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Trussrod and Stiffening bars placement!

Discussion in 'Luthier's Corner' started by FunkyMan, Mar 27, 2014.

  1. FunkyMan


    Nov 27, 2007
    Hey folks, i'm about to start my own brand of basses, and i have the first custmer that is supporting me and he bougth everything for a new bass, i think i have most of things clear to start, but there's always some details like this one:

    I'm going to use a two-way truss rods and 2 stiffening bars, and i'm concerned about how i should place them, so look at this ilustration:


    The trussrod will have a bottom acces, so in order to avoid the neck screws to hit the stiffening bars, i guess i should install them further in the headstock's direction as shown, so the quiestions are: (this will be a 5 string neck)

    -1 This is the way you do it normally?

    -2 Will this approach work properly?

    -3 It will affect the way a neck work nomally?

    -4 How far should be the bars from the trussrod(parallel)??

    -5 If everything is possible, i'm planning to place the rods with their ends getting like under the first fret, it will make the truss rod work differently this way?

    -6 Any suggestion will be very appreciative

    Thanks (sorry for bad english and horrible headstock):bag:
  2. syisrad


    Oct 28, 2008
    You have the right idea and are on the right track.

    I install carbon fibre bars in all the necks I make.

    A few things to remember when installing them,

    -The thinnest section of your neck being toward the headstock will have the most flex. The main purpose of installing carbon fibre bars is to make the neck more rigid (stiffer neck = more energy going into the strings and producing more sound rather than energy being absorbed flexing the neck). With that in mind, make sure they are installed toward the headstock end. You are on the right track starting at the first fret however mine are all installed on the other side of the nut where possible.

    -Keep in mind the weakest part on the neck is around the nut where the string tension breaks at an angle hence why volutes are used to ad strength to that section. If you can use your carbon fibre bars to strengthen that area it's a plus. May even save the bass from a catastrophic drop one day.

    -As far as installing them parallel to the truss rod, (you're winning installing them in the first place) I've always installed them parallel to the string direction rather than the truss rod. This means the 2 rods will be closer together at the headstock end and further apart at the body end. The reasoning behind this is most carbon fibre bars you'd install are rectangle in profile. If you took one in your hands you can flex in on the flat face, twist it a little end to end but try to bend it holding the profile upright and you get little to no movement. Given when they are in the neck, it's the strings that are trying to bend them you want to them in-line with the strings to get the most out of them.

    -Make sure you're fully aware of their depth in relation to the profile of the neck you'll be shaping. With that in mind you'll be installing them closer to the truss rod rather than the outer edges of the neck simply because you really don't want to expose 2 nasty looking carbon fibre bars on the underside of your neck.

    -Make sure the channels you route are a perfect snug fit. You don't want to rely on glue to fill the gaps either side of the rod because glue won't stop the bars trying to find the path of least resistance and attempt to angle flat and twist when the pressure is put on them. If you can't get it perfect, at least use a good epoxy glue to hold them in place.

    To answer some of your other questions,

    -Yep, bottom access for the truss rod is perfect because as already covered you want to keep as much material mass (wood) where the neck is weakest (on neck-thru's it's simpler to have headstock access. I always included a volute on an angled headstock for that reason)

    -re neck screws, already covered why it's best to have the carbon fibre bars installed at the headstock end, but hypothetically if you had bars long enough, you could install them the entire length. The neck screw placement wouldn't mater as the bars can be drilled into.

    -Will if affect the way the neck would normally work? Regarding the approach to the rest of the build and set up after you install them the best answer is..No.
    It is simply going to improve the strength of the neck, minimising the chance of it warping and twisting, minimising the input the truss rod will have and improving the sound and feel of the bass.

    -The truss rod will still have to do some work. Make sure your neck is still nice and level once the carbon fibre bars and truss rod are in before you glue the fingerboard on. Even with the carbon fibre bars installed and especially with a 5-sting you can just about guarantee you'll still need input from the truss rod. That's a good thing as you don't want a dead hunk of un-tensioned metal knocking around in the neck, not just because of rattles but because it will kill the neck vibrations and = crap sound.

    -Lastly, make sure you wear a masking when cutting or sanding the carbon fibre. It's nasty stuff. (I've assumed you're using carbon fibre as it's the most common used for it's good strength:weight ratio and availability)

    Hope some of this has helped.


  3. FunkyMan


    Nov 27, 2007
    Pure gold information thanks a lot Syisrad!
  4. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    i'm just a fixer, not a builder, but in my experience way too many bolt-on bass necks develop a problem kink right where you have those bars stopping, right where the neck flattens out in back to meet the body!

    i'd do what it took to have the bars extend further to the end of the neck, maybe by moving them or the screw locations in or out.
  5. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Your best bet is to get a full-length truss rod, with or without full-length CF bars.
  6. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I concur with Walter and PilotJones; To me, you have the neck structure exactly backwards. The stiffening bars (if you are going to use them) should start back at the heel of the neck, and end at around the 5th-6th fret area. The truss rod needs to extend out to between the 1st fret and the nut. If you go with heel access, then the truss rod assembly needs to be longer than you have shown.

    The thing to understand is that the most highly stressed area of the neck is centered around the 10th-14th fret. That's where the neck will tend to stretch and kink over time.

    The outboard end of the neck, from the nut to the 5th fret, is the area that you want to pull into a slight curve under string tension. The truss rod's job is to adjust the amount of that curvature over a small range from none to a little bit.

    Strung up and in playing condition, the curvature of the fingerboard should look like one-quarter of an ellipse. Most of the curvature should be out at the outer end, gradually flattening out as you get to the heel.

    The way you have it drawn, the fingerboard will be flat at the outer end, and then take a kink right in the middle, which the truss rod probably won't be capable of flattening out.
  7. WorktheWood

    WorktheWood Supporting Member

    Mar 27, 2014
    Do any of you feel graphite bars are not needed? I know of several builders who don't use them.
  8. that's a good question. I asked a famous builder once about why he did not use them in his instruments and he said that in order to put in the graphite rods, wood is removed first and that already weakens a perfectly good neck. His take was that if you find a piece of maple that was rigid enough, it should not need any reinforcement. His take was that high volume production manufacturers just use graphite in order to be less picky on the wood.
    Now he could be wrong and just biased against mass produced, but it made sense to me
  9. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Just so you know, I'm a pro instrument neck builder. That's literally what I do for a living, and have been doing for over 20 years. I also build my own line of basses, but about half of my business is supplying truss rods and custom neck structures to other Luthiers. I've built around 1000 custom necks over the years, and I'm constantly experimenting with the structures and reinforcements in instrument necks.

    I don't like carbon fiber reinforcing bars, and I don't use them in my necks. I'm not saying that they are awful and horrible, but to me they are just a poor way to do the job. You are routing out a large slot of wood and gluing in a block of plastic. The bars that they sell aren't solid graphite or carbon fiber, they are cast or pultruded polyester resin with some percentage of carbon fiber stranding inside. It's a reinforced plastic bar, which you are then trying to glue to a piece of wood. Unnecessary weight, problems with differential expansion, and the carbon fibers end up in the wrong place, structurally. And I don't like what they do to the sound. Which will probably start the beer bottles flying around here.....

    I like carbon fiber, but I buy it in the raw strand form, a thin thread that is called TOW. Where I want to do reinforcements, I rout a small slot, and lay in some West Systems epoxy, and a bundle of the TOW strands. Much better bonding to the wood, higher strength, less total plastic added, and I can put the reinforcement right where it's needed.

    For example, to resist the classic 12th fret stretch/kink problem on bolt-on necks, I add a "back strap". It's a small bundle of TOW strands that's set in under the truss rod, running from the back of the heel up to about the 5th fret. It's down deep in the neck, about 1/8" from the back surface. That's where the tensile strength is needed. In cross-section, the installed "back strap" is about 1/16" thick x 1/4" wide, but the result is far stronger than putting a pair of bars up under the fingerboard.

    The carbon fiber bars are popular because they are really easy to install, either in factory production, or by hobby builders. That's their big advantage. Manufacturers like them because, like Joeyl points out, it allows them to be less fussy in selecting boards for their necks. Slap in the plastic bars and fewer necks come back on warranty. And they are a great marketing hook. Carbon Fiber Stiffening Bars for Increased Stability! I mean, everyone wants Increased Stability, right?

    But they aren't needed in well-built necks. They aren't needed for strength, and a well-built neck won't have stability problems. There are many ways to make necks stable without gluing in plastic bars.
  10. Jeff Bonny

    Jeff Bonny

    Nov 20, 2000
    Vancouver, BC
    What do you think about CF rods as a fix for a badly bowed neck with no truss rod...something like an old Dano or Supro? I have these basses both with a lot of bow and I'm considering my options for making them playable again. I didn't want to install truss rods because I think part of the sound of these old instruments is that they don't have a big steel rod sitting in a hollow trough. I thought a tight fitting piece of CF might be the lesser evil.
  11. steve_rolfeca

    steve_rolfeca Supporting Member

    High-end luthiers were using graphite reinforcements in the bracing and necks of experimental acoustic guitars at least 20 years before anyone thought to do it in mass production.

    I first put carbon fiber reinforcements in a neck in 1993, on the basis of a Bass Player magazine article by Rick Turner, who certainly qualifies as a famous bass builder. Even at that time, there was still no-one using graphite in mass production.

    I hate to do the "my dad can beat up your dad" thing, but your builder was incorrect. The idea that graphite would be weaker than the piece of wood it replaces, is ridiculous.

    I was using high-grade, old-growth Canadian rock maple in my build. It was unusually stiff and dense, yet the graphite (Luthier's Mercantile 1/4" by 3/8") was MUCH stiffer than the same volume of wood. I could easily flex and even snap the piece of maple between my thumbs and pinkie fingers. In comparison, the C-F stick felt like trying to flex a ruler.

    It's perfectly possible that some manufacturers may be using C-F reinforcements to make up for sub-standard lumber, but that's certainly not why so many high-end luthiers have jumped onto the bandwagon over the years.

    EDIT: Bruce's comment about poor-quality "graphite" rods being sold as real carbon fibre wouldn't surprise me. But once again, my personal experience with one particular product (the rods sold by Luthier's Merc back in the mid 90's) was very different. Those rods were feather-light, with a high proportion of carbon to resin. They cut and shaped more like a very hard grade of wood, than plastic. I'll check with Tony Karol, my luthier buddy, but I think he continues to use their products in his harp guitars and baritones...
  12. that's why I withheld the name of builder :p
  13. TapyTap


    Apr 26, 2005
    If you were building a headless bass with a Maple neck, what would your approach be regarding neck reinforcement?
  14. That's precisely what I was thinking. :meh:

    Again, so glad to have your experience and wisdom around here Bruce. :)

    Re CF rods; I've avoided them, because I believed them to be unneeded in a well made neck. As an aside, I wonder if we all want necks that are to shallow? My tendency is to build necks with more heft.
  15. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Oh, yeah, I definitely agree that carbon fiber is much stiffer than the wood that it replaces. That's pretty obvious. I think that builders' comment was misunderstood.

    I don't like the bars because they are overkill. Waaaay more reinforcement than needed, at a price. If I wanted to build necks that were like blocks of granite, I would. But, that's not the goal.
  16. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I'd use a single truss rod down the center. As long as it's sized correctly (in terms of its mechanical ratio) and located correctly, that's all you need.

    Whether to add some kind of stiffeners or reinforcement down at the 12th fret area will depend on how thin you intend to cut it around there. The thinner it is, the more it will need some kind of a strap or bar to combat long-term stretching and kinking.

    Building a neck to be stable is a whole subject by itself. First, there are two kinds of stability problems: Long term stability and short term stability. They are really two different things.

    It's also important to understand that stiffness and stability are not the same thing. You can build a neck to be very stiff, and still have it be unstable, if you aren't careful. And you can build a neck to be soft and springy, while staying very stable. One doesn't guarantee the other.

    Long term stability is the "warping". As the wood dries out slowly over months and years, it will shrink. But, it doesn't all shrink evenly, so it will bend into a curve. It slowly changes shape by itself, having nothing to do with string load, temperature, humidity, brand of paint, or anything like that. It's just the wood slowly dying.

    Preventing the neck from warping over time is mostly about lining up the growth rings and grain lines of the wood around the centerline of the neck, so that the inevitable movement will be balanced and counteracted, or at least go in a predictable direction. And, you dry the wood carefully for a long time to get most of the movement out of it before you use it.

    That's obviously a quick description, but the point is that making a neck stable over the years is mainly about preparation and selection of the wood, and alignment of the grain.

    I've got to run.....more later.....
  17. TapyTap


    Apr 26, 2005
    All great points! This is my favorite:

    Sticking with the headless theme, let's add the variable of a singlecut body shape to this equation. How might this affect your opinion?
  18. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    I just want to point out that from a mechanical engineering perspective, the logic here is faulty. The individual strings do apply a bending force on the neck that is not in the plane with the neck centerline (except perhaps for the middle string on a 5-string), but the net effect of all the strings is a bending force directly in the plane of the neck's centerline. So, installing reinforcing bars parallel to the neck's centerline, rather than parallel to the string it sits under, would be the best way to avoid any twisting or lateral vector to the force applied to the rod by the string tension on the neck.
  19. syisrad


    Oct 28, 2008
  20. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Well, actually, I didn't mention that, but I did factor it in. It seems to me that even from the observations you made yourself, you would conclude that you cannot predict what the individual string tensions will be for whatever strings the user ultimately installs. So, it seems to me that it would be best to not make any assumptions beyond the most basic: There will be string tension. String tension among all strings that I've looked at are not necessarily the same, but they are generally within just a few pounds of each other.

    Thus, I believe my earlier post is completely valid.

    Also, if you insist on assuming that thicker strings will be tuned to higher tensions, and that that makes a difference in how you should orient reinforcing rods that I have to ask: Have you seen many necks (particularly, ones that have no reinforcing rods) that are twisted like a corkscrew because they thicker strings pulled on it harder than the thinner strings?