Acoustic Guitar Construction

Francis,

Neil Harpe says Leadbelly's Stella ladder braced 12-string had just 3 braces (plus a bridge pad and Popsicle brace) on the top, same as 6-string Stellas, and a ~26.5" scale.

Diagrams:

http://www.laguitare.com/images/ladder-braced_plans.jpg

http://s127.photobucket.com/user/michae ... s.jpg.html

Here is a clip of a lightly built 12-string in the Stella tradition:




The double X you show should certainly be strong and resist the tension of the strings, but it may be too strong if the braces are "normal" sized. The double X is intrinsically stiff. At some point we must decide between how long we want an instrument to last and how responsive we want it to be. The solid body electric is the ultimate in strength, but has almost zero acoustic response.

Thanks John,
Not considered ladder bracing at all so the links are very interesting. The Grossman guitar details need further study and the other one seems to have so little of what I would call conventional bracing.

I think at present I will go with twin tone bars behind a bridge plate and beef-up the main X-brace by 20-25% to compensate for the extra loading. I think I can get away with that as I'm using a short scale length and light gauge strings. This seems a more tried and tested route - but don't be surprised if things change between now and start time!

Cheers
Francis

Francis M wrote:Thanks John,
... I will go with twin tone bars behind a bridge plate and beef-up the main X-brace by 20-25% to compensate for the extra loading. I think I can get away with that as I'm using a short scale length and light gauge strings. ...!

Looks like a fun project. A 25% increase in the X height nearly doubles the stiffness as stiffness is proportional to the cube of the height. So it seems like your 20-25% target is in the right ball park. I would go with the twin tone braces instead of the double X as well. The guitar will have enough treble as it is, allowing the top move as much as possible once it is structurally sound seems like the direction to go.

Francis,

The point of showing the ladder braced designs was that it does not take all that much bracing to make a 12-string hang together, and how good they can sound when lightly braced.

On the other hand, one must be respectful of the 12-string's predicament. Pete Seeger's "trademark" 12-string with the triangle shaped sound hole was actually replaced quite a few times because he was so rough on instruments that he would break them. When one broke, he just had another made, or so the story goes.

Just a little side bar -- Gibson's introduction of the double "X" generally is not considered a highlight in the evolution of acoustic guitars, to the contrary it delivered a crippling blow to their acoustic market share and was rather quickly rescinded.

ken cierp wrote:Just a little side bar -- Gibson's introduction of the double "X" generally is not considered a highlight in the evolution of acoustic guitars, to the contrary it delivered a crippling blow to their acoustic market share and was rather quickly rescinded.

Yes I've seen some of the bad press relating to the double x bracing Gibson used. That said others seem to be making it work. I'm guessing its the relative sizing of the two x braces. The secondary x brace to me could be quite light structurally, probably lighter than twin tone bars used conventionally.
Many thanks all for the feedback on this.
Francis

John Link wrote:Francis,

The point of showing the ladder braced designs was that it does not take all that much bracing to make a 12-string hang together, and how good they can sound when lightly braced.

On the other hand, one must be respectful of the 12-string's predicament. Pete Seeger's "trademark" 12-string with the triangle shaped sound hole was actually replaced quite a few times because he was so rough on instruments that he would break them. When one broke, he just had another made, or so the story goes.

John, Guess a lot comes down to confidence and spare finances. I'm quite happy that I could build the ladder braced design and would suspect that a great deal of the strength in the design comes from the glue lines between the thinner timbers. Bit like laminating sides or the thin double tops with nomex sandwiched between them. Individually not having much strength but add some glue lines and presto controlled ridgidity.

Francis