How does a truss rod get "maxed out"?

Discussion in 'Hardware, Setup & Repair [BG]' started by Bluesbreaker5, Jan 27, 2009.

  1. Bluesbreaker5


    Mar 24, 2006
    Maybe I don't quite understand how a truss rod works. But if you get a good relief set, they why would you need to keep tightening the truss rod until it can't tighten any more? Doesn't it keep the neck from moving once it's set? Does it move on its own? Does it bend?

    Seems to me the only time you would re-adjust it would be if you wanted to purposely change the amount of relief from where you had it set it before.

    Now I've read here that seasonal changes, temperature, humidity, different strings, etc will make a neck bend. So then the truss rod bends too? Or does it loosen itself? If that's the case why not make "unbendable" truss rods with locks? Is that easier said than done?

    I just can't see why a truss rod would "max out".

    Help me, help me....I'm melting.....
  2. 62bass


    Apr 3, 2005
    Wood can shrink a lot as it dries out. Also the pressure of the nut against the wood can crush the wood and force the nut deeper, thereby limiting the amount of turns you can take on the nut to tighten it. Search around for some diagrams or illustrations of truss rods and when you study them you'll see how this can happen. Luckily it can usually be cured by adding a few washers under the nut. I've done this with many basses and guitars.
  3. JLS

    JLS Supporting Member

    Sep 12, 2008
    Albuquerque, NM
    I setup & repair guitars & basses
    String tension is trying to pull the headstock into the bridge, so to speak;
    if, for example, you you let this go on long enough w/o adjustment, the fibers of the wood will take a set, at which point you will be thinking that the cost of a yearly setup wasn't so much, after all.

    Some necks, are, as well, just weak from the get go. I don't like to have to use a lot of trussrod to get a neck to the right relief--which is DIFFERENT, from bass to bass, and player to player--because I can hear the tonal quality of a very tight trussrod, and don't like it.
  4. EADG mx

    EADG mx

    Jul 4, 2005

    Can you explain?
  5. 62bass


    Apr 3, 2005
    Yeah, I want to hear what that sounds like too.
  6. Yeah, but it'd be nice if they sold basses with dry necks. (mutter, mutter, mutter . . . ) Part of the game though; like fret edges wanting a bit of filing in winter for the first 2 years.

    THAT's good to learn. I was wondering about the recent thread about a 64 Jazz needing a washer. Slow compression over decades makes a lot of sense. At least, it does now ;) Thanks.
  7. JLS

    JLS Supporting Member

    Sep 12, 2008
    Albuquerque, NM
    I setup & repair guitars & basses
    Not even going to try, online.
  8. I'll try. A very tight truss rod= a very stiff neck. Very stiff neck= more sustain, brighter tone(usually). Not always desirable.
  9. 202dy

    202dy Supporting Member

    Sep 26, 2006
    Tightening the truss rod counteracts the strings pull on the neck that causes a forward bow (relief). It accomplishes this task by compressing the neck. There is less material on the fingerboard side of the neck than there is on the back of the neck where your thumb is. Naturally, it is less resistant to the force of the rod than the opposite side of the neck. When compressed along it's length the neck flexes out of relief.

    The truss rod controls the flex and geometry of the neck. It does not affect the stiffness of the neck.
  10. 62bass


    Apr 3, 2005
    In fact, the first bass I ever did this on was a 64 Jazz in 1980 or so. About 16 years of compression.

    It's dry as a bone here in Ontario this time of year. I don't think any factory could dry their neck stock as dry as it gets in my house. And static electric sparks everywhere. I was taking off my sweater with the lights off last night and got quite a light show.
  11. Less dry in Nova Scotia, but not a lot of difference; little difference in absolute moisture content of -10 degC & -30 degC air when it warms up to 20 degC.

    Kiln drying can get wood pretty dry. But it costs more.
  12. Was thinking of you today Dave. Had one of my jets frozen to the ground in Halifax, with your Winter wonderland your experiencing.
    Velocibasser likes this.
  13. Neat! I'm in the first village between the airport & downtown.

    Horrid driving this morning!! Icy, icy, icy!

    Hey, if you get stuck here sometime, get in touch!
  14. JTE

    JTE Gold Supporting Member

    Mar 12, 2008
    Central Illinois, USA
    The wood can also be compressed by someone tightening the TR nut too much at once. I never tighten the nut until after I've pre-stressed the neck in the direction the rod is going to pull it. That will go a long way towards preventing compression of the wood.

  15. Well, here's how I see it. If you have to tighten the rod significantly to counteract dry weather fingerboard shrinkage, the rod is placed under a fair amount of tension in order to straighten the neck back out. And so is the fingerboard, as it is usually what's shrunk, creating excessive relief.. Two elements under increased tension, pulling in opposite directions=increased stiffness. Not sure where I've gone wrong here.
  16. 62bass


    Apr 3, 2005
    Good point.

  17. You have not gone wrong. The physics involved in all of the stresses placed on the neck are well beyond most peoples mathmatical skills.

    202's explanation is the most common that is used, and is the best way to explain it, for most people.

    The instruments neck can also be thought of as an adjustable tension spring mechanism. Which it is.

    The supplied numbers are only for an example.

    Take all tension off your trussrod. String up a set of strings that put 120lbs of tension, on your neck. Measure the relief. You get a number of .050".

    Your aiming for .012" of relief. Your neck must be stiffer so that it will flex less under load. When you adjust your trussrod, you are applying a preload to the neck. Adjusted properly for the correct preload the neck will flex to a .012" relief number.

    The preloaded neck has potential energy stored, because of the compression placed on it, by the bending of the neck. Just as a bows limbs store energy that is released to propel an arrow.

    There are several factors that contribute to the strength, or stiffness of a neck without the aid of the trussrod. Wood grain orientation, and the number of growth rings cut through during profiling are prominate in this process.

    There is lots more..
  18. Good point, indeed!

    Does the 'pre-shape' serve to have the compression occur over the whole neck more than just where the truss rod contacts the neck at it's ends?
  19. Not really Dave,

    How the neck bends is dependant on how the neck was carved, the growth ring orientation, and the final thickness overall, along the length.

    When a load is put on the neck the thinner areas will want to flex more than the thicker areas. The trussrod only adds additional support. It is limited to how much it can do. The neck has to be thought of as a whole, in regards to ultimate strength.

    Take your neck, with no trussrod tension, and set to zero relief. Flex it so that you have .012" of relief. Measure the bend and how it occurs over the length of the neck.

    Now string it up, and apply the appropriate trussrod setting for a .012" relief figure with a, low or med tension string set. You should see that the neck basically follow the same overall curve pattern along it's length. Put a set of strings with really high tension, and the thinner areas towards the headstock will bend a bit more.

    I noticed a variance with my Geddy jazz. That neck bows more at the headstock end with flats than I liked, even with the relief set to .015".

    Final thickness plays a large part in neck strength, even with the trussrod support. The trussrod can only do so much. This is evident when a bass has not had the trussrod adjusted properly for many years. The neck takes a set towards the headstock end. You can set the trussrod under no load, but the neck will show a curve at the headstock end under load.

    Setting your relief to as tight a tolerance as possible will go a long way to keeping your neck from developing a permanent bad set.

    Not everyone will agree with my findings on this subject. Thats ok. If we all agreed on everything, the world would be pretty boring.
    petrus61 likes this.

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