[CAUT] Wire Stretch

[Keith Roberts]

The ribs are shorter in the 20% of bridge length and the frequencies are
much higher.

I don't think of it as downbearing pressure but contact or
coupling pressure. The bridge is actually pushing up so we could call it up
bearing or down-bearing resistance. Coupling pressure should be around 2# to
4# per string. You have some leeway but too much coupling pressure cuts into
the bridge and too little loses sound production. Yes the unisons spread out
but the overall load should be less on the longer ribs and you can't change
the coupling pressure that much anyway.

I just did all the rib shaping and board diaphraming and cut off bar in and
S&S A at Dale Erwins shop. He finished it up and it's glued in and it's time
to hear it. Next time I'll be there to set the bearing, hopefully.

Keith Roberts




On 4/28/07, Bob Hohf <rhohf at centurytel.net> wrote:
>
>  Why would you ever want to set the same downbearing force per unison when
> the unisons are so unevenly distributed along the length of the bridges?
> The top two sections of a typical concert grand contain 43% of the strings
> on 20% of the total bridge length.  A smaller piano may have 46% of the
> strings on 29% of the bridge.  Setting a uniform downbearing per string will
> set up a tone-stifling imbalance in the downbearing force along the
> bridges.  For 20+ years I have been setting downbearing on the principle of
> uniform force per unit of bridge length.  This produces much less force per
> string in the high treble and much more in the tenor.  I don't measure
> angles.  In Parts 5 & 6 of "Recapping Bridges" in the 1999 Journal, I
> present a discussion of downbearing as force, not dimension, and describe a
> method of setting a more uniform load distribution.  I'm sorry it's too
> involved to go into detail here.  I know of one other rebuilder besides me
> who does something along these lines.
>
>
>
> Bob Hohf
>
>
>
>
>
>
>  ------------------------------
>
> *From:* caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] *On Behalf Of *
> RicB
> *Sent:* Saturday, April 28, 2007 3:04 AM
> *To:* caut at ptg.org
> *Subject:* [CAUT] Wire Stretch
>
>
>
> Yes they do what Bob ?  Graduate downbearing angles so as to get exactly
> the same amount of downbearing force on each unison ?
>
> RicB
>
> >Course the problem with this notion is that equal tension by no means
> (in fact dictates the opposite) of equal amounts of downbearing pressure
> across the scale unless one compensates by graduating the downbearing
> angle appropriatly...which I dont think anyone does.<
>
>
>
> Yes, they do.  Check out your July and August 1999 Journals
>
> Bob Hohf
>
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