Official Luthiers Forum!

If the forces on the top are to buckle the guitar upwards around the sound hole why isn't back bracing done differently? Why isn't the back braced from top to bottom rather than side to side? Guess I'm not understanding something about stresses on the guitar.

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A higher purpose for wood.
Rich Smith
Issaquah, WA

I'm not a pro and I don't even play one on TV, but my guess would be that the width of the rims provide strength to the back from the stresses you are describing. Further, with the guitar being longer from neck to tail, the rim provides a pretty strong ring-like structure to prevent that from happening. Just my guess though.

Darrin

Well;
The back does have a role in the neck stresses on the top!
BUT to brace the back top to bottom-which i take it to mean the length of the back??

If you design a guitar to handle and desperse the tension then all should be fine!
NOT copy a FACTORY GUITAR!!!
If not you've not done your homework !

Mike

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Mike Collins

I take it you mean that the tops forces are pulling up on the back edges (ie top behind the bridge), but pushing down around the soundhole ??? In my view, the neck is certainly pushing the top downward, but depending how stiff the UTB is, the top and fingerboard may actually look like its rising right there. I have seen guitars where the FB ext rises, others where it sinks, all depends how that area is built/braced. Most sunken guitars seem to have thinner, less stiff UTBs, and on some the UTB brace ends arent tucked and have sharp end scallops - read stress risers (and most often loose UTB brace ends as well)

Essentially the neck is pushing the top downwards under the FB ext, and this forward motion of the top of the neck block (towards the soundhole) usually means the bottom of the neck block is moving outwards towards the nut - thus stretching and flattening out the back. The simplest (and I think best long term) solution is Rick Turner's method of tying the upper neck block area to the lower waist via CF tubes/bars. I havent done this on any yet, but will be shortly.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Exactly right Tony as the strings pull rotationally arond the crown of the saddle the front of the bridge wants to sink and the back of the bridge wants to rise. Conversely the stings rotationally load on the nut caauses the peghead end of the neck to want to rise and the FB extension end to sink. the job of the UT braces is to resist the force of the FB extension and the Xbtace/bridgeplate to resist the bridge rotation. If the UT brace is properly sized it resists the downward motion of the FB extension and the X-brace in conjunction with the bridge plate resists the upper ward and inward rotation of the bridge while still allowing that are to a good fulcrum fro string energy transmittal and wave motion.

There are lots of other ways to support the loading. The key is to do so in a way that does not diminish the function of any of the components ability produce it required affect.

Michael would a double X brace pattern be as strong as conventional bracing? I plan on building a guitar with upper bout sound holes rather than a center sound hole. The bracing pattern for this calls for a double X config.

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A higher purpose for wood.
Rich Smith
Issaquah, WA

Tony,

I think Rick now uses the back centre graft topped with carbon fibre to resist the back-flattening pull and the pair of cf rods for resisting the push of the neck at the top of the neck-block.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010