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glueing in a truss rod.....

Discussion in 'Luthier's Corner' started by martinedwards, Jan 17, 2006.

  1. Ok, I searched and had no luck for exactly what I want.

    I have a 23" u channel truss rod. I have a bass with a slot cut for it.

    How much do I glue it in place?

    just the ends?

    All along?

    suggestions welcome (that don't involve formica fingerboards........)
  2. tjclem

    tjclem Supporting Member Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
    Hopefully the truss rod has a covering over most of the rod? I use the stew-mac truss rods and the LMI ones. I use calking in several sopts along the slot well away from any of the exposed threads of it.....t
  3. Ok, thanks, but what is all this talk of epoxy I hear? isn't it needed?
  4. Suburban


    Jan 15, 2001
    lower mid Sweden
    Never tried myself, but GMC says that the channel should be extremely tight, and then the U should be epoxied all the way.

    My thoughts is that you don't want epoxi in the U-channel, so you need a tight channel. But with such a tight channel, the epoxi will not penetrate very well. Hence, you need to glue the whole way, and most of the glue will join only the top.

    But for more experience, you should talk to GMC. He is a really good guy, and ditto luthier.
  5. Phil Mailloux

    Phil Mailloux

    Mar 25, 2005
    Brisbane, Australia
    Builder: Mailloux Basses
    The only place where you need to use epoxy is to glue carbon fiber rods in the neck.
    You shouldn't use epoxy on your truss rod.
  6. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Gluing in a truss rod will change the action it produces.

    A rod-rod, or channel-rod system, if left free to float in the channel, will directly apply side forces (towards or away from the fretboard) to the interior of the neck, at various points, producing the bending. This is AFAIK the way it is normally done. To the best of my knowledge, dual rods are usually installed with a dab of silicone at each end, so that they can float.

    If a double rod, or channel-rod system has one of its two members glued full-length to the neck, this changes. It will then act basically the same as a single, non-curved rod, in that it will produce tensile or compressive forces applied longitudinally (the long way) at the end anchor points of the non-fixed rod. Depending on which side of the neutral axis these forces lie, this with forward- or back-bend the neck. The only differences between this setup and a single, straight rod are that the neck is "reinforced" by the fixed (glued) member, and that in most cases pressure can be applied in both directions (dual action, producing front- or back-bow).
  7. Basschair

    Basschair .............. Supporting Member

    Feb 5, 2004
    Stockton, Ca
    You know, I could see using a little bit of caulking in the channel to avoid any rattle from the rod, at least for a dbl action one. Just to help get the nice snug fit. I made the mistake of using a bit that created a flat-bottomed TR channel on my first bass, so it rattles a little bit. :scowl:
  8. If the truss rod rout is tight then there should be no need for glue on a dual action LMI rod. If you go the right width and depth it will fit snug as if it was glued. IMHO of course YMMV. :)
    The LMI rod has a covering on the rod that prevents rattling if it bends that far. Oh I forgot to mention tape between the rod and the fretboard to prevent any glue from going into the channel.
    I could see if your rout is loose perhaps carefully glueing each end.
    Good luck,
  9. gyancey


    Mar 25, 2002
    Austin, TX
    The aluminum U-channel rods work better if you don't glue the channel in the slot. Just make sure the recess is very tight. I've done it both ways and without glue is better.
  10. Suburban


    Jan 15, 2001
    lower mid Sweden
    Hm, I'm not sure I agree with this...
    Yes, the action will change, very little.

    The two-part truss rod works by a straight, adjustable rod that bends a non-adjustable rod.

    If the 2-part is left floating in a trench, and then tightened, pressure will be applied to the top and bottom (fingerboard and neck, respectively). The pressure will be applied more or less at three points: upwards in the middle of the fingerboard, downwards at the ends of the rod. IRL, the load on the fingerboard is very much spread over the entire fingerboard, since the dpeth of the trench=the height of the rod.

    If the non-adjustable part of the rod is glued to the neck, the action will be accordingly: that part of the rod is now just as much a part of the neck.
    The adjustable part of the rod will have to bend that entire neck. But still, due to the design of the rod set with the end lugs posing a torque to the ends of the rods, the action will be bending the fixed rod. Thus bending the entire neck. The bend will, theoretically, be more even along the lenght of the neck.
    Aslo, a u-channel will, theoretically, put a more even bending pressure on the neck if left floating, than a floating dual-rod will.

    A dual-rod or U-channel rod will never act by compressing the neck, always by exciting a torque in the non-adjusable rod.

    Normally, I would say that the optimum is a flat neck with some upbend of the top fret . or two. This will be easier to achieve with a floating truss, but I fail to see any problems with a glued-in, if the more even action is desired by the builder/user.
  11. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Here's where I believe you are in error, or at least not quite correct. If the non-rotating rod is bonded to the neck, no torque can be generated on the non-rotating rod alone; it must be generated on the non-rotating rod / neck wood system. See below.

    I respectfully disagree.

    I am thinking of it this way:

    First, the double-rod floating case. In figure A, we see the non-rotating rod (NRR), including its attached end blocks, in tan. The rotating rod (RR) is blue, and semi-transparent. Figure B shows the same, diagrammed in side view.

    We'll assume an application of tightening the RR, to draw the two lugs attached to the NRR towards each other. In our free body diagram we'll consider the NRR as the system under stress; the stress is applied by the RR. We place the boundary of the NRR system (red, dashed) to intersect the lugs at the thread points, and place the tension forces (T) (blue) at these points. See figure C.

    The system has a neutral axis (NA) (black, CL), which runs through the centerline of the NRR. The tensile forces are offset to this. Because of this offset, a pair of torques are created at each end of the rod, bending it into a "frown" shape. Because the rod is floating (free to slide longitudinally) in the channel, this produces forces away from the fretboard at its two ends, and towards the fretboard at the middle, producing backbow in the neck (to counter the frontbow caused by the strings).

    Next, the double-rod glued-in case. Figure D shows the trussrod mounted in a neck. Here we will assume the NRR to be bonded to the neck wood. Because of this, we must redraw the free body diagram, to have the "system" being acted on include the NRR and the neck wood. See figure E. Now torques are still generated, due to the applied tensile forces being offset to the NA. But, in this case the NA is no longer through the centerline of the NRR. It is in a position determined by the complex product of the individual neutral axes of the NRR and the wood neck.

    Depending on the physical configuration, this combined NA could be in different positions. For example, looking at the contribution by the wood alone, having an all-maple neck of a certain shape will put its NA in a certain position. Using a stiffer rosewood fingerboard at the top will shift the NA in that direction; it will also increase the overall stiffness. The first of these effects alone would result in more bending with the same amount of trussod adjustment; the second would result in reduced bending; the combined effect could be more, or less, than the bending produced by a floating rod, which bends based on the tensile force/NRR NA interaction, rather than the tensile force/combined NA interaction.

    There is another factor not being accounted for, but which I will ignore at this point in order to keep the complexity down. This is the force being applied, in both cases, to the back of the neck by the RR, as it "tries to stay straight".

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  12. Don't worry about a flat bottom TR channel. I cut mine that way for my rod/bar TR and I use small neoprene O-rings around the rod at even spacing to hold it firm.
  13. Suburban


    Jan 15, 2001
    lower mid Sweden
    Peter Pilotjones, my dear and esteemed friend, I feel a need to apologise for my rudimetary skills in pedagogics.
    Or, in other wirds:
    I most respectfully agree!:cool:

    Your elaboration shows exactly what I was trying to say.
    If you join the non-adjustable part to the neck, the two will effectively be one part.

    What I was disagreeing with earlier was that a dual rod (based on torque) would convert to to a compression rod (based on tension). And your elaboration shows that this is correct: the dual rod still works with torque when glued in, but the non-adjustable part is now not only the rod, but the entire neck!

    So I believe that you and I, once again, agree but fail to communicate that in full clarity. To the possible confusion - or enlightment - of our fellow TB'ers:D
  14. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001

    Agreed except in one possible area. If anyone out there uses a straight single compression rod (in a non-curved channel), then the bending is produced by offset tension on the neck structure--same as the glued-in double rod. The main advantage is that the compression in the second case (glued-in double rod) is largely carried by the NRR, instead of the neck wood.

    Pete :)
  15. Suburban


    Jan 15, 2001
    lower mid Sweden
    Hm, that's true, of course.
    Never saw anyone try that straight single rod, though...probably due to the risk of compression breakage of the wood.
  16. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    The descriptions I've read hint that Carl Thompson angles the rod deep down towards the back of the neck at the body, which sound like it could be that. Not sure though.