what is downbearing?

[Stéphane Collin]

Don,

I don't know how much accuracy I need.  I would like to, though.  I'm only
suspecting the whole debat about downbearing to be irrelevant, due to
unaccurate measurements.  But I would like to change my views also, and know
exactly how much downbearing I should put on my next rebuild job, and how I
could accurately (enough) measure it.
All I know, is that a 0.6 mm error on a 2 mm measure is a 30% relative
error.  And a 30 % relative error in offset of a string of 55 mm like the
last trebble string of a piano, results in a disaster when indirectly
measuring the angle it makes with the bridge, thus the downbearing force.
One more source of unaccuracy : talking about the carpet thread method, on a
piano which has felt on the plate at the hitch pins.  Should you put
pressure on the thread, and compress the felt at that place, like a string
does ? but how much pressure ? I can tell you there is a huge difference in
reading the moment the string just leaves the bridge, especially in the
trebble, where string length is short.  Should I believe that downbearing
varies very much if the string is well seated at the hitch pin ? And then,
if the string is not well seated on the bridge, it's natural deflection
would it have also incidence on the calculated downbearing ? all this I feel
is not so important in the bass section.  But then, I never have problems
with the bass section : it is always ok.  Problems arise in the 3 last
octaves of most pianos I see, and there any reading (even string length) is
subject to caution.  Help !

Stéphane Collin



----- Original Message ----- 
From: "Don Gilmore" <dgilmore@kcmpi.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: Friday, February 20, 2004 11:13 PM
Subject: Re: what is downbearing?


> Well, I suppose small angles produce a larger change in downbearing force
> relative to large angles, but with a piano, small angles are all we should
> ever have to deal with.  So we get what we get.  Comparing the error with
> the imaginary instance where we pull the string down at an 80 deg. angle
> doesn't really tell us anything.
>
> The question is: how much accuracy do you need?  Does a difference of a
few
> grams matter, or does it have to vary by a kg before there is any
noticable
> change?
>
> Don A. Gilmore
> Mechanical Engineer
> Kansas City
> ----- Original Message ----- 
> From: "Stéphane Collin" <collin.s@skynet.be>
> To: "Pianotech" <pianotech@ptg.org>
> Sent: Friday, February 20, 2004 1:51 PM
> Subject: Re: what is downbearing?
>
>
> > hi Don.
> >
> > But then, isn't relative error much more important in small angles ?
> > Or do I miss something again ?
> >
> > Stéphane Collin.
>
>
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