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Thread: Truss Rod Profile

  1. #1

    Default Truss Rod Profile

    Hi, this is my first post on the Forum. I am from Australia and have been building mandolins for the last six years. I did the Siminoff Mandolin Building Course in 2010. I have always installed truss rods in a flat profile, just noticed in my Siminoff manual that he showed them as a curved profile. Any thoughts either way? I have never had any issues with truss rods that would not perform as they should.
    Last edited by Dunn Mandolins; Feb-10-2017 at 6:37pm.

  2. #2

    Default Re: Truss Rod Profile

    Welcome to the forum Messer Dunn.
    You can take a look at my blog page if you want my thoughts on mandolin truss rods.
    http://www.apitiusmandolins.com/Oliv...og%20Page.html
    www.apitiusmandolins.com

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  4. #3
    Matt Cushman Cush's Avatar
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    Default Re: Truss Rod Profile

    I have used the Siminoff style truss rod on all my mandolins since 1999. They are quite effective, light weight and easy to make. With all those qualities I see no reason for me to change now.

  5. #4
    formerly Philphool Phil Goodson's Avatar
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    Default Re: Truss Rod Profile

    Quote Originally Posted by Oliver A. View Post
    Welcome to the forum Messer Dunn.
    You can take a look at my blog page if you want my thoughts on mandolin truss rods.
    http://www.apitiusmandolins.com/Oliv...og%20Page.html
    Oliver,
    Your description of how single, curved truss rods work and how the forces, as they are tightened, tend to move the neck forward instead of backward (as needed) sounds plausible; but if that is true, how is it that they actually work? When you tighten a single action truss rod, the neck indeed does straighten to move the headstock towards the back.
    I must be missing something?
    Phil

    “Sharps/Flats” “Accidentals”

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  7. #5

    Default Re: Truss Rod Profile

    If you look at the drawing below, you can see how this design is upside down. Virtually all modern and not so modern fretted instruments that utilize a single rod system, bend their rods the other way. I'm sure you'll agree that by tightening the nut, the effective length of the rod becomes shorter and if it becomes shorter, will want to straighten out. If the rod straightens out, it will bend the neck in the wrong direction.
    These rods can be effective for very minor adjustments as they will impart some stiffness as the nut is initially tightened.

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  8. #6
    Matt Cushman Cush's Avatar
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    Default Re: Truss Rod Profile

    Quote Originally Posted by Oliver A. View Post
    If you look at the drawing below, you can see how this design is upside down. Virtually all modern and not so modern fretted instruments that utilize a single rod system, bend their rods the other way. I'm sure you'll agree that by tightening the nut, the effective length of the rod becomes shorter and if it becomes shorter, will want to straighten out. If the rod straightens out, it will bend the neck in the wrong direction.
    These rods can be effective for very minor adjustments as they will impart some stiffness as the nut is initially tightened.

    The drawing looks like a MacRostie style TR. The Siminoff style that I am familiar with looks much different. I will try and find a drawing. I don't believe it matters what direction the rod bends. All that matters are the two anchor points of the rod and how deep it is set in the neck.
    Last edited by Cush; Feb-11-2017 at 5:12pm.

  9. #7
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    Default Re: Truss Rod Profile

    Im not a builder nor do i play one on TV but the way i look at it Oliver A would be right if there is empty space below rod. But if rod can't straighten at the bend because there is no room to then the tension would be pulling opposite of the string which would be what you want.

  10. #8
    Adrian Minarovic
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    Default Re: Truss Rod Profile

    There's a bit of confusion in this thread... Let me add some of my insight.
    SImple compression rods work by putting one side of neck into tension just like string on the bow (archery) with tiny difference that it is inside the neck.
    Straight rod works best if it is burried as deep as possible but since the fingerboard with frets installed is much stiffer that the rest of the neck (also counting with the rounded shape of neck the neutral axis of bending moves effectively right to the fingerboard joint) you don't have to go to the very limits of depth to get it working.
    Bent rod similar to Olivers drawing (with gentle bend) will work much like if it were installed straight between the endpoints. There is force that wants to straighten the rod and thus working against our wanted action but the force is nearly negligible compared to the bending force caused by the compression between the nuts of rod. (have you ever tried to stretch a rope between two points? - it will remain slightly bent just by gravity no matter what you do - and you need tiny force to deflect it even more) So the neck will bend down away from strings.
    If you do opposite bend like in the Siminoff book with the ends of rod just below fingerboard you will still likely end up with working rod but not as effective. There is small force of straightening rod wanting to bend the neck in correct direction and since the fingerboard is so stiff the tension between the ends will still add force that bends the neck the correct way, but much less effectively then rod burried deeper in neck.
    Adrian

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  12. #9
    Matt Cushman Cush's Avatar
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    Default Re: Truss Rod Profile

    I used the MacRostie style on the mandolins I made before 1999. And I had very good adjustability with that style. I just don't care for the anchor nut rather than a simple 90 degree bend in the rod. As long as the rod is placed deeply enough within the neck both rods seemed to work equally well for me.

  13. #10
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    Default Re: Truss Rod Profile

    I used the Siminoff style truss rod on the first 5 mandolins I built back in the early 90's, but 3 of them failed when the bent end that served as the anchor moved enough to ruin the effectiveness of it. After I switched to the straight rod buried deeply in the neck and anchored with a 3/8" barrel nut I have never had any truss rod problems. My favorite way to do it these days is to also use a small (1/8" X 1/4") piece of carbon fiber spaced about 3/16" out on either side of the rod. This significantly stiffens the neck but the rod still works very nicely too. I think it's the perfect combination of stiffness and adjustability. But the rod must be buried deeply or it won't be able to move the " extra stiff " neck.

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  15. #11

    Default Re: Truss Rod Profile

    Quote Originally Posted by Dunn Mandolins View Post
    Hi, this is my first post on the Forum. I am from Australia and have been building mandolins for the last six years. I did the Siminoff Mandolin Building Course in 2010. I have always installed truss rods in a flat profile, just noticed in my Siminoff manual that he showed them as a curved profile. Any thoughts either way? I have never had any issues with truss rods that would not perform as they should.
    Hello from another person in Aus, are they single action truss rods or dual action truss rods that you have been installing flat. If single action, then I would consider future builds putting them in a arched profile, they work exceptionally better.

    Steve

  16. #12

    Default Re: Truss Rod Profile

    It always seemed to me that Siminoff's rod is meant to address the entire length of the neck when what a rod is intended to do is counteract the tendency of the strings to bow the neck upward in the first 5-6 frets. I think that's the mis-understood part of the original McHugh rod. They were never meant to address every possible problem a neck might encounter, they were meant to counteract the string tension and allow for a more streamlined, thinner, narrower and longer neck.

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  18. #13

    Default Re: Truss Rod Profile

    This is from Adrian's print. What I see is the high point of the rod in that 5-6 fret area and then it drops lower from there into the peghead. I also see just how far forward it is in the peghead. To me this is to give maximun leverage over the nut side of the neck where it's the most flexible and susceptible to bowing.
    I'm not going to say I fully understand what they had in mind when they came up with this design, but I've heard it said by some very reliable sources that Loar's are very responsive to adjustment.
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  20. #14

    Default Re: Truss Rod Profile

    Quote Originally Posted by Cush View Post
    I have used the Siminoff style truss rod on all my mandolins since 1999. They are quite effective, light weight and easy to make. With all those qualities I see no reason for me to change now.
    Wow, there is a lot of great information here to go through, thank you to all who have contributed their thoughts. I have used Roger Siminoff's truss rods from his online store in the past and they have been straight with a small 90 degree bend on the end. I assumed that they were installed with that profile, with the heel end slightly lower than the nut end. But when I checked his drawings in his manual recently it showed a upwards curved profile and a long bend into the heel. Having said that, the truss rods I have installed up to date have all worked well, but I had to satisfy my curiosity. I'll go through all the information I have got here, thanks again.

  21. #15

    Default Re: Truss Rod Profile

    Single action, Steve. I install them in a slot that is relatively parallel to the fingerboard and towards the bottom of the neck.

  22. #16
    Hester Mandolins Gail Hester's Avatar
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    Default Re: Truss Rod Profile

    As Jim said, most Loar owners say the truss rod works very well and is very responsive in their Loar F5. I also believe that most of those mandolins have necks oriented on the slab making them a bit more pliable, I think.

    Here is an X-ray that's been around for along time of Loar F5 #77166, seemingly a very successful design. I don't go this deep or use this much bend and have not had any issues but here it is for discussion.
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    Gail Hester

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  24. #17

    Default Re: Truss Rod Profile

    Thats deep

  25. #18
    Registered User sunburst's Avatar
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    Default Re: Truss Rod Profile

    There have been a few discussions here about curved vs straight rods and so forth. It was a mystery to me how the Loar style curved rod worked, mostly because I hadn't thought much about it, but when Adrian (in one of those other threads) pointed out that the ends of the rod are considerably below the neutral axis of the neck and that the compression force below the neutral axis of the neck easily overcomes the tendency for the rod to "straighten out" from tightening, it all came together for me.
    The Loar style curved rod accomplishes two things that I can see.
    1. It prevents the rod from rotating in it's channel in the neck so we can tighten the nut at one end without the rod simply turning in the channel and in the nut on the other (heel) end. In other words, it makes it possible and practical to simply use a nut and washer as an anchor at the heel end of the rod.
    2. It makes it easier to move the adjusting nut up into the peghead away from the string nut, thus moving the truss rod pocket away from the weakest part of the neck into a stronger (because it's wider) part of the peghead.

    As for Loar truss rods being "very responsive", those few (maybe four or five that I can remember) that I have adjusted have worked. I wouldn't say they are "very responsive", no more than many other mandolins that I have worked on, but they work, in my experience.

    As for me, I use straight rods placed more-or-less parallel to the back of the neck rather than parallel to the fingerboard. That puts them deeper into the neck than a straight rod parallel to the 'board, and perhaps makes them more effective, being farther from the neutral axis of the neck. In reality I suspect it's about the same. I also use a fixed anchor at the heel end rather than a nut. I am able to place the adjuster nut at about the same position in the peghead as the Loar design, so all in all, the deeply set straight rod is easier for me to install than a curved rod and the advantages of the curved rod that I mentioned do not apply. Generally, I feel that my necks, with the straight rods set deeply into the neck, work about as well as the Loar rods that I have adjusted.
    My truss rod conclusion: for tension rods (not two-way) curved and straight are functionally about the same. Both work, and I think it is easier to install a straight rod, so that's what I use.

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  27. #19

    Default Re: Truss Rod Profile

    Yes, after thinking about Adrian's comments, I would agree that the compressive force between the two anchor points will overcome the force that tries to straighten the rod out. As long as the anchor points are below the center-line of the neck the rod should work well. Personally, I still like the self-contained double rod approach. Mainly because it does not add any compressive forces to the neck. Years ago, I used to use a non-adjustable steel bar 3/16" X 1/2". It worked very well but a few of those mandolins would develop some excessive forward bow and require the fingerboard to be leveled. After this was done once, they never came back for a second time. Martin guitars had a similar experience with their non-adjustable square tubes. The double rods that I use now work extremely well for me. I just tighten them up snug before the first string-up. At first stringing, they typically show a very small amount of relief, about 1 or 2 thou'. I have yet to need to adjust a single one of these. When they are snugged up, they are virtually as rigid as a solid bar (in the direction of string tension). I believe that one secret to getting them to work so well is to have them fit in their slot very well with no room to move around. That's why I glue a filler stick over top of them and clamp it down while the glue dries. I've heard that some people glue the fingerboard right over top of the double rod with no filler stick but I've never been comfortable with that idea.
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  28. #20
    Adrian Minarovic
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    Default Re: Truss Rod Profile

    The double rods exert similar force to neck as well. If you tighten the rod outside of the neck it will want to form D letter shape. when the rod is glued in, the rods also want to go apart but the channel prevents that. From this perspective, the best double rod design would be the U channel style with enough space for the rod to move (1/32" 1/16" clearance between the rod and bottom edges of the U would be more than enough, it would require some soft foam/rubber inserts against rattle).
    Gail, the mandolin in you x-ray is an A4. But Loars rod was similar, just longer, see scan. (sorry, I don't have bigger size of pic now).
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    Adrian

  29. #21

    Default Re: Truss Rod Profile

    Always wished someone would do a CAD analysis of the different rod configurations. (hint hint)

  30. #22
    Registered User sunburst's Avatar
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    Default Re: Truss Rod Profile

    Quote Originally Posted by Oliver A. View Post
    ...Personally, I still like the self-contained double rod approach. Mainly because it does not add any compressive forces to the neck.
    I've thought about that too, and here's what I've concluded. I don't mean this to sound like a lecture, this is just a basic review of the thought process that I went through.

    We can expect a neck lying on the bench, or on the instrument before ever being strung up, to remain straight because there are no forces "trying to" bend it or otherwise distort it (if it is stable, well chosen wood... and we're lucky). When we add string tension to the mandolin we add a compressive load to the neck concentrated at the fingerboard surface. We could think of the strings as eight little truss rods pulling above the neutral axis of the neck and compressing the wood. Heck, they are so far above the neutral axis that they are completely outside of the neck! With all of those "truss rods" compressing the fingerboard we can expect a little bit of a bend to show up in the neck. Fortunately, we luck out and it works out to call it a little "relief" in the fingerboard, and we are generally glad to see it. If the relief, however, is excessive, what can we do about it? We can compress the other side of the neck, the part below the neutral axis, an equal amount. We can't put strings or a tension rod outside of the back of the neck and still have good playability, but we can put one inside the neck well below the neutral axis. Once again, we get lucky because of the shape of and construction of the neck establishing the neutral axis well toward the fingerboard surface, and a tension rod set anywhere reasonably deeply into the neck is below the neutral axis.
    So, although we are adding some compression to the neck when we tighten the tension rod, we are actually countering the compression we added when we put the strings on, and we can think of it as balancing the compressive load on the neck.

    Double acting rods, or two-way rods, on the other hand work by applying a bending force to the neck to counter the bending that results from the strings compressing the fingerboard. They generally work by pressing against our glue joints. Either they are directly under the fingerboard pressing upward in the middle or they are deeper and under a filler stick so that they are applying a sheer force upon the filler stick glue joints. Either way, the glue joint is generally up to the challenge and the rods work. (I usually use Martin style U-channel rods for guitars.)
    So once again I concluded that both tension rods and two-way rods work. My (home made) tension rods weigh 13 grams. I don't have weights for any two way rods, there are several designs of them available, but I can't imagine that any of then don't weigh more than 13 grams. Since mandolins with f5-length necks tend to be neck heavy, I prefer to keep the weight of the neck down as much as I practically can, and a lighter truss rod is one way to help with that.
    My conclusion: Tension rods work, two-way rods work, tension rods are usually lighter, so I use tension rods in mandolins.

    (For the record, I don't recall ever having to correct a back bow in a mandolin neck, with one possible exception where the neck wood was cross grained and there was so much run out and "weird" grain that I feel that the wood was not suitable for a neck and never should have been used for one. Banjo necks of very curly maple are the only necks where back bow tends to be a problem on any kind of regular basis, at least in my experience, so I've concluded that we don't need two-way rods in mandolin necks made from well chosen wood often enough to make it worth the extra weight, IMO.)

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    Registered User fscotte's Avatar
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    Loar's foresight always intrigues me. The common question in his design is, how did he know? How did he know that a truss rod would hold 100 years later. How did he know that the hollowed out area around the neck wouldn't twist under the constant pull of the G side? That question can be used for the whole F5 design.

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  34. #24

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    Well, Ted McHugh had the trussrod patent and I've always figured that he and other hands-on guys who had a good grasp on the physics of instruments did most of the legwork. Loar probably said the necks needed to be longer and thinner and Ted figured out how to make that happen.

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  36. #25
    Adrian Minarovic
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    Default Re: Truss Rod Profile

    Quote Originally Posted by Jim Hilburn View Post
    Well, Ted McHugh had the trussrod patent and I've always figured that he and other hands-on guys who had a good grasp on the physics of instruments did most of the legwork. Loar probably said the necks needed to be longer and thinner and Ted figured out how to make that happen.
    I don't think the longer neck of F-5 was the reason to add trussrod. They started a year before F-5 was introduced and was used on most models insluding the short necked A styles and they even retrofitted the already maple-reinforced mahogany neck blanks with the trussrod.
    Agree with Jim, that bending forces in beams are pretty basic physics and anyone who took some engineering classes knows about them. McHugh was innovator because he was the first to realize that they can be utilized in the necks.
    I'm in the same camp with John and use simple rods in my mandolins. I prefer 4mm rod as it is close to original Gibson size and adds least weigth.
    I would add to what John said that there are ALWAYS two forces in the bent long beam no matter how the bend is introduced. The outside will stretch under tension even if pressure only is applied on one side. The total of all compression forces (over the crossection below the neutral plane) and tension (over the crossection above the neutral plane) is always zero.
    trussrod works by counterforcing what the strings do. The simple trussrod adds strength against tension near the bottom of the neck and result is that in the neck assembly with rod lightly tightened the neutral axis moves down towards the back of the neck and the compression added by strings is held by much larger part of the neck below fretboard. there could be analogy in adding layer of sinew to the back of (turkish or korean) bow, the sinew shrinks upon drying and adds strength in tension to that side of bow opposing the string side. The sinew moves the neutral plane towards back of bow (outside of the bend) and leaves much more wood to "work" against the compression caused by pull of the bowstring. (wood is typically twice as strong in tension as in compression so this prevents premature breakage caused by the compression of the belly of the bow)
    On the other hand the double rod works much like additional stick tied tightly (laminated) to the bent beam. It works by countering the forces of the strings. Adjustability of this style is analogic to what happens if the additional lamination is pre-bent before adding to the beam.
    Adrian

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