Pianotech

Recently the topic of bridge roots was touched upon, with David Hughes writing that he believes that laminated bridge root heights should not be altered due the strength imbued to the whole bridge structure from the root.

I have often wondered if the bridge cap splitting along the row of front (top or speaking length) bridge pin lines in the highest treble, very common in Steinway uprights, among others, could avoided if the cap were thick enough to contain the entire bridge pin length. Perhaps only in the top octave or so. My reasoning for that thought line was my observation that the line of bridge pins in the highest end tends to all line up in the same layer of lamination, seeming to weaken the root structure and then splitting the cap. This is observed when the split cap is removed in order to recap the bridge.

Has anyone made alterations to a laminated root with the intention to avoid the line of front (speaking length) bridge pin holes aligning to a single layer within the laminated root?

And care to share experiences?

Joe Wiencek
RPT

I just recently finished my 500th soundboard so i can't speak with any authority. But i'm not seeing the correlation of the root causation of the cap cracks. What i do see quite often is the cap grain direction at the wrong angle, and the wrong choice of wood species.

-chris

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865-986-7720 (text only please)
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So are you saying that the bridge pins inserted into the same lamination line in the root are perhaps spreading the root lamination apart thus putting the cap under tension?  

I don't think that would be a likely cause.  I do see splits along the pin line in even some very high end pianos.  I repaired one not that long ago in the top end of a Bosendorfer 225.  But I'm not sure that's the reason.  Could be EMC is not controlled when the cap is glued on with different expansion rates between cap and root.  Could be poorly cured cap material.  Could be, as Chris pointed out, wrong grain angle--I think Knabe used to make that mistake routinely.  I haven't seen any correlation with varying wood species whether maple or boxwood.  It might be an argument for a laminated cap though, both for structural integrity and stability during humidity cycles.  

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David Love RPT
www.davidlovepianos.com
davidlovepianos@comcast.net
415 407 8320
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Many Steinway uprights were made with laminations of mahoghany and maple in the bridge root. Bad idea to mix specials in the bridge root with different

levels of expansion and contraction. Can cause the bridge cap to crack if it follows underlying cracks in the root.

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Parker Leigh RPT
Winchester VA
(540) 722-3865
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