Sorry I'm late.....I've been writing posts on some other subjects....
Here's a post I wrote a while back, about the Dreaded S-Curves that happen in many Fender-style bass necks, and whether this is related to the type of truss rod chosen. The discussion gets into the "ski jump" condition, which I'm calling the "12th Fret Kink".
Well, no, an S-curve in a neck isn't caused by having a vintage-type single-rod truss rod vs a modern dual rod. You'll more often see S-curves develop in Fender-style necks, which usually have the vintage-type truss rod. But don't blame the truss rod.
The S-curve is the result of two things happening, one causing the other.
The first thing is what I call the "12th fret kink". It's a very common problem on guitars and basses. The neck essentially bends a small amount in the area right around the 12th to 14th frets. It's as if you'd bent it over your knee at that point. If you put a straight edge on the neck, you'll see that it's flat from the nut to the 12th, then takes a little bend from the 12th to the 14th, then is flat from the 14th to the 20th. This is also often called the "ski jump" effect. It's often described as the heel kicking up, but what's really going on is that the neck has bent slightly right where it transitions to join the body. The net effect is a wedge-like ramp from the 14th to the end of the heel, and fret buzz at the 20th.
So, why does the 12th fret kink happen? Because that's the most highly loaded part of the neck. The wood on the back of the neck, right behind the 12th-14th fret area is under higher tensile stress than anywhere else on the neck. If you think about it, the neck is a long lever handle, and the strings are yanking on it, trying to pull the headstock forward. The stress point is where the handle joins the body, because of the leverage ratio.
What happens is that, over time, the wood at the back of the neck gradually stretches from the continuous tension. So the neck bends right there. And the truss rod generally isn't much help in counteracting that load. Most production truss rods, either vintage-style or modern dual rods, don't do much of anything beyond the 12th fret, because they are right up against the underside of the fingerboard at that point.
The truss rod is designed to support and adjust the neck, in the area from the 1st to the 12th fret. When you tighten it, the load it applies to the neck is centered around the 5th fret, and has tapered off to almost nothing at the 12th.
And this is how the S-bend happens: First the neck develops the 12th fret kink from stretching over time. This causes the action to go high, with too much relief and buzzing down at the 20th, etc. So, the owner gets out a very large wrench and cranks on the truss rod, trying to get the neck to flatten out. But it doesn't, because the truss rod isn't pushing against the area that's bent. But the owner keeps cranking, causing the neck to backbow between the nut and the 12th. But it still has the kink at the 12th and the ski jump.
There's the classic S-bent neck. Low at the nut, high at the 5th, low at the 12th and high at the 20th. It's a common problem on old Fenders and basses with that style of neck, but it can happen on other style necks too, even neck-thru designs.
The heart of the problem is the neck being too thin (front to back) at the area where it transitions to the body. We Luthiers have various tricks and ways to counteract the kink, from better truss rod geometry, to carbon fiber inserts, to multilaminate construction. Lots of different solutions. But that's the area you need to look at in your design.
Here's a follow-on post, about what to do to counteract the dreaded 12th fret kink, in the design of a new neck:
The main thing is to look at that transition area, where the neck goes from the thick part of the heel, down to the main part of the neck. That's the critical area. Look at the depth and cross section right there. You want to keep that spot as deep (that is, thick front to back) as you can. Ideally, from a structural standpoint, you'd like the neck to have a smooth taper, increasing in thickness from the 1st fret up to the heel thickness. But, most players don't like that feel. So, we cut most necks so that they are only a little thicker at the 12th than at the 1st, with a curved transition to the heel. That's what players want. But the thinner it is at the 12th, the higher the stresses and the more likely that it will kink over time.
And obviously, the more strings you have, the higher the load will be.
Things you can do to beef it up:
Make the transition longer, more gradual.
Play with the truss rod geometry. One of the cool things about the old-fashioned single rod in a curved channel is that you can cut the shape of the channel to modify how it takes the load. In this case, sink the anchor end down deep in the neck, so the rod curves down from the 1st to the 5th, and then runs almost parallel to the back of the neck, all the way to the end of the heel. That reinforces the critical area.
I'm not crazy about carbon fiber bars myself, but if you are going to use them, put them where they are really needed. Run them from the 5th fret down to the heel, and set them in deep. That's where you need the strength and stiffness.
Going with a laminate of quartersawn strips doesn't really increase the strength of the neck, but it does help a lot with the stability. And the more highly stressed the neck is, the more likely you are to have stability problems. So it's a good idea. Thin little ribbon strips look pretty, but they don't do anything for the structure.
Then, of course, someone else asked why I don't like carbon fiber bars. So I replied:
I personally don't like the carbon fiber bars because they are essentially a block of plastic. The bars are not solid strips of carbon fiber; they are bundles of carbon finder stranding which are formed into a rectangular shape by pulling them through a bath of polyester resin. The process is called pultruding, which is like continuous casting. My point is that the cool looking black bar is basically a block of plastic with some carbon fiber stranding inside. It's made in rectangular strips to be real easy to install.
So, when you install them, you are routing out strips of wood from the neck and gluing in bars of plastic. I don't like to do that. If you need to stiffen up the neck, there are better ways to do it which leave more of the wood in place. And there are many good ways to build necks that stay stable and straight, without using those bars.
Don't get me wrong; I like carbon fiber, and I use it quite a bit. But I buy it in rolls of the raw stranding, which is called TOW. When I want to reinforce an area of a neck, or a repair or something, I make up a bundle of the TOW strands and set them in epoxy in a small groove or hole. I can put the carbon fiber right where I want it to have the most effect, and it's very strong and compact. A little bundle of carbon fiber, less than 1/8" diameter, placed down deep in the neck will increase the stiffness more than one of those big rectangular bars.
That's one of the ways that I prevent the 12th fret kink in my necks. I set my truss rod down deep in the neck, where it passes through that area, and I fully surround it in epoxy. I also lay in two bundles of carbon fiber TOW down in the epoxy, under the truss rod. The bundles are about 12" long and 1/16" diameter, running from about the 6th fret to near the end of the heel. The important thing is that the carbon fiber ends up within 1/8" of the back surface of the neck. That's where the highest tension loads are in the wood, and the carbon fiber is perfect for keeping it from stretching over time.
The bars are popular because they are easy to install. Rout or saw a slot and glue 'em in. And you have instant Carbon Fiber Reinforcement for Increased Stability! Which is great for marketing. I mean, Carbon Fiber is cool and high tech. And everyone wants Increased Stability, right? That's why the manufacturers like them. It lets them be sloppier in choosing and processing the wood, with fewer rejects for warpage. Plus, it's a new feature to sell.
The truth is, if you want to reinforce a neck to make it stiffer and more stable, you can do a better job with aluminum. A strip of 7075 aluminum, 1/16" thick x 1/2" deep, standing up vertically, will increase the stiffness more than the carbon fiber bar. And it's lighter weight and cheaper. I've done it, and it works very well. Like I said, there are many better options.
But Carbon Fiber Bars are just so cool, I guess.
Back to the topic of Ski Jumps and 12th Fret Kinks, that's my quick rundown on what causes the condition and how I prevent them in my necks. All of the necks that I build for my own basses, and many of the neck structures that I build for other Luthiers, include what I call the "back strap". It's a bundle of carbon fiber strands in epoxy that's about 10" long. It starts at the 5th fret and runs almost to the back of the heel. It's located down under the truss rod, within 1/8" of the back surface of the neck. It runs at a slight angle, from the side view, from near the heel surface, tapering up until it meets the truss rod in the middle, at the deepest point of the rod's curvature.
That's how I solve the problem. A hidden stiff cable to keep the neck from stretching at the back. No stinkin' ski jumps on my necks! I first started using these carbon fiber back straps back in 1993, on a development project that a group of us were doing with SWR. Those basses had unusually thin (front to back) necks at the 12th, and the first prototypes had terrible, immediate ski jump problems. The back straps were mandatory on those necks.
So, can this strap be added to repair an existing Fender-style neck? Sure, I've done it. Rout or mill out the skunk stripe, about 1/4" deep, from the 5th fret back to the middle of the heel surface. Lay in a bundle of carbon fiber stranding in an epoxy bed, covering it with a replacement skunk stripe. That will stop the stretching and kinking. Of course, you still have to resurface the heel fingerboard to take out the existing kink. I don't think that it's practical to bend the kink back.
Someone may ask, well, why doesn't Fender just slip a little carbon fiber stranding bundle under the skunk stripe, in the factory? And eliminate the problem from the start? I don't know. They haven't called.
