Curly sitka spruce

Ask your wood and other materials questions here. Please DO NOT post pictures and ask us to identify your wood, we have found that accurate ID is nearly impossible, and such discussions will be deleted. Thanks.
Post Reply
Andy Bounsall
Posts: 120
Joined: Thu Feb 02, 2012 12:29 pm
Location: Ottawa, Ontario, Canada
Contact:

Curly sitka spruce

Post by Andy Bounsall »

I’d never seen curly spruce before but have come across it twice recently. Anyone have experience using it for an acoustic soundboard? Wonder if/how the inherent runout in the curl figure might affect stiffness, stability, etc. I’ve used curly redwood before without any issues but that had a much tighter curl than this.

CCDB22BD-A993-4A6A-920C-6D61A60EE778.jpeg
Alan Carruth
Posts: 1264
Joined: Sun Jan 15, 2012 1:11 pm

Re: Curly sitka spruce

Post by Alan Carruth »

Curl is recurrent run out. It reduces the long-grain Young's modulus because of the angle the fibers make to the axis of the piece. You probably should make the top a little thicker than usual, but it's hard to say how much unless you get a reading on the actual Young's modulus of the piece. There are various ways to do this.

The bigger risk, IMO, is that of damaging the top if (when) the bridge comes up. With a book matched top the run out shifts across the center join. One side of the bridge, where the run out goes 'up' toward the head, is more likely to peel up. As the peel reaches the center join, and the run out starts top go 'down' into the top the bridge will pull up wood, which splits down into the top.

Whenever I use a piece of top wood that has that sort of run out change, due to a knot shadow or 'bear claw', I try to lay out the top in such a way that the bridge is on a spot that has no run out, where the fibers run along parallel to the surface. It obviously helps if the stock you have is fairly long, so you have some leeway. I'd exepct the issue to be even more pronounced in redwood, since it has such low splitting resistance and peel strength.

One thing that can help a lot is to make the bridge footprint larger. It's particularly good if you can enlarge the width of the bridge along the line of pull, as this disproportionately reduces the maximum stress along the back edge. That's the idea behind the 'belly' bridge, and it works. If you're worried about the bridge becoming too heavy you ca always make it from something like walnut.
Andy Bounsall
Posts: 120
Joined: Thu Feb 02, 2012 12:29 pm
Location: Ottawa, Ontario, Canada
Contact:

Re: Curly sitka spruce

Post by Andy Bounsall »

Thanks for that Alan.
Post Reply

Return to “Wood and Materials Q&A”