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How much pressure is on a Truss rod?

Discussion in 'Luthier's Corner' started by Arnie, Feb 8, 2013.

  1. Arnie

    Arnie

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    As I am building my bass and turned the truss rod slightly.. it is moving but I can't tell how much pressure is involved. It feels as if there is a LOT of pressure couldn't the fret board pop off?

    Here's how I think of it, when a truss rod is turned it flexes either up or down and then you leave it that way.. would over time the rod push the fret board off?

    Obviously I have NO experience with this.. and was just wondering what is happening under the fret board..
  2. Phendyr_Loon

    Phendyr_Loon

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    Any glue joint, provided it's made with an adequate amount of the proper adhesive, is stronger than the wood surrounding it.
    So if anything is going to "pop", it will likely be due to structurally compromised material.
  3. Arnie

    Arnie

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    Ok, thank you..
  4. ctmullins

    ctmullins Registered 8er Supporting Member

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    As an aside, you shouldn't use the rod to move the neck. Move the neck using clamps or whatnot, then snug the rod, then release the pressure on the neck.
  5. Geoff St. Germaine

    Geoff St. Germaine

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    I have honestly never seen the use of clamps advocated in adjusting a neck unless it was either a non adjustable neck or I may have seen it referenced in response to extreme bow or back bow. Why would you not adjust a neck using the truss rod?
  6. christopherogut

    christopherogut Supporting Member

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    I've seen a lot of advice for certain basses which state you need to bend the neck by hand to the desired amount and then crank on the truss rod while it's being relieved of tension.
  7. gidbass

    gidbass Supporting Member

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    ^

    early ric 4001's were this way, but not sure of any others.

    Good Luck!
  8. ctmullins

    ctmullins Registered 8er Supporting Member

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    My point is that the truss rod should be used to hold the neck in a certain position, not to move the neck to that position. It takes much more force for the latter than the former.
  9. Joedog

    Joedog

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    How much pressure on a truss rod? That depends on a LOT of variables, and there is no one way to respond. Stiffness, thickness of the neck, strings, ect. all determine that. I have one bass w/HEAVY stiff flats I had to REALLY CRANK on the TR to get a fairly low action (no problems whatsoever). I also have a custom 4 string w/a beefy neck, running medium rounds, and the truss rod is so loose it basically is doing nothing. Sorry, I can't help w/your question. If you built it right, I dont't see a problem. If you didn't......don't know what to say, other than try and get action where you want it, and pray.
  10. Geoff St. Germaine

    Geoff St. Germaine

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    How does it take more force to move the neck to the position than to hold it there? The rod is tightened and this creates a bending force in the rod. This force acts against the pull of the strings and with the bending force of the neck (in most cases). This force imbalance causes the neck to move until these forces balance. It takes exactly the same force to bend the neck as it does to hold it in the new position. If you applied less force with the truss rod than what moved it there, the neck would move back toward where it had been initially moved from.

    I really don't see any advantage to using clamps to do work that the truss rod was designed to do.
  11. christopherogut

    christopherogut Supporting Member

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    You raise some good points. One of my suspicions has to do with the truss rod threads. I imagine that an older truss rod that's been adjusted countless times, neglected, etc... has some friction that exists between its threads and those of the threaded nut. Perhaps a 'stuck' rod that is being adjusted will start to undergo some twisting due to thread friction and is more apt to break than if you moved the neck by hand and relieved the truss rod tension... in that case you might be able to free up a lot of the friction on the threads, 'break them free' easier, and adjust it easier. I've never broken a truss rod but I can imagine that thread binding plays into it a bit. Just my two cents. Feel free to correct me if I'm completely wrong... ;)
  12. ctmullins

    ctmullins Registered 8er Supporting Member

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    You're right, Geoff - poor explanation on my part. Yes, my argument is really about putting unnecessary stress on the threads.
  13. christopherogut

    christopherogut Supporting Member

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    ... also, it's a question of tensile strength versus torsional shear strength. A truss rod is designed to be in tension, as you said, to counteract the string tension. It may function perfectly when at rest to counteract the string tension which is realistically about as much as the combined string tensions, but it may have a lower strength when twisted. With 125lb-200lb+ force being provided by the strings parallel to the neck, the truss rod can counteract this... but if there's some thread binding and the whole system is put in motion... the process of adjustment considering thread binding could start to twist the rod, and perhaps it could fail due to exceeding the truss rod's ultimate torsional shear strength. Again, just my two cents based somewhat on a Bachelor's in Mechanical Engineering and a paper a few years ago about truss rod safety factors, haha.
  14. Beej

    Beej

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    Got a link to that paper christopherogut? A few engineers on here that would undoubtedly like to look it over.

    I always move the neck by hand and then tighten the rod to hold the position. Just the way I was taught in order to reduce stress on the threads when adjusting and sustain the life of the rod.

    I know the material of the neck is not "force neutral" but it feels like the strings cause forward bow, and we correct that with the truss rod. Stiffening the neck with a solid rod (or more than one) can also correct any potential forward bow, but it also leaves you with a neck that cannot be adjusted for relief. We know that the strings come up to around 150-200lbs of force (I know you physicists will be reeling with my poor jargon :D ), but the single rod is more than enough to compensate.

    Regarding a rod coming through the board or the back, it does happen from time to time, but all other build factors considered, it's rarer than a stripped nut or broken trussrod...
  15. miziomix

    miziomix Über on my mind Supporting Member

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    Highly unlikely. The sheer width of the glued area prevents the fingerboard from popping off.

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