Shanks parallel to strings- off list

[Dean May]

>>Perhaps if we take the batter out of the picture my point will be more
clear.  Rigidly attach the bat to a fixture.  Now throw the ball at it.  If
the object is to get max transfer of energy from ball to bat, then you want
the least amount of ball rebound (the greater the rebound, the more the
ball's energy stayed in the ball).  What point on the bat will give the
greatest ball rebound?  Will it be the center of percussion?  If so, then
that doesn't seem like the spot that's giving greatest energy transfer from
ball to bat.


Hi Phil

If the bat is mounted rigidly, then it doesn't matter where the ball hits
it. Any point on the bat will give equal rebound. I think the problem here
is one of perception: you seem to be primarily considering the effects of
the speeding ball coming to the bat. But we have two things to consider: the
energy the ball adds and the energy the bat adds. For the energy the ball
adds, you are quite right: it doesn't matter where on the bat it hits. Every
spot will give the same rebound.  But for the energy the bat adds, the ball
needs to hit at the spot where the greatest force from the bat will
transfer. There is only a limited amount of energy, and if some goes into
your hands (the reaction force), that takes away from what is available to
the ball. The center of percussion is the spot where all of the energy of
the rotating mass is focused.

In our situation the strings are not speeding towards the hammer assembly. A
better baseball analogy for us might be T Ball: the ball is stationary and
the bat is the only thing in motion. Does that help you see why the center
of percussion is the best place to hit it?

I've appreciated your observations and questions. They motivated me to do a
little more research. It would be interesting to do some testing by adding a
little mass past the hammer to try to bring the CP right to the hammer. My
calculations in the other post used the 11 gm bass hammer, which gave a cp
pretty close to the hammer. Obviously, in the light treble hammers the
Moment of Inertia of the shank assy will play a more significant role, which
will move the CP closer to the pivot.

Bill Spurlock recommends in his hammer hanging pamphlet to thin the treble
shanks to remove mass, saying that will eliminate some of the "woody" sound
that the you get in the extreme treble. I wonder if the woody sound is a
result of the cp moving away from the hammer closer to the pivot. The
resulting higher reaction force could be vibrating the shank assembly
generating the noise. Thinning the shanks would move the cp back closer to
the hammer.

Somebody needs to do some experimenting with adding weight past the hammer.

I realize now the number I came up with for the cp is the same as the answer
you got, but I was confused (I get that way easily) as you didn't tell us
anything about how to get the moment of inertia. So I incorrectly assumed
you didn't use it. Mea culpa.

Blessings,

Dean

Dean May             cell 812.239.3359
PianoRebuilders.com   812.235.5272
Terre Haute IN  47802