Official Luthiers Forum!

What are the braces above the X actually for? What forces are they resisting?

I ask because I've started making these acoustic basses with a Fender style bolt on neck....




The neck bolts into a full height neck block with no downward pressure onto the top. Given that, is a UTB necessary? What about braces around the soundhole?

I somehow forgot to take a photo of the bracing pattern before closing, but here's with a lightsource: (actually I realise it's from a previous build, but no matter)



There seems to be a surprising amount of tap tone when I tap the top around the sound hole, so I was hoping to reducing bracing in that area.

Would anyone have any thoughts?

“There’s a surprising amount of tap tone…” That means the bracing there didn’t kill it. The UTB (which ought to be a hefty brace) keeps the neck block from twisting and collapsing into the sound hole. Without it, the block will quite readily break the top along the grain lines adjacent to the fingerboard and allow the neck to fall right in. The sound hole bracing helps to prevent collapse. In guitar design, the sound hole is located right where the most strength is needed. The UTB and the bracing carry the forces around it.

Aside from structural support, proper bracing doesn’t limit vibration but rather controls where it occurs. You don’t want the motion from the strings getting lost before it moves as much of the top as possible.

There's also often a brace between the UTB and the neck block called a popsicle brace because its thin and flat and looks like a popsicle stick. That brace is to help prevent the top from cracking next to the fretboard extension.

_________________
Steve Smith
"Music is what feelings sound like"

I think you still need a top brace, I copied a Bourgeois technique of leaving a little gap between the top brace and the neck block extension and after the top is on filling it with a very snug fitting shim to guarantee good contact and stability. I can access the gap via the mortise for the fretboard tenon. I wedge in a piece of mahogany tightly and hit both sides with thin ca to anchor it. (Note, in the picture the shim is not in yet and the channel for the truss rod is not fully completed.)

IMG_1889 by Terence Kennedy, on Flickr

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.