bridge cap

[Delwin D Fandrich]

Philip Jamison wrote:

> I'm recapping a Knabe treble bridge, the top half of which (as most of us
> know) often splits due to the odd grain orientation. I often wonder why
> Knabe bothered to place a cap on this part of the bridge, as the lower
> section has none and is fine. Anyway, I wonder if quartered maple is the
> only acceptable cap, or can one use straight-sawn?
>
> Philip Jamison
> West Chester, PA

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Philip,

Treble bridges split because of the relative alignment of the tri-chord bridge pin sets and the wood grain supporting them.
Consider the relationship of the lines of stress on the nearly adjacent bridge pin sets through the treble sections relative
to the grain of the wood in the cap and you'll understand why treble caps crack. In the top treble sections the tri-chord pin
sets are very close together and are nearly in a row from one unison to the next. This physical relationship changes as you
go down the scale until, by the time you get to the tenor sections, they are quite spread out. Tenor bridge sections rarely
crack because there is little concentrated stress forcing the wood fiber apart.

Bass bridges in which the bi-chord pins are all in a row -- as opposed to being staggered and notched -- suffer from similar
stresses. Here it is compounded by the relatively higher individual string tensions that are common to wrapped strings.

Quarter-sawn (actually, "vertical grain" is probably a better phrase to describe this grain orientation) lumber orients the
wood grain in the plane that is most able to resist the side bearing stress from the rows of bridge pins. Flat-sawn lumber
would have an even greater tendency to crack in this application.

Actually, a good laminated bridge cap stock will outperform either.

-- ddf