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truss rod placement

Discussion in 'Luthier's Corner' started by kimstevens, Dec 18, 2011.


  1. kimstevens

    kimstevens

    Nov 12, 2002
    I'm wondering about the differences between truss rods with adjustment access at the heel vs. at the nut: I recently bought a bass on Ebay (since returned) that had access at the heel and the rod was seemingly functional, but the neck had a forward bow at the first few frets (near the nut) that couldn't be adjusted out. Since to my understanding truss rods do not span the entire length of the neck, is it correct to say that a rod with heel adjustment works on a section of the neck from some point after the first few frets , up to the heel, while in a neck with adjustment at the nut the rod affects the span from the nut up to some point lower than the heel (say, frets in the upper teens)? Also, would heel adjustment necks then be better at staving off the upper neck hump that sometimes occurs, while nut adjustment necks would be better at keeping the area of the first few frets straight?

    Thanks
     
  2. Zach Ehley

    Zach Ehley

    Jul 16, 2009
    It kind of depends. How long is your truss rod? The ones I get from Allied are shorter than a 34" scale fb. I put the adjustment at the hs due to function over astetics. I don't really like the look of a cover on the hs, that's why I don't do it on acoustics, but do on basses. I don't really like the look of it at the body either, but you have to get in there somehow.

    The neck is thick near the body, so it's less likely to deflect there. On many basses and guitars the neck isn't thinnest at the nut, but closer to the first fret. So you don't need to get that rod all the way to the nut. I put the end of the truss rod nut at the nut, which puts the acting force of the rod in between the nut and the first fret. If I were to put the truss rod nut on the body end, the other end would be between the first and second fret. If you have a longer truss rod, no problem either way.

    Your eBay bass may just have not been build well.
     
  3. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    US-NY-NYC
    A heel-adjusted truss rod can work fine at the first fret, if it's long enough to reach there. Too short will produce the problem you had.

    You can see here how Lakland makes an extra effort to bring the TR all the way up. Unlike a Fender, their headstocks remain thick for a distance past the nut, to accommodate the TR end anchorage which is past the nut.
    b2fc7e92.
     
  4. Musiclogic

    Musiclogic Commercial User

    Aug 6, 2005
    Southwest Michigan
    Owner/Builder: HJC Customs USA, The Cool Lute, C G O
    Unfortunately most people make the assumption that a truss rod works the complete neck, when in reality the truss works to counteract the apex of the forwardbow created by strung tension. The truss rod will effectively adjust to the forward bow in either heel or headstock position if there is sufficient compression length under the apex of the forward bow. If your neck had bow at the first few frets, that is because the neck is warped or the rod is excessively short, and has almost nothing to do with rod placement.

    Truss rods are an exceptionally simple system dating back to the acqueducts of Rome, and are often given credit for more and much less than they are truly capable of.
     
  5. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    US-NY-NYC
    This was meant to be my point—that if the rod is too short, and is heel-adjusted, there is no rod at the nut end to do the job that needs to be done there (which is where the OP had a problem).
     
  6. Musiclogic

    Musiclogic Commercial User

    Aug 6, 2005
    Southwest Michigan
    Owner/Builder: HJC Customs USA, The Cool Lute, C G O
    Pete, this is the section of the question I was addressing, and not questioning anything you pointed out.:meh:

    This is the type of query that can start a misinformation boom that plagues this section, trying to expound to give a greater understanding of what a truss rod actually does.
     
  7. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    US-NY-NYC
    Ok, so I think I see — would you say this is correct: most of the string-induced bow should be closer to the the body, so warpage up at the top must have been due to warpage?
     
  8. Musiclogic

    Musiclogic Commercial User

    Aug 6, 2005
    Southwest Michigan
    Owner/Builder: HJC Customs USA, The Cool Lute, C G O
    No, the most bowing will usually occur at the thinner portion of the neck(I say usually because most of us have seen the ski slope warps that were common at the 17-20th fret areas of the 80's Japanese Fender and Squier instruments) But the way that truss rods work, if the neck over bows or warps near the last 6 inches of the rod, there will not be enough force from the rod, on the neck, at those points to correct that area. The greatest opposing force created by the truss rod will be at the center of the rod curvature and decays out from the centered pressure to the ends where the force is the least along the rod. So even in the Lakland example above, if the neck warped/Over bowed in the first few frets the rod would not create enough force in that area to correct it.

    Of course, you being more adept at mechanical engineering principles than I am, you may have a different view or equation. This is just from my experience, and understanding what effect a truss rod actually has on an instrument neck from years of working on them. Physics may suggest something different.
     
  9. kimstevens

    kimstevens

    Nov 12, 2002
    Thanks, I do have a greater understanding of the system now.
     

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