Official Luthiers Forum!

I've seen the term 'driving the bus' used to give a word picture to profiling a guitar's sides to accept the back. The concave dishes used to do that are spherical. But we brace our guitar backs side-to-side not front-to-back. The backs are arched from side to side and there's nothing to encourage the spherical shape along the length. I've seen a builder use x-braces on the back to get the back to be spherical, too. Anything wrong with that idea? I can't think of a single manufacturer do this and only one bespoke builder doing it. Usually there's a reason why somebody doesn't do something.

Can someone comment?

Thanks very much.

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Peter Havriluk

The rim together with the curvature of the ladder braces will create the spherical shape. But I prefer cylindrical back arching because it seems like it would better resist headblock rotation, since it doesn't have any lengthwise curvature that can flatten out and provide slack.

"The rim together with the curvature of the ladder braces will create the spherical shape. But I prefer cylindrical back arching because it seems like it would better resist headblock rotation, since it doesn't have any lengthwise curvature that can flatten out and provide slack."

But if it flattened out wouldn't that tend to pull the neck back?

A properly prepared rim provides curvature support around the entire perimeter in all directions. The bracing adds strength and helps in dealing with RH changes. Too there are live back considerations. And if you check it out there are indeed many back brace arrangements that look like the X and even more elaborate spider web appearance.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Not if the neck pulling up is what reeled in the slack and flattened it.
Mike

You have to keep in mind the big picture, as you see it, for the guitar you are building. Back bracing should work in unison with the top. Some designs rely on an active back while others want a passive design as Ken eluded to. The back bracing should be based on your design concept.

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Joe Beaver
Maker of Sawdust

Double post

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Joe Beaver
Maker of Sawdust

The arch or dome also accommodates the 5/16" or more of across-the-grain wood movement without damage and generates a little extra volume without needing to increase the depth of the sides. We've had a couple Traugott guitars in over the last few months which have a lengthwise spruce interior brace which ties into the top of the back braces and resists the flattening of the back due to neck and bridge loads.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Woodie - Is the lengthwise back brace you mentioned like the one in the photo below? I don't know if the back in the photo is a Traugott or not. That's a new concept to me. Interesting.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Reasonably close, if memory serves, Mr. De Rocher...I failed to take any photos, given the boss was handling the work and I was looking over his shoulder. I believe the brace was tied into the neck and tail block as well, but I did not get to spend much time peering into the boxes (this is the problem with guitars that do a fleeting pass through the shop for something minor like a new saddle or a setup tweak).

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A constellation only takes shape when one maps the whole.
- Beth Brower

Yep, that's one of Jeff's guitars. He talks about the floating back brace in detail in this presentation. Definitely worth a watch. https://vimeo.com/188390275