rod/bar/tube material that is good in compression

[Randolph Rhett]

I am doing an experiment building an instrument with a tailpiece and floating bridge. Unfortunately, if I want the top as light as possible the compression of the tail towards the neck is causing distortions. I'm getting something of an s-wave across the top as the top "crinkles" under the compression. By reinforcing the top to bottom with a post (destroying the tone, of course) I can tell it is the compression of the tail block and not the downward pressure of the floating bridge that is causing the problem. I need to counter the tailpiece's tendency of pulling the top of the tail block towards the neck.

I was thinking of running a thin rod/bar/tube from the top of the neck block to the top of the tail block to avoid this compression and distortion. I would want the rod/bar/tube to not touch the top so as not to affect the tone. I'm struggling to think of a material that would do the job.

Yeah, I know. Crazy experiments sometimes don't have solutions. But, someone may have suggestions. Most things spanning 18" with small enough diameters (say 1/8" to 1/4") bow under compression. For example, fiberglass rods (like a go-bar stay) bow in compression. Carbon fiber tubes can be very stiff (great beam strength), but are notoriously weak in compression. I've already tried carbon fiber and it buckles. I suppose I could go with a thick wood block, but that somewhat defeats the attempt at a very light guitar.

Suggestions?

[David King]

Carbon may be weaker in compression than in tension but it is still very good. CF kite tubes or arrow shafts would be obvious places to start. I'd go by the local hobby shop and check out the Midwest Products CF display. I have a 6mm o.d. tube here that will not deflect easily. Apparently these axial spars are common in Russian guitars
One way to monitor when a column is getting close to it's weight bearing capacity is to listen to it's resonant frequency as pressure is applied. The frequency will drop to 0 just as the column fails. Of course you want the terminations to be very clean, preferably a point at the middle of each end.

[Jason Rodgers]

So, to clarify: you want to span the end blocks with some sort of brace that prevents them from tipping inward.

I've seen people use Rick Turner's CF flying buttresses on both ends. In other words, CF tubes running from the top of the neck block to the waist, as well as tubes from the top of the tail block to the waist. These are much shorter spans than the full length of the box. I've seen 1/4" up to 1/2" CF tubes used.

[Mario Proulx]

Wood is at its best in compression, so take it from there.

[Michael Lewis]

I have used straight grain redwood and CF tubes for this application in various instruments. Both seem to work well and the redwood is much less expensive.

[David King]

What about wrapping a redwood spar in a CF woven sock and vacuum bagging that? Or just laminating on thin unidirectional CF panels to the 4 sides. I guess the socks are great for torsion which isn't your problem.

[Alan Carruth]

I used a plain old spruce rod in my first harp guitar, and it worked fine. Taper it toward the ends to reduce weight. Mine was about 1" diameter in the middle and maybe 1/2" at the ends, iirc.

That frequency thing applies to the longitudinal pitch of the rod. Think of a go-bar. A rod with no load but constrained at the ends has a longitudinal resonance that depends on the geometry and the material. As you load the ends the pitch drops because the load resists the stiffness of the rod trying to straighten it out. At the buckling load the stiffness is overwhelmed, and the pitch drops to zero. Since this is a linear effect it can be used to predict the buckling load on a column: you load it until the pitch drops an octave, at which point you've got half the load it can take. They're using this in aircraft design to optimize the sizes of truss members, and I first heard about it years ago from a researcher at the Navel Post Graduate Institute who's been involved it this.

Since you know the string tension on your guitar, you could take that as the max load and experimentally come up with a rod that will work. It might be interesting to see how small a rod would be needed.

Thickening the rod in the middle raises the resonant pitch, of course. So does using the same amount of material as a tube, since the stuff in the middle doesn't do nearly as much work. I read years ago about a business that makes optimized tubes for aircraft structures. You tell them the length and the load, and they design and make a CF wrapped aluminum tube to that spec.

[Alan Carruth]

I goofed! The frequency thing applies to the TRANSVERSE resonances. The only excuse I can make is that I've been severely under the weather for the past few days, and when the thermometer is as low as it's been lately, that takes some doing. Since I could not edit the post you'll have to do it yourselves.

[Randolph Rhett]

So, to clarify: you want to span the end blocks with some sort of brace that prevents them from tipping inward.

I've seen people use Rick Turner's CF flying buttresses on both ends. In other words, CF tubes running from the top of the neck block to the waist, as well as tubes from the top of the tail block to the waist. These are much shorter spans than the full length of the box. I've seen 1/4" up to 1/2" CF tubes used.

I've seen that as well. I always wondered what at the waist they were anchoring to. There just doesn't seem anything there that would provide a solid anchor. Perhaps the back itself? I'll have to mull on that...

[Jason Rodgers]

It's usually some block that runs the height of the waist. Being the most curvy area of the instrument, and narrowest across the body, I'm guessing this is the strongest/stiffest location for an anchor block.

[David King]

For what it's worth some of the steel string guitars with the longitudinal post between end blocks can generate some serious volume and tone. Well worth experimenting with the concept if you have the time and inclination.

[Steven Smith]

So, to clarify: you want to span the end blocks with some sort of brace that prevents them from tipping inward.

I've seen people use Rick Turner's CF flying buttresses on both ends. In other words, CF tubes running from the top of the neck block to the waist, as well as tubes from the top of the tail block to the waist. These are much shorter spans than the full length of the box. I've seen 1/4" up to 1/2" CF tubes used.

I've seen that as well. I always wondered what at the waist they were anchoring to. There just doesn't seem anything there that would provide a solid anchor. Perhaps the back itself? I'll have to mull on that...

Here's one I did that shows the block I installed at the waist for an anchor. Could also run rods to the tailblock for your application.

[David King]

Here's a tube that might be a good candidate. They come in huge range of sizes in 1/32" increments.
https://dragonplate.com/ecart/product.a ... 42&cID=102
You won't beat the price.

[Dennis Duross]

I couldn't tell you how it sounds because I haven't finished it yet, but here's a take, below.

The bar is poplar, 1.5" wide x 1.125" deep, so yes, it has some heft to it.

Going to have a floating bridge, and an off-set soundhole.

[Randolph Rhett]

I have ordered some wrapped (as opposed to pullstruded) tubes. Let's see if they hold up. I'll probably do two at top and one centered below.

I can see I'm not alone in working on this problem. Thanks for all the ideas!

[Mark Swanson]

Looks like clean work Dennis but I don't think it needs to be half that massive. Why so large?

[Dennis Duross]

I'm sure you're right, Mark. But the neck bolts onto the beam, so I thought I'd just go ahead and run it the whole length at the full dimension.

How much smaller would you feel comfortable with?

[Alan Carruth]

I actually used a through bar from the neck to the tail block a few times years ago on a version of a semi-hollow archtop. The bar was large, similar to what Dennis used. Aside from allowing me to use very shallow sides, I mounted the pickups on the bar, which kept the weight of them off the top. I was able to get a nice 'acoustic' sound from a very small box, which minimized feedback issues. I stopped work on that line of things when UPS destroyed a prototype, and refused to pay off the insurance. Pretty discouraging.

[Dennis Duross]

Mark, that last line of mine may read like sarcasm, but it's not.

[Mark Swanson]

Not at all, Dennis!
I would want to consider the whole design before I could say how I'd do it. I guess if your bar is what is holding the neck in place then maybe it's fine...I was just wondering about it and wanted to hear your reasons, thanks.