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Thread: Truss rod angle and depth

  1. #1
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    Default Truss rod angle and depth

    I haven't found anything about this in prior threads, so if it's been discussed before I apologize.

    Happened to flip open Siminoff's Construction Manual today just because it was handy at lunchtime. The book opened to his section on making and fitting truss rods, which was a coincidence because I had been milling a bunch of octave mandolin truss rod slots and nut pockets. Never paid any attention to this before, but Roger says the truss rod slot should be angled through the neck, and be significantly deeper at the neck heel end than at the headstock. He specifies angled filler strips to accommodate the design, which makes sense. But he doesn't say why, he just says to do it that way.

    I've never used anything but straight truss rod slots in whatever the instrument. Never had any trouble, either. The question(s): Is an angled truss rod important/better? If so, why? Is this peculiar to Roger Siminoff? Is he just stating Lloyd Loar orthodoxy?

    It was a surprise to read this, and it aroused my curiosity. If anyone can offer enlightenment, I'd sure appreciate it.
    "And the days that I keep my gratitude higher than my expectations... Well, I have really good days."

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    Registered User sunburst's Avatar
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    Default Re: Truss rod angle and depth

    This has been discussed here quite a bit, and it can get pretty involved. This applies to "standard" tension truss rods. Two way rods, stiffeners such as CF rods and so forth, those are different.
    Bottom line: as long as the rod is well fit, well designed and well installed, it will work as long as it is positioned below (toward the back of the neck) the neutral axis of the neck shaft. Deeper below the neutral axis might increase efficiency, and that is the reason many of us install a straight rod deeper at the heel end. It parallels the back of the neck more closely than it does the fingerboard so it is deeper overall than if it paralleled the fingerboard.
    Some rods are curves. Some curve up, some curve down. Either way, they work the same way as a straight rod as long as they are set below the neutral axis of the neck shaft.

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    Default Re: Truss rod angle and depth

    Thank you for the comprehensive explanation, John. Most helpful.
    "And the days that I keep my gratitude higher than my expectations... Well, I have really good days."

    -Ray Wylie Hubbard

  5. #4

    Default Re: Truss rod angle and depth

    I asked this question pretty extensively a few years ago among the best builders in the world. Ten different people gave me about 25 conflicting answers so......I took the top ten designs, depth, angle, curve up, curve down, straight, et cetera, and made ten different necks from the exact same boards, complete with fretted fingerboards attached.....

    They all worked almost identical.

    So much for hype and mythical loar fairy dust and unicorns; now I generally use a straight channel because it is easy to manufacture. My least favorite is a straight rod with a right angle bend at the heel because the stress load at the bend is different than a straight pull under tension and almost all of the ones I have had to replace in repair work for others have been the right angle bend at the heel.....
    Spruce dork

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  7. #5
    Adrian Minarovic
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    Default Re: Truss rod angle and depth

    I agree with above. I posted about this in one of many previous threads on MC. Typical Loar rod was curved (that is also visible in the original patent granted to McHugh https://patents.google.com/patent/US1446758A/en) and in Loar necks (pic below).
    The most important part is where nuts (anchors) are located and the line connecting those must be below neutral axis. (that holds with smoothly curved rod) The deeper below axis, the more efficient it works (less tension will move the neck).
    I do a compromise between curved and straight rod. I rout straight slot but drill from heel into the slot with long drill to burry nut deeper in the heel end and slightly smooth the transition bend with small gouge (I lose some effectivity of smoothly curved rod but the bend prevents turning of the rod) and I slightly deepen slot towards headstock to get the nut pocket 1" away from nut and thus reduce the weakening of headstock.
    I think these two factors were also important for Gibson mandolins but with factory setting and one set of templates you can easily cut the slot of whatever shape you want.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Adrian

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  9. #6

    Default Re: Truss rod angle and depth

    What is the position of the neutral axis? Where is it measured from? Or how is it determined?

  10. #7

    Default Re: Truss rod angle and depth

    Quote Originally Posted by Will Williamson View Post
    What is the position of the neutral axis? Where is it measured from? Or how is it determined?
    The neutral axis will correspond with the center of mass ,the centroid of the cross section. With a rectangular section, like a 2x4, it is in the middle. With a neck it will be somewhat closer to the fingerboard. If you are below the middle you are fine.

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    Registered User sunburst's Avatar
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    Default Re: Truss rod angle and depth

    The neutral axis of a neck turns out to be very close to the fingerboard glue joint. That is partly because the fingerboard material is generally a stiffer, more dense material than the neck. The half-round to V shape of the neck also contributes.

  12. #9
    Adrian Minarovic
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    Default Re: Truss rod angle and depth

    The whole deal is a bit more complicated by the fact that woods stiffness is different in compression and tension (though I found some conflicting numbers)

    Generally the neck behaves kida like T bar. The fingerboard being a bit stiffer than neck wood corresponds to the wide top of the T while the neck shaft is the leg. The neutral plane of T bar is up high on the leg towards the T.
    If we assume that the wood is stiffer in compression than in tension (it takes more force to change length in compression than it takes to change the same amount by tension within its elastic limits) and the neck is rounded and not rectangular (unlike the T) then the neutral axis moves up even higher.
    See some data here:
    http://www.ukuleles.com/Technology/neck.html

    The truss rod (standard tension rod) works just like another set of strings that would be tensioned on opposite side of neck to counteract the tension o strings. Since it is relatively close to the neutral axis it has to act with greater force that's why we use steel rod with strong nuts.
    Adrian

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    Default Re: Truss rod angle and depth

    Name:  DSC07569.jpg
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    Jonathan, this is a xray of a couple years old Collings MT, don't know if this will help with the discussion, but here it is.

    d

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  15. #11
    Adrian Minarovic
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    Default Re: Truss rod angle and depth

    Quote Originally Posted by darylcrisp View Post
    Name:  DSC07569.jpg
Views: 119
Size:  4.1 KB

    Jonathan, this is a xray of a couple years old Collings MT, don't know if this will help with the discussion, but here it is.

    d
    If you compare the two x-rays you can see how much further up the headstock the TR nut is on Gibson (thanks to curved shape of rod) than on Collings. The Gibson headstock is definately stronger against breakage.
    Adrian

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