Less expensive carbon fiber reinforcement

[Beej]

Well here's my first swack at this. Not sure if I'm doing it right but there's three strands of 12K TOW in each channel and a ton of epoxy. Once it's dry and sanded I'm moving on to fretboards and just assuming this is going to be equivalent to putting in Dragon Plate carbon fiber bars. Fingers crossed!

View attachment 4339845
View attachment 4339846

I'm still very curious to hear your thoughts on this experiment once it's a built, functioning neck. In the interest of learning, how did you do these? How deep and wide are the slots and were all three layers of CF laid in the bottom of the channel?

Gotta say I like the experimental spirit.

 

[Rob Page]

How deep were those slots?

6mm

 

[Rob Page]

I'm still very curious to hear your thoughts on this experiment once it's a built, functioning neck. In the interest of learning, how did you do these? How deep and wide are the slots and were all three layers of CF laid in the bottom of the channel?

Gotta say I like the experimental spirit.

Slots are 6 mm deep, roughly 1/8" wide. I didn't have any specific reason, I believe that's what the Dragon Plate bars I was using were so I just stuck with it.

I really fumbled around at first making a big mess but what I finally landed on was, stuff a TOW strand down into the channel, mix up a small batch of epoxy, pour ("pour" I should say cause the stuff's pretty viscous and doesn't really pour so great) then stuff another strand on top of that one, pour more, stuff another strand on top, etc.

The hardest thing was the quick working time. In hindsight I should have used West Systems slowest hardener, 209 cause here in Southern California, the 205 "fast" stuff is just wayyyyy too fast.

 

[Rob Page]

You are embedding a small super stiff cable in there, that will go in tension when the strings start trying to bend the neck forward. These cables are acting like guy wires holding up a pole.

I recommend filling the rest of the slot, above the 4 strands, with a strip of wood.

I had always thought the CF was to reinforce the neck and keep it from bending forward. The last few necks I've built, I actually had to use the truss rod to provide relief cause the strings weren't doing it. (And these were SIX STRING basses!) That's how strong my necks are. So I began questioning the point of even having the CF. Then someone—perhaps in this forum, can't remember—said to me the real reason for the CF is to keep the neck from twisting...which, of course, can't be fixed with a truss rod.

That made a lot of sense to me—when using those long thin bars which do not twist easily—but I'm not sure this method is really giving me any of that.

So here's my actual question: will embedding a couple strands of TOW in epoxy like this help keep the neck from twisting?

 

[Bruce Johnson]

@Bruce Johnson
Would something like this work? epoxying the TOW between neck lams during construction and then epoxying the whole structure at once?
View attachment 4340403

Sure, that would work. You could also rout a slot into the side of the center strip, and drop in some TOW and epoxy. That way you can position the CF bundle exactly where you want it, and it doesn't even have to go in a straight line. You can make the path curved, tapered, go around a corner, whatever.

 

[Bruce Johnson]

I had always thought the CF was to reinforce the neck and keep it from bending forward. The last few necks I've built, I actually had to use the truss rod to provide relief cause the strings weren't doing it. (And these were SIX STRING basses!) That's how strong my necks are. So I began questioning the point of even having the CF. Then someone—perhaps in this forum, can't remember—said to me the real reason for the CF is to keep the neck from twisting...which, of course, can't be fixed with a truss rod.

That made a lot of sense to me—when using those long thin bars which do not twist easily—but I'm not sure this method is really giving me any of that.

So here's my actual question: will embedding a couple strands of TOW in epoxy like this help keep the neck from twisting?

This method of using small bundles of CF TOW is using them as tension cables. Picture them as acting like guy wires holding up a pole. Like a length of steel cable, they don't do anything if you push on it or move it sideways. They only have strength and stiffness when you pull on both ends.

When you put them in the neck down deep, parallel to the centerline, they are stiffening the neck against bending forward from the string tension.

In resisting twisting, one small CF bundle down the middle won't do anything. Two parallel CF bundles down deep will resist some, but not much. To really resist twisting, you'd need to use two CF bundles, which cross over in an X pattern. Those bundles would be most effective right up under the fingerboard. Think of what you'd do with guy wires to stop a pole from twisting.

The rectangular extruded CF bars do have a good resistance to twist, proportional to how tall they are. Fitted into a tight slot, they will provide some resistance to twist. But not as much as the small CF bundles in an X pattern going the full width of the fingerboard.

What you did on your necks, two parallel slots stacked deep with CF TOW and epoxy, will mostly resist against the neck bending forward. But it will provide some resistance to twisting.

 

[Rob Page]

Has anyone tried using carbon fiber+epoxy as black-ish divider between neck laminates?

 

[MPU]

It would be very messy when sanding. Cf dust gets everywhere. I mean everywhere. It colours every lighter coloured woods. For stiffening the neck it would be overkill and easily make the neck too stiff to bend.

 

[dwizum]

Carbon dust also contains a lot of sharp fragments that can wreck your skin (not to mention your lungs). When I have to do any significant sanding or cutting of carbon parts I do it in a respirator and face mask and long sleeves, then immediately vacuum the whole area, vacuum myself, vacuum my mask, vacuum the vacuum for good measure, then put the shirt directly in the laundry. Not super fun.

From a structural perspective it's so easy to over-stiffen a neck that I don't think we need the stiffness of actual full carbon laminations like that. And from an appearance perspective they're going to be so thin that you won't know it's carbon. So, might as well use black dyed veneer.

 

[Rob Page]

From a structural perspective it's so easy to over-stiffen a neck that I don't think we need the stiffness of actual full carbon laminations like that. And from an appearance perspective they're going to be so thin that you won't know it's carbon. So, might as well use black dyed veneer.

It isn't stiffening I'm after with CF, it's twisting I want to curtail. Anyway, point taken. Just idle musing...

Oh, and that is black dyed veneer in the pic!

 

[RichterScale]

I don't know the answer to any of it,...but that's a really nice bass.

 

[Rob Page]

I don't know the answer to any of it,...but that's a really nice bass.

Oh thanks man!
@PipeDreamFretworks

 

[Brich]

@Bruce Johnson
Some really great info here. So 3 years ago I built my first neck from curly maple that developed a twist. I'm currently on build 4 and that worries me. I'm using a less figured wood (Birdseye maple) and want to add a bit of assurance. Would any hardware store epoxy suffice? Also is this what I should buy for TOW?

Thanks for any recommendations

 

[Rob Page]

I actually went back to bars just for that reason—twisting. My necks don't really need reinforcement to keep them from bending forward. In fact, I usually have to use the truss rod to add relief to the neck cause the strings aren't doing it. But I do worry about twisting. Bruce mentioned earlier that this method won't do anything to prevent that.

 

[Bruce Johnson]

@Bruce Johnson
Some really great info here. So 3 years ago I built my first neck from curly maple that developed a twist. I'm currently on build 4 and that worries me. I'm using a less figured wood (Birdseye maple) and want to add a bit of assurance. Would any hardware store epoxy suffice? Also is this what I should buy for TOW? View attachment 4409490Thanks for any recommendations

Brich;

Yes, that's the carbon fiber TOW that we've been talking about.

You need to use an epoxy that's low enough viscosity that it will soak into the TOW. Most of the hardware store epoxies, particularly ones that cure in 1 hour or several hours, are too thick. Some of the 24 hour epoxies are okay. Look at the packages in the store. Does the resin in the bottle flow like liquid, or is it a thick syrup?

 

[Bruce Johnson]

I actually went back to bars just for that reason—twisting. My necks don't really need reinforcement to keep them from bending forward. In fact, I usually have to use the truss rod to add relief to the neck cause the strings aren't doing it. But I do worry about twisting. Bruce mentioned earlier that this method won't do anything to prevent that.

If you specifically want to use the carbon fiber TOW to resist twisting, then you need to put it in the horizontal plane, right under the fingerboard, but in an X pattern. One line from one side of the heel to the opposite at the nut area. And another the opposite way, crossing over at the middle. Same installation; cut a small groove and lay in the TOW in a bed of epoxy in the groove. This forms crossed guy wires, like you were trying to stabilize a telephone pole.

But, of course, the best way to avoid twist is careful selection and cutting of the wood, to get the grain lines and rings in the right orientation. As we talked out on the other thread.

 

[dwizum]

Some brands of epoxy carried in hardware stores, craft stores, or hobby shops have a specific "laminating epoxy" product. That's the one you want. Bob Smith and Zpoxy are common brands.

 

[JayGunn]

+1 on Z-Poxy.

I got some for grain filling, where it is also best to have low viscosity (worked great, set up hard enough to sand in about 4 hours). Since then I have used it for laminating veneers and for general wood glue-up, and it has worked fine. Because it is thin I use up some of the allowable "working time" so it can soak into the grain in a joint, and I add a bit more if it seems to need it, then clamp, not too strongly...

 

[eadbass]

I have a question for @Bruce Johnson.
First of all thank you for all the precious advices you share here.
I'm tempted to try your carbon backstrap method but I'd like to clear up a few things.
You drill the truss rod slot a bit deeper and put 2 or 3 strands of TOW carbon fibers in epoxy. Do you sit the truss rod simply just above the carbon fibers/epoxy?
Do I need to wax the truss rod?
I assume the truss-rod acts pushing in the direction of the fingerboard side of the neck (for a dual truss-rod deflection near head and heel in one direction, and near the middle of the neck in the other direction) but doesn't the backstrap added stiffness somehow limit the action of the truss-rod?

 

[Bruce Johnson]

I have a question for @Bruce Johnson.
First of all thank you for all the precious advices you share here.
I'm tempted to try your carbon backstrap method but I'd like to clear up a few things.
You drill the truss rod slot a bit deeper and put 2 or 3 strands of TOW carbon fibers in epoxy. Do you sit the truss rod simply just above the carbon fibers/epoxy?
Do I need to wax the truss rod?
I assume the truss-rod acts pushing in the direction of the fingerboard side of the neck (for a dual truss-rod deflection near head and heel in one direction, and near the middle of the neck in the other direction) but doesn't the backstrap added stiffness somehow limit the action of the truss-rod?

Hello eadbass;

I'll try to answer your questions. The carbon fiber backstrap is only in the inboard half of the neck, from about the 5th fret to near the end of the heel. And it's set down low in the neck, as close to the back surface as you dare. I try to place it about 1/8" from the back surface, right down the center.

The backstrap doesn't interfere with the action of the truss rod. It actually helps the truss rod control the shape of the neck into the best relief curve. The ideal relief curve isn't a constant curve along the length of the neck. It's nearly flat on the inboard half of the neck, increasing curvature closer to the nut. The backstrap makes the inboard half of the neck stiffer, with most of the curvature happening out between the nut and the 5th fret. That's how you want the neck to bend under the string load. The backstrap braces the inboard half of the truss rod, allowing the outboard half to resist the string load bending. While it's doing that, the backstrap is also preventing the back of the neck around the 12th fret from stretching and causing the dreaded "12th fret kink". The truss rod and the backstrap are working together to do the same thing.

If you are installing a single-rod truss rod, you'd normally rout a slot in the neck for it which curves down deeper in the middle and comes up at either end. That distance that it curves down in the middle is called the Droop, and is normally around 0.180". That is, the slot is 0.180" deeper in the middle than it is at the two ends. If you are installing a backstrap, you cut that slot so that it slopes down on the outboard end to the correct amount of droop in the middle, but then continue that slot straight at the same depth in to near the heel. This forms a shallow wedge of space underneath the inboard half of the truss rod, as it curves back up. That space gets filled with the backstrap; the carbon fiber TOW strands and epoxy.

If you are installing a double-rod truss rod, you would normally cut a straight slot down the neck, of the right depth to fit the truss rod. To add the backstrap, that slot takes a step down deeper (about 1/16" to 1/8") at around the 5th fret, and continues straight back to the heel. That's where the carbon fiber strands and epoxy goes.

Yes, you need to wax the truss rod, if you are going to cast the whole truss rod in epoxy. That has risks, advantages, and tradeoffs. You may not want to do that. If not, you can install the backstrap into the slot by itself first. Lay the carbon fiber strands and epoxy down into that lower slot, neatly, and let it cure. If necessary, clean up the main part of the slot with another pass of the saw or router. Then install your truss rod "dry" as normal in the slot, right on top of the backstrap.