Key Leads

[Stephen Birkett]

Dennis Johnson wrote:
 [...]
> I've been reinstalling leads as needed from the front (as the factory is
> actually doing again now) but lately I've been wondering if perhaps the
> idea wasn't completely foolish. I'm still trying to tell if its my
> imagination, but I really think that I can feel the difference between one
> lead at the very front and *slightly* more lead towards the middle, so
> tough weight is the same. As much as I am irresistably attracted to using
> less lead, I think that maybe the other example actually starts smoother.
> Was this really a sound idea that unfortunately got blasted due to other
> weight/geometry problems?
>
Once again I will raise the dreaded moment of inertia to answer this
one. There is a difference. The static (touchweight) is based on the
first moment of the key about the pivot and varies as the distance
between lead mass and pivot. The dynamic (feel) of the key is based
on the second moment (moment of inertia) and varies as the square of
the distance between lead mass and pivot. Thus when you move the
leads to the middle of the key and *slightly* increase to balance the
touchweight you reduce the moment of inertia significantly
(even though the leads are a bit heavier)...hence the key feels
different.

Another analogy...golfclubs are (or used to be) sold in sets which
were statically matched (i.e. the first moments about the grip were
the same)...called `swing weight', which is identical to D.Stanwood's
strike weight! Recently some manufacturers started making sets that
are dynamically mateched (i.e. the second moments are the same). You
can't match both first and second moments, because one varies in
proportion to distance whereas the second varies as the square of the
distance.

My 1937 Hamburg Steinway has had key leads added sometime before I
acquired it...these are quite near the front of the keys and the
action has always seemed a bit `clunky' to me, presumably because of
the position of the leads. Would like to remove them altogether, but
not being a (modern piano) technician I'd probably mess up the whole
action doing so.

Stephen Birkett (Fortepianos)
Authentic Reproductions of 18th and 19th Century Pianos
Waterloo, Ontario, Canada
tel: 519-885-2228
fax: 519-763-4686