strange back problem

[bob531]

Hi,
just finishing my first build for over 30 years and ive got a problem in that the back has sank (concave). What causes this ?? Is it the same as a top humidity or what?
The weather in the uk where i live has been very dry only rained once during the whole of April.
The back was buiilt with a radius ,as was the back braces.Sides were radiused in a dish to fit the back . All was fine when i glued it a couple of months back.
Bob

[JonWint]

Humidity change. Transverse shrinkage/expansion of the back is greater than the longitudinal change of the braces.

Happened to me before I had better monitoring/control of shop humidity. A top went flat before assembly. I removed and reinstalled the braces.

Will be harder to fix after the box is closed. Has binding been installed yet? Best fix is to remove back. Compromise is to live with it.

[bob531]

Hi , Thanks for the reply.
I guess i `ll have to live with it. Im at the almost finished stage and until i noticed it was quite happy with the build (only my 3rd acoustic build and first since around 1988).
The guitar is for my own use (actually building 2 almost identical guitars) .
Now i know the back problem is there ,its a p.......sing me off a bit.
Had a dropped front on the other one which remedied itself after 3 days of humdifiying the inside .
Ive searched all over google for dropped ,sunken back and found nothing at all ,only concerning tops.
One mention on the Taylor guitar site about the back going flat due to being to wet(but thats not the problem here).

[dnf777]

Thats odd. My guess is a big change in humidity as well, but thats still pretty dramatic. Any reason to suspect REALLY wet or dry wood at time of build?

[bob531]

Yes its very odd, top wood has been seasoning for years , back /sides wood i`m unsure how dry it was but certainly a few years ,i cut it from a plank of Goncalo Alves that i had in my wood stock . It certainly felt on the dry side when used.
The back went from around 3mm above the sides in the middle of the radius to about 2mm below in the middle. Affects the whole back starting around 4 inches from either end.

[Bass.swimmer]

It's happened to me a lot actually. I tend to work with not quartersawn wood, which is really prone to move.

If it's any help, one of my favorite guitars I built has a concave back. It's holding fine after some years.

[charles Tauber]

Originally Posted by bob531

Yes its very odd, top wood has been seasoning for years , back /sides wood i`m unsure how dry it was but certainly a few years ,i cut it from a plank of Goncalo Alves that i had in my wood stock . It certainly felt on the dry side when used.

Not odd at all: that's what wood does.

Don't confuse "dry" with "seasoned". "Dry" refers to the moisture content in the wood: it changes in response to its environment. "Seasoned" refers to permanent changes to the wood that happen with age.

Wood is hygroscopic, meaning it absorbs and desorbs moisture to attain equilibrium with its environment. As it does so, it changes size and shape. Wood changes length only a negligible amount, but can change considerably in thickness and width.

If you glue a brace with its long grain (length) running across the width of another piece of wood, the brace inhibits the movement of the side to which it is glued on the other piece of wood, since the brace will not substantially change length in response to humidity changes. In response to humidity change, the differential movement between the constrained side and the unconstrained side of the braced back is what causes the change in radius of your guitar back. To prevent this, you must maintain the humidity at a constant level during the building process at least until after assembly.

Non-quartersawn wood will move more across the width of the back than quarter sawn wood, exaggerating the issue.

[John Arnold]

The wood is drier that it was when the braces were glued on.

[bob531]

John,
Thats certainly possible as i cut it from a much thicker plank of goncalo alves. The spruce braces were from 100 year old spruce.
bass.swimmer,
Good to know as its for personal use ,so no need to sell it.
Charles ,
I do know a bit about wood having been a violin and bow maker for almost 30 years. Though arching etc... is carved in in those not made out of flat plates.
I guess i didnt expect that the back would bend so much in the opposite direction,i thought the braces would at least prevent so much movement.

I mentioned ive made two guitars almost identical ,the one with the problem is quarter sawn ,the other without the problem is slightly off quarter , but both from the same thicker plank.

[John Arnold]

For all practical purposes, wood does not move in the grain direction. With that in mind, moisture content in the braces is not nearly as important as in the back. The reason the back flattens or goes concave is because the braces have not gotten shorter as the back wants to get narrower. A flat or concave back is more likely to crack, since it will be in tension from the shrinkage.
The whole purpose for arching the back with curved braces is to prevent cracks by allowing the back to move without creating enough tension to crack it.
Cross-grain construction is discouraged in most woodworking, but arching of thin, flexible plates allows it to work, more or less.
The lesson should be to get the plates as dry as possible before bracing them. It is much better to have a little swelling instead of shrinking, since that puts the wood in compression.

[charles Tauber]

Originally Posted by bob531

...i thought the braces would at least prevent so much movement.

First rule of any (fine) woodworking is you can't prevent humidity-related wood movement. Best you can do is slow down the rate of movement.

I mentioned ive made two guitars almost identical ,the one with the problem is quarter sawn ,the other without the problem is slightly off quarter , but both from the same thicker plank.

Interesting. Assuming all else equal, it demonstrates that even two pieces of wood from the same board won't necessarily behave the same way.

[JonWint]

Feeling lucky?

Guitar top down on padded bench, restrain the body edges at the brace ends, insulate the center seam and rim beyond braces, heat (lamp or heat gun) until glue softens, jack up center of braces from inside, slip the braces until convex shape returns.

What could go wrong? You built it once, you can rebuild it if required.

That's my idea of fun.

[printer2]

I did two bodies when the RH was at 20% or less. I did the backs flat and when life got more humid the backs developed an arch. A little different than the 15 foot one I put in with wood that is in the 4-45% range. Will be interesting to see how they are in a few years.

[bob531]

All good points but why doesnt humidifying the guitar fix the problem as it does with the top?
Does the finish drying/ shrinking, in this case shellac increase the problem? Though i would think shellac as quite flexible as finishes go.

[JonWint]

The back's moisture content is out of sync with the rest of the wood. It may require more humidity than the rest of the wood "wants".

Shellac, that is French polished, is so thin that it probably doesn't provide membrane tensile strength to the fabricated back system.

(I'm not a luthier, just a hobbyist guitar hacker/builder/restorer.)