performer
lowering the wippen enough to get the jack under the
knuckle, but not enough to disengage the rep lever
from the drop screw?  A sensitive performer should be
able to feel that point of simultaneous
let-off/drop-screw engagement and stay there, no? 
Actually, I suppose lowering the drop screw would
slightly separate the ideally simultaneous contact.  

If, in the bass, let-off was 1/8th, and drop another
1/8th, with 1/16th rise during aftertouch, Would a
3/16th inch "trill blow distance" be possible and
repeatable?


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