Curve on Bridge Bottom

[Erwinspiano@AOL.COM]

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In a message dated 4/29/2002 7:52:54 PM Pacific Standard Time, 
mfarrel2@tampabay.rr.com writes:


> Subj:Curve on Bridge Bottom 
> Date:4/29/2002 7:52:54 PM Pacific Standard Time
> From:<A HREF="mailto:mfarrel2@tampabay.rr.com">mfarrel2@tampabay.rr.com</A>
> Reply-to:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> To:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> Sent from the Internet 
> 
>           Terry

      If you're rebuilding a Stwy or Mason these pianos both cut crowns in 
the bridge bottoms already. SO no extra cutting needed. To check this lay the 
bridge on the floor, press down on the ends of the bridge and the middle will 
come up a bit or with the board dry fit in the case lay the bridge on it and 
see if it conforms to the boards curve. Usually it will. 
    These makers intended there to be some crown along the grain and it's 
quite easy to induce if you wish. Steinway refers to this as double crowning 
and there contention is that it supports crown along the grain. I'm not sure 
it helps that much after the bearing load goes on but it is measurable before 
that especially if you're using a jig such as a torsion box with a 60 to 80 
ft. radius. With the board layed down on the box the bridge is then glued on. 
There are 2 by 4 blocks placed on the box to fit between the ribs to give 
support when the pressure is applied. I use my old go bar deck.  The curve in 
the box being greater than the hollow in the bridge bottom allows the bridge  
to be bent a  little bit more than its cut crown. At least this is how I do 
this. The resultant crown along the grain is significant and measurable with 
a straight edge. I personally have done it both ways and frankly  In my 
humble opinion I like the sounds of the treble better this way all be it 
subjective. Others I know who do it this way have stated similar opinions. 
Also with the crown along the board/rib it does require less force to glue 
the board in at the top.
  I can still measure the crown along the grain after the board is installed 
by the way. 
    It may well be possible that in some designs it's an advantage and in 
other it's not.
       Clear as mud? Hey you asked>>>>>>>>.Dale Erwin>>>..

> 
> I know some rebuilders cut a radius curve on the bottom of new bridges. I 
> know some folks here have indicated that they do not. I will not dispute 
> that it may well not make any significant difference which way one goes.
> 
> Is there any good technical argument either way? It sure seems to me that 
> the biggest, stiffest, longest rib on the board should be an integral part 
> of shaping the board. Has a straight-bridger measured crown on an installed 
> board without the bridge, and then measured it after the long bridge was 
> installed? Does it not flatten the board a bit? Is some of the ribbing 
> theory (tighter radii) designed to compensate for the small (??) flattening 
> effect of a straight bridge?
> 
> It can't hurt to cut a nice little curve into the bottom of a new bridge - 
> can it?
> 
> Terry Farrell
>   


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