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Thread: truss rods

  1. #1
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    Default truss rods

    I am getting my pop and back profiled for a new build, and have the neck glued up. now, I need to make an order from stew mac for some supplies. one , being a truss rod. The different times, I have looked at truss rods, to understand how they work , I see many that have a bow to them, and that like of makes sense to me. the straight rod from stew mac seems like it would just give a straight dead pull, and achieve nothing. I also understand , they would sell none of these if that were the case could somebody explain the straight versus arch TR and which might be better. I guess i could buy some rod, and make my own, but then buy taps to thread, which lead me to wonder, why not just buy a threaded rod? thanks. and as per usual, I may be giving this way too much thought
    Mike Marrs

  2. #2
    Registered User sunburst's Avatar
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    Default Re: truss rods

    A simple, tension truss rod works the same way whether it is straight of curved. The rod compresses the neck wood below (away from the fingerboard) the neutral axis of the neck shaft and thus counteracts the strings, which are compressing the neck above the neutral axis. When installed correctly in good quality necks, tension rods work whether they are straigt or curved. If there is an advantage, it seems to me that the ease of installation of a straight rod might be it.
    (I don't see either of these mentioned in your OP, but just for the sake of info; two way rods work differently from tension rods, CF rods stiffen but do not adjust.)

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  4. #3
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    Default Re: truss rods

    I've always built my own as per Siminoffs' plans.. my channel is tapered , being 5/8" deep at the heel and 3/8" at the nut.. works great for me..
    kterry

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    Adrian Minarovic
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    Default Re: truss rods

    The important thing in compression truss rods is that the anchors need to be as deep as possible. Depth of line connecting the anchors is the most important factor. Curve has some effect but rather small. (imagine how easy it is to bend string that is stretched with 20lb) From the three common designs the vintage Gibson curve (following bottom of neck) should be most effective and the Siminoff "reverse curve" least effective. For first timer the straight rod is first choice as is easiest to install and works. I prefer the Gibson curve as it allows the nut go higher up the headstock (with higher headstock angles like on old Gibsons), but I simplified the design and drill angled hole through heel into straight routed channel in the neck and slightly deepen it towards headstock with shaped scraper.
    Adrian

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  7. #5

    Default Re: truss rods

    For anyone who hasn't built a fair number of necks, I'd recommend using a dual action trussrod. It can address a number of complications that shouldn't happen but sometimes do.

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    Default Re: truss rods

    I've always wanted to see a CAD simulation of all the different kinds of trussrods.

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    Default Re: truss rods

    Quote Originally Posted by buckhorn View Post
    I've always built my own as per Siminoffs' plans.. my channel is tapered , being 5/8" deep at the heel and 3/8" at the nut.. works great for me..
    I have hogos prints, but find Siminoffs book very good for reference, but somehow, I am missing print 10 and 11 which I think have the truss rod diagram, not sure , its gone,lol
    Mike Marrs

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    Default Re: truss rods

    Siminoff believes the Gibson curved rod is poorly concieved because as tension is applied it is trying to straighten itself which would cause the neck to upbow. I think that's wrong because to get the nut end which is the most pliable to slightly flex shouldn't require the amount of force that would cause the upbow plus the body end is so firmly anchored in the most massive part of the heel. Also the curved rod actually is at it's highest point mid neck and then drops lower as it enters the peghead and is also anchored far forward in the PH to give maximum force at the front of the neck. Would like to hear others take on this though.

  11. #9
    Registered User John Bertotti's Avatar
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    Default Re: truss rods

    Out of curiosity just how important is the truss rod to start with? Neither of my mandolins has one, old Vega Bowl Back and My Old Wave A oval hole. Neither has any issues with bowed necks. The A has had constant tension on it since I got it so many years ago. I honestly can't remember when I got it somewhere between 2005 and 2008. I assume here it is an F model you are working on, is it more susceptible to bow?
    My avatar is of my OldWave Oval A

    Creativity is just doing something wierd and finding out others like it.

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    Default Re: truss rods

    It was when the F5 was introduced with the longer thinner neck that the trussrod was introduced. Prior to that the ovals had a short neck with 12th fret joint, were much wider with a more robust neck as well as a maple re-enforcement built in. They proved to be very stable.
    Not familiar with bowlbacks but I think they have very short necks. Can't speak for Bill but he might be using CF. several builders forgo the rod for CF re-enforcement.

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  14. #11
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    Default Re: truss rods

    Quote Originally Posted by Jim Hilburn View Post
    It was when the F5 was introduced with the longer thinner neck that the trussrod was introduced. Prior to that the ovals had a short neck with 12th fret joint, were much wider with a more robust neck as well as a maple re-enforcement built in. They proved to be very stable.
    Not familiar with bowlbacks but I think they have very short necks. Can't speak for Bill but he might be using CF. several builders forgo the rod for CF re-enforcement.
    I honestly can’t imagine a neck smaller then I have one either of my mandolins.
    My avatar is of my OldWave Oval A

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  15. #12

    Default Re: truss rods

    I've become a fan of the two way truss rod ever since I bought a guitar from Florida. When I got it here in California, the action was so out of wack, the truss rod was adjusted to neutral, then a turn and a half beyond. Over the next six weeks, I gradually turned it back to where it started. I have not adjusted a mandolin truss rod after the initial setup, but I don't travel. But it's good for me to know I have the ability to adjust both ways.
    Silverangel A
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  16. #13
    Registered User sunburst's Avatar
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    Default Re: truss rods

    Quote Originally Posted by John Bertotti View Post
    Out of curiosity just how important is the truss rod to start with?
    If a mandolin neck turns out to be very stable, stiff wood (we can't always tell when building) and stays straight and never needs a truss rod adjustment, I guess it turned out that the truss rod was of no importance.
    If that seemingly stiff, stable piece of neck wood turns out to pull into a forward bow from string tension and a simple truss rod adjustment puts it back into excellent playing condition, then the truss rod was very important.
    (Do you feel lucky? )

    If the amount of relief in the neck is exactly what you want and it stays that way with no truss rod adjustment, the rod is not important. I you sell the mandolin to someone who want's a different amount of fingerboard relief and a simple truss rod adjustment makes it ideal for the new owner, the truss rod has suddenly become pretty important.

    If the neck is great for the current owner and has CF rather than an adjustable rod, I suppose it had better not be sold to the person who want's a different amount of fingerboard relief.

    As for two way rods, sometimes they come in handy in banjo necks (long and thin), in particular curly maple banjo necks. Occasionally they even come in handy for guitar necks, but they so seldom are needed in relatively short, thick, stiff mandolin necks that I think the extra mass in the neck is not worth the potential benefit since the chance of needing a backward adjustment is so low.

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  18. #14
    Adrian Minarovic
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    Default Re: truss rods

    I wouldn't put double action truss rod into mandolin as they are very heavy and (F or A) mandolin is not very well balanced to start with. I use 4mm rods in my mandolins that are closer to original Gibson size. Also deeply buried thinner rod removes less material from the vulnerable "throat" of headstock and moves the nut pocket higher up the headstock leaving more wood where it is needed for strength.
    Adrian

  19. #15

    Default Re: truss rods

    The rod that came with my Arches kit was double action. The Mandolin doesn't seem neck heavy to me. Actually I haven't touched it except to put a tiny bit of tension on it so as not to rattle.
    Silverangel A
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  20. #16
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    Default Re: truss rods

    this year Martin just released its American Deluxe models that incorporate some changes. one of these being a lighter weight titanium adjustable truss rod.

    from Martin:
    TITANIUM TRUSS ROD
    One of the ultra-modern features that is new to this series is a two-way titanium truss rod, which makes the neck super easy to adjust, and it is 64% lighter than a traditional truss rod.
    full story here:
    https://www.martinguitar.com/modern-deluxe/

    wonder if/when someone will design the same for mandos.

  21. #17
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    Default Re: truss rods

    I had to replace a truss rod in a mandolin for a customer who owns an industrial metals business. He had easy access to titanium, had the equipment and welders/machinists to do the job of making a truss rod from titanium, but after we weighed the advantage (mass) against the disadvantages (expense, difficulty of machining and higher coefficient of expansion as compared to steel) we decided that it wasn't worth the slight mass saving even though it would have cost him nearly nothing.

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  23. #18

    Default Re: truss rods

    The point is that you don't know what is going to happen to a neck over time. Given that string tension is all on one side, the chances are better than 50:50 that the neck is going bend a bit forward. Having the ability to adjust the relief does a lot to make an instrument play well over the years. If you've got an old mandolin that plays very well and doesn't have a trussrod, consider that there are probably quite a few brothers and sisters manufactured at the same time that did not fare as well and have been tossed or are now unplayable.

    Dual action trussrods exist for the sole purpose of increasing productivity efficiency. Without having having a factory manned by legacy trained employees with generations of experience and the skill of selecting wood, all bets are off. Acquiring the most suitable wood is more difficult and expensive. The days of going out in the world and cutting down the most appropriate and suitable trees are over. The population wants to be able to go to Walmart and buy an instrument for cheap in total disregard and ignorance of what is actually required to construct a decent instrument. If you build an instrument by hand, you are generally going to be far more selective of materials than an overseas factory cranking out a hundred instruments a day but still, very few people can look at length of mahogany or maple and have a clue to what that length of lumber is likely to do in ten years.

    Most necks are going to react to string tension by bending forward and a conventional Gibson style trussrod will allow for adjustment of optimal playability. If the neck behaves as it should and only bends forward from string tension, the Gibson style trussrod works great by giving control of this predictable movement. But a few necks are going to go south due to the imperfect nature of everything. If for what ever reason the neck develops a backbow, the conventional trussrod cannot address the problem. I'm going to guess that probably 15% of the necks leaving a large scale factory building mainstream consumer level instruments are going to go south. By employing a dual action trussrod, these uncooperative necks do not end up as warranty returns and company profits are higher. But its not only the hidden and unpredictable natural mojo of the lumber that determines whether or not a neck is going to behave. There are a number of things in the manufacturing process that can cause problems.

    If fret slots are slightly too narrow, the combined effect of all the frets can induce backbow. This unfortunate result can happen from a number of things. Inferior fretwire or a wornout fret slot blade, harder than typical fretboard wood are things I have encountered over the years. There is also an odd thing that can happen when gluing a fretboard in place where there is a temporary expansion of the wood from the moisture of the glue. Despite the neck being perfectly flat before clamping the fretboard in place, there is a backbow when the glue has cured. This probably happens a lot less on the comparatively short neck of a mandolin but its common for guitar makers who use water-based glues such as Titebond.

    I don't like an instrument that needs to employ the reverse action of a dual action trussrod but at least they remain playable without needing to resort to more expensive procedures. For less experienced builders, I encourage using a dual action trussrod so the end result of their hard work and early experience is more likely to be positive.

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    Default Re: truss rods

    Of all the mandolins that I have worked on over the years I can only remember 2 that had a back bow in the neck, and they were both carbon fiber mandolins. I can't remember if the fretboards were wood or composite, but they were seriously back bowed. I e-mailed the manufacturer thinking they would want to know of the problem, but I never received a reply. I used the method that Siminoff described in his book on my first 5 mandolins, and I had to replace 3 of them within 7 years. Since then I've only used a straight rod set deep in the neck and never had.another problem. Sometimes I will add a small piece of carbon fiber on either side of the truss rod on especially weak necks (repair work) and the added stiffness improves the response and helps the truss rod move the neck more uniformly. I make sure to put some tension on the rod before I level the board and install frets, that way I can release some tension if there's not enough relief under string tension.

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  26. #20

    Default Re: truss rods

    Quote Originally Posted by Ward Elliott View Post
    Since then I've only used a straight rod set deep in the neck and never had.another problem. Sometimes I will add a small piece of carbon fiber on either side of the truss rod on especially weak necks (repair work) and the added stiffness improves the response and helps the truss rod move the neck more uniformly. I make sure to put some tension on the rod before I level the board and install frets, that way I can release some tension if there's not enough relief under string tension.
    Please excuse my ignorance, but your post is a perfect opportunity to try to get a better understanding of what's going on. I've built exactly one mandolin, an Army/Navy style, and embedded an aluminum bar for stiffness because I wasn't prepared to take on the complexities of truss rod installation.

    When you say you set a truss rod deep into the neck, does that mean it's BELOW the neutral axis of the neck, and applying TENSION moves the neck toward back bow? That's what I picture when you say you tighten the rod before flattening the neck so if the strings don't pull the neck sufficiently in the forward bow direction, you can relieve some tension and let it go that way? If so, I get it.

    Now, when a truss rod is installed just below the fret board, is it used to apply COMPRESSION to counteract forward bow induced by the strings because it is ABOVE the neutral axis of the neck? That also makes sense to me.

    And then I read about curved truss rod slots, which I'd like to just ignore until I get the above fundamentals correct.

    And should a double action rod be placed along the neutral axis of the neck if possible?

    I would really appreciate some feedback on my understanding of how these things work. They're still in the magic category for me.

  27. #21
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    Default Re: truss rods

    Yes, a single action truss rod has to be below the neutral axis of the neck or it just won't work. It might help stiffen the neck some, but it just won't bend it backward. I think the deeper the rod is in the neck the better it works. In my experience, most non-adjustable rods (steel, carbon fiber) eventually fail either because they are undersized, not fitted well, or the glue that holds them in place creeps. It kills me to have to tell a customer that the truss rod in their otherwise quality mandolin is not working, and fixing it requires removing the fretboard, digging it out and replacing it with one that works. I now have the option of compression fretting a mandolin with EVO fretwire, and that is very helpful, but it would be better in the long run if the adjustable rod worked correctly. I think the double action truss rod is intended to be installed directly under the fretboard, and I worry that the pressure against the board might cause the fretboard to come loose in some cases.

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  29. #22
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    Default Re: truss rods

    I have a Gibson from the 60's that someone X braced, but the truss rod was worthless. Removed the fingerboard and deepened the slot into the heel and it is now a functioning truss rod and the mandolin plays great. Guessing it was too shallow from the factory as it couldn't hold the neck flat at all under tension even with light strings.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

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    Default Re: truss rods

    If it's true that a straight rod deep in the neck is all thats needed, then what was Ted McHugh thinking when he went with the curved rod? My best explanation is to have the adjusting nut far into the peghead. About the only place you can put the adjuster with a straight rod is right where the rod enters the peghead. That seems to limit the leverage to me.

  31. #24
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    Default Re: truss rods

    Like this.
    And thanks to Adrian for the print.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    hank 

  33. #25

    Default Re: truss rods

    Very helpful to see that picture. So what happens when you tighten the curved rod (meaning increased tension)? It looks like it would try to straighten and also apply pressure to the bottom of the channel, which would add to forward bow. My brain hurts.

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