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The Dreaded "S-Curve"?

Discussion in 'Hardware, Setup & Repair [BG]' started by CheshireCatFun, Mar 9, 2013.


  1. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    I'm working on a new ERB bass project, and was debating using a classic vintage truss rod (to capture that classic Fender P-Bass neck look, with the skunk stripe) vs. using a double expanding truss rod, when I ran across the concept of the "s-curve" warp.

    Could any of you shed light on what exactly it looks like and entails. The source in particular attributes it to the vintage truss rod found in Fender P and J Bass necks.

    C.
     
  2. zortation

    zortation

    Dec 26, 2011
    Toronto, ON
    S curve and warp are two different afflictions of a bad neck. S curve is when a neck has two bows instead of one, rendering it useless.

    A warped neck is a neck with a lateral twist in it, rendering it useless...like what an old suspension bridge does in a hurricane.
     
  3. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    And that is most often caused by a vintage truss rod? And, if so, how common is it?

    Any known pics?

    C.
     
  4. zortation

    zortation

    Dec 26, 2011
    Toronto, ON
    I tired google images but this is all I got...this and pics of curvaceous women in revealing dresses:

    2686909815_eb6e8541bf_z.
     
  5. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    Pics of curvaceous women in revealing dresses, and you went with a pic of a crane? :D

    C.
     
  6. zortation

    zortation

    Dec 26, 2011
    Toronto, ON
    Ok...here ya go

    from googling "s curve neck"

    [​IMG]

    No word on whether she has a vintage rod installed...although I think I may have one handy.
     
  7. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    That's a bit more like it . . .

    So, s-curve, fact or fiction, and is it caused by a vintage rod?

    C.
     
  8. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    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.

    I hope this helps!
     
    Stefan Verbeeck and Bluebard like this.
  9. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    Brilliant analysis and explanation. Makes perfect sense. Very illuminating!

    Immeasurably! Thank you so much.

    So what would be some viable solutions like you listed? The neck was going to be 4"-5" wide, quartersawn maple, with up to three carbon fiber rods, and a possible laminate down the middle, and a generally thicker neck, since I don't need to wrap one hand around it. Perhaps also I could extended the neck heel out a bit further to make a smoother transition, kinda like a PRS?

    C.
     
  10. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    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.
     
  11. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    what don't you like about CF stiffeners?

    american standard fender necks have them, and over the years these have been blessedly free of the dreaded S-curve you described so perfectly (which is really that spot where the back of the neck transitions from round to square and bends up into the ski ramp, followed by tightening the rod 'til it backbows between this kink and the nut).
     
  12. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    *gears are definitely turning*
     
  13. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    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 fiber 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.

    Rant over.
     
  14. BruceBass3901

    BruceBass3901

    Oct 17, 2009
    Wickham, UK
    Bruce Johnson, might I just say that your definitions and descriptions have been really imformative. Thank you very much for taking the time to post them.

    Might I ask if you have a picture or diagram of how you install the TOW carbon section in your necks? I am having a few problems visualising exactly what you mean
     
  15. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Thanks, Bruce!

    Have you seen this set of web pages?

    http://www.xstrange.com/Building/Buildingneck1.html

    It's a series of pages showing how I build my Scroll Bass necks, step by step. These are actually from about seven years ago, and a little outdated.

    Down at the bottom of the first page, you can see the carbon fiber stranding along with the truss rod components, before they are cast into the neck. The carbon fiber ends up in the routed slot, under the truss rod, embedded in the epoxy.

    I first started using the carbon fiber like that in the neck back in 1993, on a development project I was doing for another company. That particular neck design had serious S-bend problems, and the combination of my truss rod design with the carbon fiber solved the problems. All of my own bass necks that I've built since 1996 have the carbon fiber in that location. It's cheap insurance against the dreaded kink.

    The carbon fiber stranding is officially called Carbon Fiber TOW, and you can buy rolls of it on ebay. The last roll I bought cost me about $30 for 5000 feet, which is a lifetime supply. It comes in various sizes, which are called 2K, 4K, 6K, 12K, etc. The designation refers to the actual number of carbon fiber strands. I mostly use the 6K size which, if you look at it with a big magnifier, is actually 6,000 tiny strands. It comes off the roll as a loose ribbon, but if you twist it up, it's about 1/16" diameter. It's very handy stuff to have on hand for all kinds of reinforcement and repair work.
     
  16. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    Before we begin . . .

    BRUCE, MEET BRUCE!!

    *I always find those moments amusing . . . small world*

    <hr>

    Alright, first off, how deep would one go with installing reinforcement material, or the truss rod. You mention some of these materials being as close as 1/8" from the back profile.

    Next . . .

    A strip of 7075 aluminum? Tell me more about this. I suppose you could have several of these in the neck.

    Also . . .

    I'm getting a rough idea of how this works, but some visuals would be very helpful.

    C.
     
  17. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    aha, distinction noted!

    fender supposedly uses the CF by wrapping it around two wooden dowel rods, with the CF controlled to be stiffer where they want and more flexible where they want, then they inlay the wrapped rods into the neck (no idea how deep).

    the reasoning i heard kinda matches yours, the goal is trying to keep as much wood as possible still in the equation.
     
  18. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier

    Hmmm, that's interesting. I don't follow everything that Fender does, but I haven't heard of them using that technique before. If so, that's very sensible. They must have spies in my shop! (just kidding!) Disclosure: I do design/consulting/custom work for various instrument companies and Luthiers, but I haven't yet done any work for Fender, even though they are nearby.

    There are many ways that you can use the carbon fiber TOW to reinforce a neck. For example, you can rout a pair of slots on either side of the truss rod, down deep to within like 1/8" of the finished back surface of the neck. Lay in a bundle of TOW in the bottom in a bed of epoxy, with a nice-fitting wood filler strip above it, filling up the slot. Now the carbon fiber strands are right where they are supposed to be to handle the tension loading. The whole installation is much stronger, lighter weight and (dare I say it on TB?) better sounding.

    I've also built necks where the carbon fiber TOW bundles were set into slots in the side surfaces of the center strip of the neck, before the strips were laminated together. That's another way of placing the carbon fiber where you want it, to do the most good. If you've seen the infamous Banjozilla on my web site, that's how I made its neck, way back in 1990. It has carbon fiber TOW bundles set into the sides of the center strip, and no truss rod.

    I didn't mention it before, but another reason why the small bundle of carbon fiber TOW is stronger than those danged bars is the glued surface area. When you set the raw fibers down into liquid epoxy in a channel in the neck, the epoxy is surrounding and bonding to the outside of every one of those teeny little fiber strands. Add it up and it's a lot of surface area. In comparison, when you glue to the bars, the only surface area you've got to bond to is the flat sides of the plastic resin.

    Reinforcing a neck with aluminum strips is done in a similar way. Rout a slot, like 1/4" wide. Then fit in a thin aluminum strip, standing up vertically, with a wood filler strip beside it to fill the remaining space. The slot and the strip don't have to be constant depth. You can make them taper or curve in depth to vary the amount of stiffness in different areas of the neck. And there's no need to fill the slot with solid aluminum. A strip of 7075 aluminum 1/16" thick by 1/2" deep will provide all the stiffness you want, with minimal weight.

    Lots of other possibilities.

    For the record, I'm not just pulling these ideas out of the air. I've been a pro Luthier for almost 20 years, and I specialize in custom instrument necks. I've built around 900 instrument necks over the years, of all different types and sizes, for many clients and my own instrument lines. I've done a lot of serious experimenting with truss rods and reinforcements. I have actually built necks with the techniques I've described above.
     
  19. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    First of all, thank you soooooooo much for this excellent information, and a much bigger response forthcoming, but is this the kind of thing you were talking about when ordering a tow of carbon fiber? Also, where would I get some of that 7075 aluminum in 1/16" by 1/2" strips? Also, would they be set deep, and then covered with a wood filet?

    C.
     
  20. CheshireCatFun

    CheshireCatFun

    Mar 9, 2013
    PS. - If I'm using all these methods, how thin can the neck get? It doesn't need to be terribly thin since, afterall, I'm mostly tapping and don't need to wrap my hand around it, but, still, I want to see where I can maximize comfort and facility in playing.

    C.
     

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