Carbon Fibre capped scalloped bracing

[Judd Holt]

My goal was to develop a different spring effect in the top; thinking the CF would function as a reciprocating spring, owing to the fact that it was sprung opposite to the string pull on the top. I don't know if my theory panned out as intended, but, I post this as yet another technique for producing a very impressive sounding instrument (my opinion aside). With a top just under .120, 1/4 wide bracing with a 3/16 minimum height, this top had about 20% of what is normal deflection on my instruments. I worried that it may have been overbuilt, and maybe it is, however, the sound would not suggest so. I am anxious to produce an instrument using this method which achieves my normal deflection....we'll see.

[Barry Daniels]

Nice clean work. Can you tell us how you did it and what materials you used?

[Craig Bumgarner]

Nice work! I was struck by your comment that top deflections were 20% of your normal. Just to be sure, that is 80% less deflection than normal? So the CF reinforced bracing is much stiffer (as would be expected I suppose).

If so, that raises some interesting questions. If both your "normal:" and CF reinforced instruments sound good, then deflection testing would seem to be of little value as a means of nailing good sounding tops. Or at least, I would be having to reconsider what deflection testing means in this case. Not being snarky, I use deflection testing too and am just curious how you think this all fits in the deflection testing model?

Craig

[John Hamlett]

Some thoughts:
According to my friend Dr. Dave Cohen, (who has done some laser Interferometry and various other research on instrument tops, backs, and boxes) he recons that a brace, and therefore a braced top can be made to the same weight and stiffness as laminated braces by carving more-or-less traditional spruce braces to whatever profile it takes to get similar results. If that is the case, striving for the same deflection using the laminated braces may well result in a lighter top of similar stiffness to the usual.
If there is an advantage to laminating with CF, it may well be the lack of "memory" that the material displays. A wooden brace could be made with the same amount of "spring effect" (not at all a new concept) and mass, but a CF laminated brace may well maintain the effect longer.
So anyway, it seems to me that a similar top could be built by springing spruce braces into place and leaving them taller to get the same stiffness and mass as the CF laminated braces, but the CF laminated versions may maintain their "spring rate" over time better than spruce alone.
Deflection testing is a good way to gain information from our top wood and braced tops, but it is not a full measure of the top's stiffness, and we needs to take mass into consideration in order for our deflection data to have much meaning. In other words, if we measure stiffness (with deflection testing) and mass we will likely see a difference between CF laminated braced tops and spruce braced tops, but not necessarily.

[Craig Bumgarner]

Some thoughts:
According to my friend Dr. Dave Cohen, .... a brace, and therefore a braced top can be made to the same weight and stiffness as laminated braces by carving more-or-less traditional spruce braces to whatever profile it takes to get similar results. If that is the case, striving for the same deflection using the laminated braces may well result in a lighter top of similar stiffness to the usual.

I'm sorry, I must be misunderstanding what you are saying. The first and second sentences seem to contradict each other. I would disagree with the first sentence, as I understand it, and agree with the second.

If there is an advantage to laminating with CF, it may well be the lack of "memory" that the material displays. A wooden brace could be made with the same amount of "spring effect" (not at all a new concept) and mass, but a CF laminated brace may well maintain the effect longer.

Not to say the lack of "memory" is not an advantage, but isn't having a lighter top (using CFR) of equal thickness, at least possibly, an advantage in of itself. Isn't that largely what we strive for, lighter tops of stiffness equal to what is known to work?

So anyway, it seems to me that a similar top could be built by springing spruce braces into place and leaving them taller to get the same stiffness and mass as the CF laminated braces, but the CF laminated versions may maintain their "spring rate" over time better than spruce alone.

You're talkin' over years, right?

Deflection testing is a good way to gain information from our top wood and braced tops, but it is not a full measure of the top's stiffness, and we needs to take mass into consideration in order for our deflection data to have much meaning. In other words, if we measure stiffness (with deflection testing) and mass we will likely see a difference between CF laminated braced tops and spruce braced tops, but not necessarily.

Agreed. I would expect a CFR braced top to have less mass than a traditional spruce braced top for equal deflection. I would expect them to sound different as a result. Whether the difference makes a "better" sound or not is another matter.

[Alan Carruth]

Whether you can get information that correlates to about sound from deflection testing a top depends on how you do it. If you just measure, say, the 'dip' at a point 2" in front of the bridge under string load, that's telling you something about the static stiffness. If the usual rules apply it will be a measure of how long you can exepct the guitar to hold up, more or less. If you do the sort of mapping that Dave Hurd talks about in his book, where you find the deflection contours over the whole lower bout, that may very well correlate well with the sound. That's telling you about the distribution of stiffness and mass in the top, which will set up the conditions for the vibration modes that actually make sound.

Alan Carruth / Luthier

[Steve Senseney]

I think they look very pretty , and well done.

One of the reasons to use the CF reinforcement is to avoid changes in shape with the humidity.

Is this better (or worse) with this position as opposed to placing the CF vertically in the center of the brace?

[Judd Holt]

Barry- Top braces glued in normal manner, i.e., go-bar, hide glue, 25' radius. Scallop braces at full width. Glue .020 CF with T-88 epoxy. Profile braces with chisel and diamond file.

Craig- For me deflection is not what is tested, but, what is noted. The process in my building is more touchy-feel y. As far as the 'model' is concerned, I regard that as an 'all depends' proposition. The noted deflection in my tops occurs after stringing. After 2 months movement seems to abate; the instrument having found its set. This instrument moved about 80% less (had to do a bit of neck resetting). Of coarse one has to regard stiffness by location, and we're all about relativity in the build, so there is much I know not.

John- Dr. Dave is way more analytic than I am. My goal was this:to equalize the pull between the strings and the sprung braces, and in so doing, achieve greater amplitude in top movement. The axiom of greater strength being achieved with reinforcement furthest away from the axis point is why I chose this method. I make no pretense about really knowing what I am doing, but, like many here....searching in my own way. I agree with all you wrote.

My original experiments were gluing CF to the tops of same width, different heights, strips of wood. Flection testing was done with fingers against same size wood strips without CF....how's that for scientific?

[John Hamlett]

Craig, I can see how my first two statements can seem contradictory, but what I'm saying is something like:
You can get similar stiffness to mass ratios using spruce bracing alone or with CF laminated braces, but they must be carved differently. If braces are made similarly, the CF laminated ones are likely to be stiffer at a similar mass.
As for the lack of "memory", yes, talking many years.

[Jason Rodgers]

Yes, VERY clean work! And just to be clear, the black seen in the photo is the ONLY carbon fiber you used? Why just there? Why not on the upper X arms, as well? Why not over the X intersection?

[Judd Holt]

Yes, VERY clean work! And just to be clear, the black seen in the photo is the ONLY carbon fiber you used? Why just there? Why not on the upper X arms, as well? Why not over the X intersection?

Jason-Since most top movement takes place in the lower bout and the X-brace being the primary component of that engineering, this is where I thought best to test. The points that John Hamlett brings up are good ones, regarding mass. CF is heavier than the spruce, so, in keeping with the notion of 'lighter more responsive', other components were conventionally dealt with. As John suggests, I could have achieved the same stiffness with less weight, but, I would not get the same 'spring action' I sought. And again, I can't know what is really taking place physically , but, I'll definitely be taking some more cracks at it.