Crown in Bass Area & Downbearing

[Delwin D Fandrich]

> -----Original Message-----
> From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
> Behalf Of J Patrick Draine
> Sent: May 13, 2004 3:52 AM
> To: Pianotech
> Subject: Re: Crown in Bass Area & Downbearing
>
>
> On May 13, 2004, at 6:18 AM, Farrell wrote:
>
> >> From PTG Annual Convention classes.
>
> And which instructors? Papal Infallibility is not yet a feature of
> instructors at PTG events.
>
> Patrick
> _______________________________________________


I do, for one. And, while I make no claims to infallibility, I try to have good
reasons for what I teach.

Regardless of design intent there is going to be very little, if any, measurable
crown around the bass bridge of a small piano. There might well be lots of crown
in the soundboard assembly before it is installed in the piano, but once it is
glued to the inner rim it is going to be pretty much flat.

Now, before the purists among us start a big shooting war over this, I would
suggest that you make a soundboard for something like a Steinway Model M or
Model L, measure the crown across the #1 rib and then glue the thing into the
piano and measure again. Forget theory and just see what actually happens. There
will be virtually no crown along this rib. Why? Well, look at the soundboard
grain angle and then look at the proximity of this rib to the inner rim. Which,
unless it is a recently built Kimball, is quite flat. Forget the bevel. In this
regard it is a non-starter.

Now put the plate in the piano and consider how close the bridge is to the plate
hitchpin riser. Consider also the configuration of the bridge cantilever. Both
of which effectively prevent any bridge motion at low frequencies. Well, the
shortness of the string backscale prevents the motion and the cantilever
prevents any that does show up from getting through to the soundboard.

Adding any string bearing to this mix is simply adding insult to injury. It
makes the situation worse by pre-loading (distorting) both the bridge cantilever
and the soundboard around the area of the cantilever mounting foot.

>From experience most rebuilders have learned it is best to avoid loading these
bass sections with any string bearing at all. As the backscale gets longer and
the bridge gets further away from the inner rim (whether by making the piano
longer or through enlightened rebuilding) the bass section can and should be
given a bit of string downbearing.

Del