Brace dimensions for live backs

[JamesO]

Hi all!

Going by the book, back braces for a live back should be 2cm tall x 1cm wide. After shaping mine ended up being 15mm tall x 1cm wide (5/8" in American terms). I planed them off before class this morning and plan to finish scraping off the residue this afternoon.

My brace stock is .75" tall, which converts to 19mm, so I probably lost some height in general stock prep and shaping. Should I laminate another layer of stock to make sure the final height is 2cm as specified, or is 19mm OK?

Before feathering the ends of the braces or working the center of the lower bout brace, I was getting 150hz or so when I tapped it. I don't have the book in front of me for reference, but I think the target was 180hz or so.

[kiwigeo]

Hi all!

Going by the book, back braces for a live back should be 2cm tall x 1cm wide. After shaping mine ended up being 15mm tall x 1cm wide (5/8" in American terms). I planed them off before class this morning and plan to finish scraping off the residue this afternoon.

My brace stock is .75" tall, which converts to 19mm, so I probably lost some height in general stock prep and shaping. Should I laminate another layer of stock to make sure the final height is 2cm as specified, or is 19mm OK?

Before feathering the ends of the braces or working the center of the lower bout brace, I was getting 150hz or so when I tapped it. I don't have the book in front of me for reference, but I think the target was 180hz or so.

My experience with my Gore builds to date is to err on the side of caution and make bracing on the high side. Its easier trimming down top and back resonances than trying to bump them up. Trevor's back brace dimensions assume a back radius of 10'. If your back radius is different then this will affect the main back frequency. If it was me I'd certainly not scoop out the centre of the back brace at this stage and being the anal person I am I'd probably re-make the whole brace and make it a tad over 20mm height.

Have you done any stiffness measurements on your bracing stock? This allows you to chose the stiffest pieces from your stock for use as bracing.

Trevor will chime in with his sagely advice at some stage.

[JamesO]

Thanks for being helpful as always, Martin. I ended up prepping some blanks a little over width yesterday, and nearly full height. They're 1.8cm tall x 1.2cm wide. I understand the implications of the cube rule and now the added width won't make up for the lost stiffness, but I'll give them a try.

[JamesO]

I realized I'd been measuring the frequency incorrectly anyway. See here.

[jeffhigh]

It's the scoop in the middle which has the most effect on lowering the frequency.
Even though its a pain in the arse, I leave that scoop till after the guitar is built.

[Trevor Gore]

I'd warn against using undersize braces unless they happen to be really stiff stock. As the guys have said, it's a lot easier to go down than up.

You can get an idea of the B(1,1) by tapping the back once it is glued to the sides and this will give you an idea of where the T(1,1)3 will land, so you can pre-do some of the scalloping. I've been trying a different type of scallop that is a bit easier to adjust and it seems to work OK. I just file a notch in the centre of the lower back brace using a piece of aluminium tube about 140mm long by 65mm diameter with coarse sand paper on one side and fine on the other. I file a starter notch in before I close the box and then deepen it working with the same tool through the sound hole to fine tune things. The tube will just fit through the sound hole of most of my guitars.

DSCF6440s.jpg (132.04 KiB) Viewed 18127 times

[JamesO]

I like it! Thanks, Trevor. I noticed that scoop on the back you posted to my thread on the OLF, too (the thread regarding using different species for back joint reinforcement strips).

[Woodsy23]

I've been trying a different type of scallop that is a bit easier to adjust and it seems to work OK. I just file a notch in the centre of the lower back brace using a piece of aluminium tube about 140mm long by 65mm diameter with coarse sand paper on one side and fine on the other. I file a starter notch in before I close the box and then deepen it working with the same tool through the sound hole to fine tune things.

Trevor,
Is there a minimum height of brace at the notch that you wouldn't go below? There must be some point where the depth of notch weakens the back too much. Do you find that you always get the back frequency you want with sufficient brace depth remaining?

[Trevor Gore]

Is there a minimum height of brace at the notch that you wouldn't go below? There must be some point where the depth of notch weakens the back too much. Do you find that you always get the back frequency you want with sufficient brace depth remaining?

I've never had to go particularly low. I'd probably start to broaden things out if I got below where the gabling starts. How strong does a back need to be? It's not taking much load!

Here's one I broadened out a bit using the same sanding tube I mentioned before:

DSCF7344s.jpg (129.29 KiB) Viewed 18086 times

I can't remember where it finished when fully tuned, but I took some more out of the middle for sure.

[JamesO]

How strong does a back need to be? It's not taking much load!

I've wondered this. Using a floating brace to connect the four ladder braces has become popular, and Rick Turner used carbon fiber to reinforce the arch of the back on a number of guitars, though I'm not sure if he still is. The purpose of both approaches is supposed to be to strengthen the arch and reduce the need for a neck set in the future, but I'm not sure how the floating brace reinforces the arch, or how reinforcing the arch prevents neck resets.

[Trevor Gore]

Apart from that dumb Martin design where they chamfered away most of the bottom of the neck block, I've never seen neck angle failures associated with the back of the guitar "stretching". All my guitars have a back dome of 3m radius, so, if anything, they should be more prone to exhibiting problems of this sort. They don't. Nothing. Ever. Usually it is the head block folding into the sound hole that causes problems. Why? There's a big hole in the support structure (the sound hole), often with inadequate bracing around it, with a panel made of thinner, less stiff material with much greater load on it. So where's it going to fail?

Anecdote: I just made a replacement neck for a guitar that is ~ 5 years old. Not that there was anything wrong with the original neck, it was just about perfect with a buzz free 2.2mm/1.8mm action, but the owner wanted a different nut width and neck profile. The panel curvature templates fitted over it like the day if was made, so no measurable distortion. It was a bolt-on/bolt-off neck, of course, which is pretty well engineered with the primary falcates and sound hole braces let into the UTB which butts the neck block top plate. Ain't goin' nowhere. So I'm not seeing the need for floating back braces or truss rods between the end blocks, etc.