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I just read a small note in a new OM plan (GenOne) I'm using that suggested maple as an alternate wood for the inside center strip between the back braces. Wondering what would be the downside of that (passive back)? A curly maple strip would certainly be attractive, and the mass/ stiffness difference is virtually negligible for a passive back. Thoughts? I'm cutting braces this week and was seriously considering using some curly maple...

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Measure Twice,

Karl Borum

Go for it. There's no rule about what to use for that. Grain direction is probably something to consider.

I would think that the maple would have to be very curly in order to "read" as curly on such a narrow strip. Keep in mind it is cut across the grain so there wouldn't be too many curls in a strip an inch wide. It might end up looking muddy instead of blingy.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

The function of the back strip is to convert a tension load on the back seam when the back shrinks into a shear load in the glue line. Most glues don't hold up well under tension loads, and do much better in shear. The actual load involved is not enormous, and the strip doesn't need to be very thick to take it up. Most of the ones I've seen were much thicker than they needed to be.

This can be a problem in itself. Most woods are much stiffer along the grain than across, so a back strip (or stud on a crack) that is too thick will concentrate any bending load, or tension from shrinkage, along the edge of the strip. You either end up with a crack along the edge of the reinforcement, or the strip peels up when it can't bend with the wood it's glued to. The friend who pointed this out to me said that he'd seen lots of these problems, but never a broken stud or back strip.

You can probably safely assume that the wood you're using for the back strip is at least six or eight times as resistant to cracking along the grain than splitting across it, so it only needs to be 1/6 or 1/8 as thick. Taper it down gradually from the center to each edge so that the edges are as thin as you can make them.

Yet again, I learn something from Alan. It seems so inherently obvious that a thick center reinforcement could concentrate stress along the grainline of the back, but I never thought of it that way until Alan pointed it out. I do taper by reinforcement strips on the edges but probably not enough, and they are probably way thicker than needed. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.