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Fret leveling and trussrod problems!

Discussion in 'Hardware, Setup & Repair [BG]' started by FunkyMan, Dec 5, 2013.

  1. FunkyMan


    Nov 27, 2007
    Hello folks, i work as a guitar tech and i'm having this problem lately, i have some theories about what is happening but wonder if this happened to you aswell:

    -The main concern is that when i do a fret leveling job, i try to first reach the most straight position possible on the neck, so the fret leveling would be less drastic (as you know), ok so i level, bevel, polish the frets, and check for straightness and everything turns out ok, but then (in some cases) no matter how straight was before, when the neck gets tension with strings on, it gets some odd shapes, humps, hills, "S" shapes (looking from bridge to nut) etc, which make them almost impossible to get a decent action, i will like to blame it to a possible bad trussrod, or bad wood, but at the end i can't finish the job and in some cases i don't charge anything to the customer because the problem wasn't solved.

    I know that the next thing you can do is removing the fretboard and check if the neck needs a trussrod replacement.. so, i wonder if this is a common problem? or i'm doing something wrong in the fret leveling proces?

    Suggestions would be appreciate! Thanks
  2. rfslick


    Dec 31, 2008
    Benicia, CA. USA
    Are you running into this on more than one guitar? or just one. Maybe you have one neck that has bad wood.
    One thing you might do, is to remove the nut and lube the threads and the inside surface of the nut. That will allow it to move smoothly. It could be grabbing and twisting as you tighten and adjust it.
    That would be far less severe than removing the fretboard.
  3. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Yep, fret leveling can get complicated when you are working with necks that are somewhat flexible and springy. The thing to understand is that, in a flexible neck, the truss rod needs to carry part of the load in order to remain stable. It's part of the structure. If you trim the frets/fingerboard to level with the truss rod loose, then when you string it up it will change shape. Sometimes in odd ways.

    When you get a neck in for fret leveling, the first thing you should do is take the truss rod to about halfway. That is, fairly tight, but not as tight as it will go. You may have to tighten it or loosen it from the condition it arrives in. Now, check the flatness with the strings up to tune, and then with the strings loosened. That will give you a reading on what the neck's overall shape is, and how much it changes between tension and no tension.

    If it changes a lot between tension and no tension, then you need to be extra careful, because it's a springy neck. The important thing is to level the frets/fingerboard with the truss rod kept in that halfway-tight condition. Whatever that requires. It may mean pulling the frets and re-leveling the fingerboard surface, then refretting. All while keeping the truss rod half tight.

    You need to get the neck to the condition where the truss rod is half-tight, with no string tension, and the frets are level. Then, when you apply string tension, it will pull forward in a predictable curve. So then, you tighten the truss rod a little more to the flatness or small relief that you want. And it should come back to flat and stay that way, because the truss rod is supporting the load, as it is supposed to.

    If you cheat by loosening the truss rod to make it easier to level the frets, then when you apply string tension, the neck can move all over, because it isn't being supported.

    If the neck is fairly stiff, and doesn't move much between tension and no tension, then this technique of pre-loading the truss rod isn't as important. Still a good idea in general practice, but not critical.

    S-bends are a more complicated problem. There's a long thread on TB from sometime in mid-2013 where we went over them in detail.
    the V likes this.
  4. Have you seen Dan Erlewine's setup bench?


    I'd look seriously into build one of these if I was having your issues. It allows you to set the guitar up in an "undertension" position to do all your fretwork.

    BTW, if he doesn't chime in on this, you want to PM Bruce Johnson. The guy is a legend. [edit, thanks Bruce]
  5. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Yeah, that's the advanced level, where the neck is pulled up to string tension, and held in that fully-loaded position while you do the leveling. That's the "PLEK before computers" method. I've built several of those specialized neck-tensioning rigs over the years, which are even more complicated than Dan's setup. (BTW, Dan and I are friends). Those fixtures are a whole 'nother long story. That level of effort is only needed when you are dealing with very flexible necks. Or super-finicky customers.....

    But start with the basics: You need to pre-tension the truss rod properly while doing the leveling, as I described above.
  6. kohntarkosz

    kohntarkosz Banned

    Oct 29, 2013
    Edinburgh - Scotland
    Sorry to butt in. I understand the need to pre-tension the truss rod to a degree, but surely you risk introducting a back bow to the neck, which in turn results in more metal coming off the middle frets than the outer ones? I always read that you need a little bit of truss rod tension on the neck but not too much. I don't doubt your word, Bruce, I'm just curious. Dan Erlewine's book is essential bed time reading in these parts!
  7. The issue Bruce is about is one I had with my last bass. The intention with the trussrod is to get the neck as close to level as possible. This allows you to properly level the frets evenly with the 'support' of the trussrod holding the neck stable. Once everything is level and happy. You can loosen the rod off and install the strings, re-adjusting the rod the wherever is necessary.

    Does that make sense?
  8. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier

    Yes, it may do that. If the neck goes into a backbow with the truss rod halfway tight, then it's showing you what its problem is. You need to fix it by removing that lump in the middle. That may mean cutting more metal off of the frets in the middle, or it may mean pulling the frets and trimming the wood fingerboard to remove that lump in the middle.

    I'll say it again: If you cheat and loosen the truss rod to make the neck flat, just to make it easier to level, then the rod won't be loaded correctly when you bring the neck up to string tension. And it will pull to a bad shape and be unstable. In a flexible neck, the truss rod needs to be partially loaded while the neck is flat. If it isn't, you have to correct the problem in the neck to do the job right.


    You're close, but.... The primary intention of the truss rod is to adjust the flatness (level to relief) of the neck over a narrow range of travel. But, that assumes that the neck is flat with the truss rod adjusted to the middle of its range. That is, it assumes that the neck is right and healthy and within its original design specs.

    But, if the neck has warped over time (as many do), the truss rod may be operating out at the end of its working range to get it flat. If the neck is fairly stiff overall (not too springy), it may be fine. But if the neck is springy, it can be a problem.

    Because the secondary job of the truss rod is to brace the neck; to provide a metal strap along the back of the neck, to control and reduce the movement of the wood. It doesn't stop the flexing of the neck under load, but it helps to limit it.

    Picture the Golden Gate Bridge. The truss rod is doing the job of those cables running up over the top. It's not a good idea to slacken those cables, just to get the roadway flatter with no traffic on it. If there are lumps in the roadway, keep those cables tight and flatten the roadway by grinding it.

    Does that help?
    the V likes this.
  9. tjclem

    tjclem Supporting Member Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
    I would like to have one of these but I hear the version they are selling now is not as good as what has been doe in the past?
  10. kohntarkosz

    kohntarkosz Banned

    Oct 29, 2013
    Edinburgh - Scotland
    Hello Bruce. Thanks for taking the time to draft out a response. I've seen levelling beams on Ebay that claim to work whilst the instrument is under full string tension. Maybe there is some good to them, but they look like some guy's garage DIY project. Now if StewMac came up with something like that we might be onto a winner; a manual PLEK machine if you like.
  11. FunkyMan


    Nov 27, 2007
    Bruce, it's an honor to have a response from you, thanks for all the advices, an thanks to the other TB'ers too!

    Actually i had the problem kohntarkosz is mentioning, zero trussrod tension, and a backbowed neck, in that case i had to pull out the frets and trim the fingerboard, this is the most safe way to solve the problem i guess, but the technique you gave it's amazing, i'll definitely use it from now! Do you remember how was the title of that thread about "S" shape neck? Thanks
  12. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I'm glad to help! Instrument necks can be surprisingly complicated, when you get into the fine details. I don't have much time to write here on TB these days, because I'm right in the middle of moving my whole shop to a different building, different city.

    I couldn't find that specific thread on the S-bends, but here's a quick summary piece that I wrote about the subject:

    Question: What causes the common S-curves that happen in many Fender-style bass necks, and is this related to the type of truss rod chosen?

    Answer: Well, no, an S-curve in a neck isn't caused by having a vintage-type single-rod truss rod vs a modern dual rod. You'll more often see S-curves develop in Fender-style necks, which usually have the vintage-type truss rod. But don't blame the truss rod.

    The S-curve is the result of two things happening, one causing the other.

    The first thing is what I call the "12th fret kink". It's a very common problem on guitars and basses. The neck essentially bends a small amount in the area right around the 12th to 14th frets. It's as if you'd bent it over your knee at that point. If you put a straight edge on the neck, you'll see that it's flat from the nut to the 12th, then takes a little bend from the 12th to the 14th, then is flat from the 14th to the 20th. This is also often called the "ski jump" effect. It's often described as the heel kicking up, but what's really going on is that the neck has bent slightly right where it transitions to join the body. The net effect is a wedge-like ramp from the 14th to the end of the heel, and fret buzz at the 20th.

    So, why does the 12th fret kink happen? Because that's the most highly loaded part of the neck. The wood on the back of the neck, right behind the 12th-14th fret area is under higher tensile stress than anywhere else on the neck. If you think about it, the neck is a long lever handle, and the strings are yanking on it, trying to pull the headstock forward. The stress point is where the handle joins the body, because of the leverage ratio.

    What happens is that, over time, the wood at the back of the neck gradually stretches from the continuous tension. So the neck bends right there. And the truss rod generally isn't much help in counteracting that load. Most production truss rods, either vintage-style or modern dual rods, don't do much of anything beyond the 12th fret, because they are right up against the underside of the fingerboard at that point.

    The truss rod is designed to support and adjust the neck, in the area from the 1st to the 12th fret. When you tighten it, the load it applies to the neck is centered around the 5th fret, and has tapered off to almost nothing at the 12th.

    And this is how the S-bend happens: First the neck develops the 12th fret kink from stretching over time. This causes the action to go high, with too much relief and buzzing down at the 20th, etc. So, the owner gets out a very large wrench and cranks on the truss rod, trying to get the neck to flatten out. But it doesn't, because the truss rod isn't pushing against the area that's bent. But the owner keeps cranking, causing the neck to backbow between the nut and the 12th. But it still has the kink at the 12th and the ski jump.

    There's the classic S-bent neck. Low at the nut, high at the 5th, low at the 12th and high at the 20th. It's a common problem on old Fenders and basses with that style of neck, but it can happen on other style necks too, even neck-thru designs.

    The heart of the problem is the neck being too thin (front to back) at the area where it transitions to the body. We Luthiers have various tricks and ways to counteract the kink, from better truss rod geometry, to carbon fiber inserts, to multilaminate construction. Lots of different solutions. But that's the area you need to look at in your design.

    I hope this helps!
    the V likes this.
  13. FunkyMan


    Nov 27, 2007
    Wow this is gold information, i'm taking note and save it! Thankyou verymuch!
  14. JLS


    Sep 12, 2008
    Emeryville, Ca
    I setup & repair guitars & basses
    I use aluminum U-channel, with the strings at pitch; this idea comes from Rick Turner and Bishop Cochran.
    I'll put my fretwork up against a plek any day of the week. I can't, however, print you out a nifty graph of the neck.

    Furthermore: many items that Stewmac sells, came from outside the company; they have no problem with running with someone else's concept. I'm not knocking Stewmac, they are an excellent company to deal with, and their return policy is unparalleled, I wanted to clarify, " StewMac came up with something like that we might be onto a winner".