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Headblock Support Struts
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Author:  Terence Kennedy [ Thu Nov 07, 2019 12:12 pm ]
Post subject:  Headblock Support Struts
After talking with Tim McKnight at a recent guitar show I decided to try headblock support struts. I am using 3/8" hollow carbon tubes from Dragonplate as Tim does. I am using a double tenon Bourgeois type neck joint and have modified my upper bout structure so settling in is not too bad but still settles a little more than a guitar with a glued extension. I am looking for more stability.

Tim says he sees very little movement at stringup and it stays that way seldom requiring later tweaking with settling in. He has lightened his upper bout bracing considerably. I am excited to see what happens. I am wondering how many of you folks have tried some sort of headblock strut support and what your experiences have been.

Thanks

Terry

Image

Image
Author:  Hans Mattes [ Fri Nov 08, 2019 1:27 pm ]
Post subject:  Re: Headblock Support Struts
I've been using headblock support struts for a while now. I've found that additional struts between the waist blocks and the tail block reduce (but don't entirely prevent) "settling in." Rather than carbon fiber tubes (which I've tried), I now use hardwood struts, typically ½" x 2", to resist neckblock rotation.

My motivation has been freeing the upper bout to allow additional resonances to support string harmonics, so I use an elevated fretboard and no UTB. All of this frees the soundboard from structural duties so bracing can focus on tone issues. The only structural responsibilities of the soundboard relate to preventing excessive rotation of the bridge, which I manage via lightweight X-bracing and a somewhat tall (north-south) bridgeplate.
Author:  Terence Kennedy [ Fri Nov 08, 2019 3:07 pm ]
Post subject:  Re: Headblock Support Struts
Got any pictures Hans?
Author:  Jim Watts [ Fri Nov 08, 2019 4:48 pm ]
Post subject:  Re: Headblock Support Struts
Terrence,
Looks pretty beefy and I'm sure it'll help. Unless I'm thinking about this wrong (always possible!) it seems like you'd want to reverse the orientation of the tubes so they attach at the bottom of the neck block. That way the fibers go into tension as the bottom of the neck block tries to rotate away, instead of compression in the current configuration. That said the forces are pretty low so it probably doesn't make any difference.
Author:  Hans Mattes [ Fri Nov 08, 2019 7:52 pm ]
Post subject:  Re: Headblock Support Struts
Terence: since you asked,
Attachment:
IMG_4514.jpeg


And before you ask, the grey material surrounding the soundboard kerfing is grout (yep, regular, sand-filled tile grout). On a couple of builds I installed steel weights in the sides, ala Trevor Gore. But I got to thinking -- if the purpose of the weight is to modify the acoustic impedance of the sides in order to reflect the energy in the soundboard back to the soundboard (rather than letting it dissipate in the sides), the impedance change should be located as close as possible to the edge of the sides. Ergo the grout. I know that's not what this thread is about, but I thought I should address the issue if I was going to post the picture. And, to complete the off-topic commentary on the picture, the white dots in the soundport block and the waist block are dowels because I cut the blocks on my CNC from 1" walnut (because my longest end mill wouldn't cut through a 2" piece of wood). And the oval soundhole (surrounded by an oval rosette) allows more soundboard to connect the upper and lower bouts. FWIW.
Author:  J De Rocher [ Fri Nov 08, 2019 8:45 pm ]
Post subject:  Re: Headblock Support Struts
In the past, whenever I've seen extra structures added to the neck block/upper body of acoustic guitars, the stated motivation for doing has typically been to prevent neck rotation over time that would lead to needing a neck reset. If that's the intended purpose, I continue to wonder about the real world need for adding reinforcement structures like these. Does anyone have anything like ballpark numbers for the actual frequency with which acoustic guitars suffer enough neck rotation to require a reset and how much that frequency increases over time? So what percentage of guitars need a reset after 10 years, 20 years, 30 years, etc.? Will all steel string acoustic guitars eventually need a reset? Or if a guitar makes it to something like 20 years without one, is it likely to never need one? I realize that there are structural differences in the neck body joints of guitars from different builders so the need for resets likely varies between builders, but I'm talking ballpark.

It seems like the argument for taking such structural measures would be more compelling if, for example, 50% of guitars need a reset after 20 years than if only 5% of guitars need a reset after 20 years.

On the other hand, the idea of reinforcing the neck block/upper body so as to reduce the structural role of the soundboard and allow it to contribute more to the sound is interesting.
Author:  Colin North [ Sat Nov 09, 2019 8:55 am ]
Post subject:  Re: Headblock Support Struts
Hans Mattes wrote:
Terence: since you asked,

And before you ask, the grey material surrounding the soundboard kerfing is grout (yep, regular, sand-filled tile grout). On a couple of builds I installed steel weights in the sides, ala Trevor Gore. But I got to thinking -- if the purpose of the weight is to modify the acoustic impedance of the sides in order to reflect the energy in the soundboard back to the soundboard (rather than letting it dissipate in the sides), the impedance change should be located as close as possible to the edge of the sides. Ergo the grout. I know that's not what this thread is about, but I thought I should address the issue if I was going to post the picture. And, to complete the off-topic commentary on the picture, the white dots in the soundport block and the waist block are dowels because I cut the blocks on my CNC from 1" walnut (because my longest end mill wouldn't cut through a 2" piece of wood). And the oval soundhole (surrounded by an oval rosette) allows more soundboard to connect the upper and lower bouts. FWIW.


It's good to see well built Martin type clones, but I find it very interested to see someone trying something quite a bit "out of the box".
Thanks Hans, hope you post some more pics.
Author:  Colin North [ Sat Nov 09, 2019 8:59 am ]
Post subject:  Re: Headblock Support Struts
J De Rocher wrote:
In the past, whenever I've seen extra structures added to the neck block/upper body of acoustic guitars, the stated motivation for doing has typically been to prevent neck rotation over time that would lead to needing a neck reset. If that's the intended purpose, I continue to wonder about the real world need for adding reinforcement structures like these. Does anyone have anything like ballpark numbers for the actual frequency with which acoustic guitars suffer enough neck rotation to require a reset and how much that frequency increases over time? So what percentage of guitars need a reset after 10 years, 20 years, 30 years, etc.? Will all steel string acoustic guitars eventually need a reset? Or if a guitar makes it to something like 20 years without one, is it likely to never need one? I realize that there are structural differences in the neck body joints of guitars from different builders so the need for resets likely varies between builders, but I'm talking ballpark.

It seems like the argument for taking such structural measures would be more compelling if, for example, 50% of guitars need a reset after 20 years than if only 5% of guitars need a reset after 20 years.

On the other hand, the idea of reinforcing the neck block/upper body so as to reduce the structural role of the soundboard and allow it to contribute more to the sound is interesting.

Most of the guitars needing neck resets I see are between 20/30 years old, some 10 YO, but I can't say as a percentage of guitars built.
Author:  Ken Nagy [ Sat Nov 09, 2019 9:54 am ]
Post subject:  Re: Headblock Support Struts
Not being taught in guitar making; well not being taught in violin making either; I am always amazed at the massiveness of the soundboxes. Thick ribs. Huge linings. Do they really have to be that stiff. If so, mine will collapse!

The one by Hans has lighter bracing on the soundboard that looks pretty good to me, but then humungous braces to compensate. It seems like a good idea, tying it all the way to the lower block, but I was thinking maybe 1" X 1" pieces of split spruce? You're not driving a car over it. I thought that maybe it was double walled, but then I saw that grey stuff, and read that it was for acoustical weighting. That's cool, but doesn't thinning accomplish the same thing? Find points where clay helps, and thin it there?

I really do like the end block design.

It seems like body flexing is the enemy; but isn't body flexing, and the stresses put on the sound plates by the instrument, part of the sound? The stress adds tension, and the sound created is different?

Just thoughts from someone who doesn't know anything, who is trying to find truth in the madness. I'm not trying to be contrary, I really just wonder how much is truth, and how much is dogma.
Author:  Terence Kennedy [ Sat Nov 09, 2019 10:41 am ]
Post subject:  Re: Headblock Support Struts
I have no answers on all this. I just know that after talking to Tim McKnight and looking at his guitars I found he was extremely impressed with the increased stability and his ability to significantly lighten his upper bout bracing.

I am basically copying his design so we'll see-- I'll report back in a year :)

If anyone else here has any real world experience with headblock support methods I'd love to hear about it.
Author:  Barry Daniels [ Sat Nov 09, 2019 10:55 am ]
Post subject:  Re: Headblock Support Struts
Here is a falcate OM where I used 1/2" baltic birch for the head block and I threw in one carbon rod for some support on the non-cutaway side.

The sides and the linings are also laminated.
Author:  Burton LeGeyt [ Sat Nov 09, 2019 2:07 pm ]
Post subject:  Re: Headblock Support Struts
I did it on a few many years ago and abandoned the method. I didn't see the improvement I would have needed to see to justify the extra work. I spent time attempting to solve the problems with my neck block and top/back radii instead.

I did play one of Tim's guitars at Artisan last year and was impressed! I see why you would want to try it :)
Author:  Trevor Gore [ Sat Nov 09, 2019 6:33 pm ]
Post subject:  Re: Headblock Support Struts
Hans Mattes wrote:

... the grey material surrounding the soundboard kerfing is grout (yep, regular, sand-filled tile grout). On a couple of builds I installed steel weights in the sides, ala Trevor Gore. But I got to thinking -- if the purpose of the weight is to modify the acoustic impedance of the sides in order to reflect the energy in the soundboard back to the soundboard (rather than letting it dissipate in the sides), the impedance change should be located as close as possible to the edge of the sides. Ergo the grout. I know that's not what this thread is about...

I think you're conflating two things here: heavy linings increase the acoustic impedance mismatch between top and sides and so reflect the bending wave in the top back into the top, with less energy dissipating down the sides. On the other hand, side mass alters the momentum equilibrium of the T(1,1)2 mode to increase the net radiating area of that mode so it produces more sound, whilst also increasing the effective mass of the mode (due to the increased radiating area) and so the side mass can be used to tune the resonant frequency of that mode. Side masses are somewhat more adjustable than grout.

Full explanation in the book, of course.

Regarding head block struts, unless the struts are pre-stressed, for the struts to take any load, the neck block has to rotate and the waist flanges have to deflect to take the load. I suspect that by the time that has happened, the "damage", so to speak, has already been done. (i.e what Burton said). However, Hans' design with the "diamond" frame looks like it would be quite effective. Meanwhile, my oldest double tenon neck joint guitars, with the "L" shaped head block are now pushing 15 years and I haven't had to re-set any of those (so far!). Filling the gap between the head block and the upper transverse brace helps a lot, as in the standard Gilet design of bolt-on neck.
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