Curve on Bridge Bottom

[Ron Nossaman]

>
>       While dry fitting a 1927 S&S L board today I was thinking about your
> question as to bridge curvature in the bottom of bridges so I thought I'd
> take a few measurements. 
>    Prior to fitting the board in the case there is no evidence of crown along
> th bridge line.Non can be seen or measured. After dry fitting in the case
> with a few clamps it took on a great deal of crown along the bridge line. I
> was so take by this I took a number of digital photos with a long straight
> edge along the bridge line.There was probably a good 5/8ths of an inch  gap
> at each end of the straight edge. This crown was nice and hemispheical . Even
> taking into account the modest diaphramizing this is alot and obviously
> intentional. The rasten/inner rim is cut this way intentionally.
>      The bridge ,not glued on yet was laid on the board in its location.
> Though it has some crown cut into it , it is not as much as the dry fit
> boards crown induced by simply clamping to the rim. Very interesting ehhh??
>     Never the less to my mind the board will flatten out less with the bridge
> in a crowned configuration than a flat cut bridge, providing the bridge is
> glued to the board on a crowned suface as I described previously. However
> nominal this bridge support may be it has to resist bearing at least to some
> degree. I'll take whatever I can get.
> >>>>>>>>>>>>>Dale Erwin>>>>>>>>>>>>>>


Hi Dale,
Look at unloaded crown heights for "standard" 18M (60') radius ribs in a
generic board. 
Rib #1, 2.6mm 
#3, 7.2
#5, 9.4
#7, 6.3
#9, 1.8
#11, 0.5

The crown height being, obviously, dependant on the length of the rib. The
numbers are a little different with graduated radii ribs, but the effect is
similar. In a free state, not clamped to a rim, the board will be flat along
the grain because the stiff long grain holds it that way. The perimeter of the
board, however, is anything but flat. If this board perimeter is clamped down
to a flat surface, a crown will form along the long grain. The very fact that
the rib crown is higher in the middle of the board where the ribs are long than
it is at the ends where the ribs are short will guarantee this, whether a
bridge is attached to the board or not. The rim would have to be cut wildly off
plane for the board to not form this crown, so I'd have to say this isn't the
result of any intentional sculpting of the rim at all. Nor is it the result of
any special bridge crowning or attachment method of the bridge to the board.
The bridge doesn't form this long-way crown, the ribs do when the board is
glued in.

Ron N