Friction/StrikeRatio

[Dennis Johnson]

At  3:20 PM 6/29/95 -0600, stanwood wrote:

>The data below shows FrictionWt as (DownWt-UpWt)/2
>Before and after moving the capstans and heels
>approximately .375" to change strike ratio from an average
>level of 7.5 to 5.5

>
>
>(chart omitted)



>The piano played liked a nightmare, now it plays like a dream.
>
>Small knuckles would not have solved this problem.
>


_______________



        I was out of town for the weekend or I'd have responded sooner to
this post.  I'm not entirely sure, but this very interesting data
documenting capstan relocation on a particular piano and the profound
improvement it obviously realized, seems to be presented as a proof which
somehow should contradict my earlier statement about knuckle sizes.  I do
not see a contradiction here at all.  There are two different kinds of
friction in the action that we must be concerned with, and there are many
sources for each kind.  I do not know the proper scientific names for each
kind of friction, but one is the type found in tight centers or bushings
that are unrelated to geometry, the other type is a  binding directly
caused by less than optimal geometry.  This is nothing new. The trick of
course, is correctly diagnosing bad geometry where it occurs, and solving
that problem without creating others.  I appreciate this revealing data
about the capstans on that piano, but it does not really say anything about
friction as it relates to knuckle size except that on this piano they are
most likely correct. If you are using the Hamburg shanks, than the knuckles
are the correct size for that leverage.  The point I tried to make is that
the pre-84 shank from Steinway has an improper-sized knuckle, resulting in
bad geometry and excess friction. Changing this knuckle to the correct
smaller size would solve at least one geometry problem and reduce friction.
I just did this on a 1984 D a couple of weeks ago. Unfortunately, I do not
have impressive data collected like what we saw on this D from Harvard, but
only changing knuckles on the pre-84 shank (which are about .410") to the
smaller one from Renner USA (about .350"), lost an average of 4-5 grams DW.
I did not notice much change in upweight, it was a nominal improvement. The
smaller knuckle on a lesser leveraged shank also gives the jack a little
more room to clear it in the window, which really helps.
        I will go further and say that if you had installed smaller
knuckles on the aforementioned Hamburg shanks, ignoring the capstan
problem, that still would have somewhat reduced total friction within the
action, but clearly in this case that would not have been the correct
solution. It would not have solved the real problem and would not yield a
satisfactory result. The interesting sistuation here is that a too small
knuckle on the Hamburg shank will reduce overall friction, in spite of that
incorrect geometry. This says that total friction is not necessarialy
always inversely related to good geometry, and that some friction is good.
I think that this apparent contradiction with the normal inverse
relationship between friction and good geometry must be observed in the
context of its involvement with so many variables.  It is difficult if not
impossible to completely isolate all of the individual contacts and lever
ratios as they relate to total friction. Chris Robinson spent quite some
time with this issue a few years ago and did some geometry classes on
knuckle sizes.  I also believe that Ken Sloane wrote an article some time
ago for the CAUT newsletter about his experience with the pre-84 shank when
he ended up with some horrendous DW numbers.
        But to be fair, if you never use the early Steinway dimension
shanks, then this is not a problem that you would ever run into.  I have
pursued this issue primarily with the hope that technicians will not
misjudge the early dimension shank from some bad experiences due to an
incorrectly- sized knuckle.  This shank does work well (with the
appropriate weight hammer) as David's  strike ratios have confirmed.
        As long as you concede that smaller knuckles have less friction
than larger knuckles, independent of other variables, I do not see any
conflict. If I misread the intent of your data, please forgive my
confusion. And thanks for sharing that data, it really is an impressive
improvement. I don't have my atlas here to look up its date, but that is an
old number. What is the oldest Steinway you remember seeing with
significantly wrong capstans?



Dennis Johnson
St. Olaf College