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(hopefully) not the same old truss rod question

Discussion in 'Luthier's Corner' started by pi_r_squared, Sep 8, 2008.

  1. pi_r_squared


    Sep 3, 2008
    I've been toying around with the idea of building as electric bass for some time. I assume it's like making pancakes - plan on throwing the first one away.

    I've even thought about building a 3d model of a bass and performing a computer stress analysis (don't ask why, maybe my definition of 'fun' is warped). Modal analysis might be fun as well (natural frequencies, etc), but modeling wood for a modal analysis might be nearly impossible.


    My truss rod question is as follows:

    If loosening the truss rod (single acting) removes the back bow in the neck, then the truss rod has to be on the back side (away from the frets) of the neutral axis of the neck. But, where the end of the truss rod is accessible at the head stock, it seems like it's either on the wrong side of the neutral axis, or very close to the neutral axis.

    If my observations are correct (they may be dead wrong), then truss rod adjustment will change the bow in the portion of the neck closer to the body much more than the top of the neck, since the truss rod will be putting a bending moment on the lower portion of the neck, but pretty much just compressing the upper portion.

    Is the truss rod slot deeper as it gets closer to the body? and where does one find the dimensions for this?

    Also, is there a good website that answers the dumb questions about building an electric bass that are second nature to those who have done this before?

    Thanks in Advance,

  2. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Your concept is not wholly accurate. If no one else answers by tonight, I'll give it a shot then. For now, back to (engineering) work.
  3. pi_r_squared


    Sep 3, 2008
    It just occurred to me...

    Once strung, the strings become part of the 'structure' and the effective neutral axis moves closer to the strings. So the truss rod may indeed be behind the neutral axis, even up by the headstock.

    I usually deal in steel, but this reminds me of finding the effective neutral axis in reinforced concrete (taking the rebar into account).

    Does this mean the truss rod groove in the same depth down the length of the neck? or does it vary?


  4. eleonn


    Aug 24, 2006
    Lima - PerĂº
    If a neck would have the same dimensions along its length (lets say @ first fret position) and you adjust the trussrod, you would see most of the change in the middle part of it. Due neck dimensions change along its length, the more close to the body, the bigger the volumens neck is, the harder is for the truss rod to make it move. When you adjust the truss rod, it will move at the easiest part first which is the closer to the headstock which is the part where the neck gives less movement resistence. I'm not sure it my explanation makes sense. My english is still a bit basic for some things. Maybe someone else join later or youll have Pilot giving you some comments about this (I like to read Pilot technical comments:smug:).
  5. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    No, loosening a single action truss rod does nothing but allow the string tension to bend the neck. If the strings are slack, it has no effect. The way a single action rod works is that the tension of the strings wants to bow the neck up, like an archery bow. When you tighten the truss rod it counteracts that tension.

    The way it works depends on the style of rod you are using. The "traditional" Gibson style compression rod is a single steel rod installed in a curved recess. Each end is anchored in the neck, so tightening the rod compresses the wood between each end by pulling them together. It's curving away from the fingerboard. When you tighten it up, it wants to straighten, and thus pulls the neck into a back bow. You can also install these more or less straight, but they are always near the back of the neck, and usually closer to the fingerboard at the nut end, since the neck is thinner there.

    The newer style rod has one rod over the other. These come in single and double action. On the dual single action rods I've used (from lmii.com), the top rod is fixed, and the bottom rod adjusts. They are welded together at the end opposite the adjusting nut. These are installed right under the fingerboard in a flat bottom channel.

    When you tighten the rod, it causes the bottom rod to shorten, and the top rod bend in a bow, causing the neck to back bow.

    There are also double action rods like this, where the bottom rod is threaded on each end. These can put an up bow into a neck with no string tension.

    The advantage to the dual rods are ease of installation, since you only have to rout a straight flat bottom channel, and they don't have to extend to the back of the neck, which can make carving the neck less risky. Also some people think the single compression rods effect the neck's sustain by overly stressing the wood.

    The advantage of the single curved rods is it takes less wood removed from the neck, and the rout is shallower at the nut end. But they are trickier to install due to having to make a curved bottom slot.

    Fender traditionally installed the curved rods from the back of the neck (the "skunk stripe"), since they didn't use separate fingerboards.

    Single action compression rod:

  6. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    So, to summarize in relation to the OP, if you are talking about a standard single-action rod, then the rod tension causes a backward bending moment not due to being on the back side of the NA, but rather due to the curve of the channel, quite possibly despite the nut end not being on the back side of the NA.

    Could you do a straight rod, fully on the back side of the NA? Sure, but it might or might not require putting in more compression than is required on a curved-channel rod.

    I read a description of CT's rods that sounded like they might be a straight rod, fully on the back side of the NA, but I'm by no means sure about that.
  7. Phil Mailloux

    Phil Mailloux

    Mar 25, 2005
    Brisbane, Australia
    Builder: Mailloux Basses

    You sound like someone I could have a great time hanging out with :D

    I've started modelling my bass designs in Inventor 3D, all the tiny little parts separately modelled and assembled. I haven't learned the stress analysis package yet though. That could be interesting. That stuff definitely is fun. lol
  8. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    I haven't done it personally, but I've heard of people using a straight rod. That's also the way Martin guitars does it, but theirs is in an aluminum U channel.

    The original Parker Fly guitars used a piece of piano wire for a truss rod! I think they now use a very thin steel rod.

    I prefer the over/under rods, but my partner and I are going to build a couple of Red Specials, and he wants to use a single Gibson style rod. I figured that the heck, it's more in the tradition of the original guitar.
  9. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    I think the fun part about lutherie is it's art, and engineering.

    We draw out everything to scale in 2D first.
  10. XylemBassGuitar

    XylemBassGuitar Supporting Member Commercial User

    Aug 14, 2008
    Durango, CO
    Owner and Operator, Xylem Handmade Basses and Guitars
    I generally install my (single action) truss rods straight (though I have done some curved ones).

    As long as they are fairly deep in the neck (below the neutral axis) they work amazingly well. I switched to the straight installations from the curved because they seem to be much more effective. The adjustment end at the headstock is a little higher, but not by much.

    Of course, don't place the rod too deep in the neck, you don't want to carve into the channel from the back of the neck, nor do you want the truss rod to break out the back of the neck.
  11. pi_r_squared


    Sep 3, 2008
    Thanks all!

    I think I have a better understanding of how the engineering makes it's way to the application as far as the truss rod goes.

    This is sort of a lateral step from the topic, but who says I can't jack my own thread:)

    Are there specific natural frequencies at which you want the body to resonate? or some you want to specifically avoid?

    Maybe that should have a new thread.

  12. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    BTW for neck bending analysis, the strings are not part of the beam under consideration, thus shifting the NA, as previously conjectured. This is a neck beam being bent by the tension force from the strings; not a combined neck/string rigid body, acted on by some other outside force.

    You can either take the neck as a cantilevered beam, or the neck/body as a beam. In the former case, the neck would be considered at rigid anchorage at the body joint; and the applied force would be that force applied by the string tension. That force is applied to the neck beam at the point it bears, the nut saddle, and in the direction of the string.

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