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DA truss rod - should the end blocks be fixed/anchored or not?

Discussion in 'Tele Home Depot' started by Yonatan, Oct 29, 2020.

  1. Yonatan

    Yonatan Tele-Meister

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    Ok, I can't figure this out:

    I'm getting ready to glue on the fretboard. I have a dual action truss rod as in the picture, and I noticed that the truss rod can slide a bit into the access hole that I drilled from the headstock.

    I'm reading conflicting things about this:

    Some people seem to think that that blocks should be fitted tightly enough so that they don't move, and some people say that it can be expected for the truss rod to slide if the fit isn't completely snug, and the TR will just be held in place by placing a bit of tension on it if necessary.

    So here's what I can't figure out: Throughout the entire range of operation of this type of truss rod, is the distance between the end blocks fixed? Or does that distance change across the range of operation?

    twowayTR_edited.jpeg
     
    Last edited: Oct 29, 2020
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  2. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    As I probably mentioned before, Stewmac originally designed these to be removed. The blocks don't need to be glued in place. a quarter turn on the rod puts pressure on the blocks holding them in position. The blocks go down...the top rod goes upward in the center, so that expands the total height some.

    In most cases the rods will be under tension/compression. I've never measured it, but the blocks probably come together one turn or so, making them closer together. They require a 7/16" deep slot IIRC.

    Here are the installation instructions again. Read number 3!

    https://www.stewmac.com/video-and-i...and-repair/hot-rod-adjustable-truss-rods.html
     
    Last edited: Oct 29, 2020
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  3. Freeman Keller

    Freeman Keller Friend of Leo's

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    To add to what Marty says, I use LMII rods that have little drops of weld on the sides of the blocks that lock them in place, but I honestly don't think it matters. The ends do pull together or apart but is is only few thousands of an inch.

    For an interesting experiment before installing the rod adjust it dead neutral and then give it one turn CCW and then one turn CW and loot at how it reacts. Then think of it doing that inside your neck.
     
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  4. Yonatan

    Yonatan Tele-Meister

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    Indeed, I had your previous response in mind here...

    Interesting, do you think that if the blocks were truly completely fixed (theoretically), it might interfere with how this type truss rod works? I've been playing around with it, but I can't figure out the mechanics of it enough to answer my question.
     
    Last edited: Oct 29, 2020
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  5. Freeman Keller

    Freeman Keller Friend of Leo's

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    No, it would not work if you totally prevented the ends from moving. The way these work is really quite ingenious. (I'm going to show pictures of an LMII rod but the stewmac works the same.) There is one bar or rod whose length is fixed, the other is a rod that is threaded on both ends, one end is a standard right hand thread, the other is backwards. Just like a turnbuckel. On the LMII rod the fixed bar is welded to the end blocks, on the SM rod is is simply threaded in.

    When you turn the adjustable rod CW the ends move towards each other, turn it CCW and they move away. The threading on an LMII rod seems to be 32 threads per inch, that is a very fine thread (which means it is very powerful but moves slowly). I took an LMII rod and turned the adjuster one turn CW, the block moved 0.026 inch, so the total movement is 0.050 towards each other. It looks like this

    LMII rod adjusted flat and neutral. The fixed bar is against the straightedge, in a real neck it is against the bottom of the fretboard. There is no pressure up or down on the straightedge

    IMG_6622.JPG

    Here it is with one full turn CW. As I said before, the blocks have moved a total of 0.050 towards each other (that is about the diameter of a low E string) but look at how much it has bowed, I measure 0.500 at each end

    IMG_6623.JPG

    In real life you never turn it a full turn, I consider 1/8 to 1/4 turn a lot, but you can see that even a small amount has a lot of effect.

    The end blocks are pushing down at the heel and nut end of the neck, the fixed bar is pushing up all along the fretboard, applying nice even pressure its entire lenght

    Lets look at one more scenario - here is one half turn CCW (looser)

    IMG_6624.JPG

    Now the black rod is in compression and is flexing but it is also flexing the bar in the opposite direction. That would be pushing up against the fretboard at the heel and nut ends, adding relief to the neck (I've never had to do this but it is one "advantage" of a double acting rod).

    So, to go back to your original question. If the blocks did not move towards each other there would be no effect. You would strip or break something. They do move but not very much and even with the little barbs imbedded into wood there is enough give to let them come together.

    A simple, powerful, and very clever way to control the shape of your neck.
     
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  6. Yonatan

    Yonatan Tele-Meister

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    @Freeman Keller
    Thanks for such a complete analysis, I hadn't found one so detailed on the entire internet!

    I was considering gluing in a small block of wood against the "anchor" (not really an anchor) on the headstock side, to prevent the truss rod from being able to slide towards the headstock by way of the access channel (the one on the other end is fixed against the heel end of the truss rod channel. So if I do, I will leave a tiny bit of play in it.

    Or I'll just leave it loose and put a bit of tension on the truss rod if needed as Marty mentioned.
     
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  7. gabeNC

    gabeNC TDPRI Member Ad Free Member

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    Thanks Freeman,

    Excellent instruction.

    That direction would be to correct "back-bow" on the neck? I've started to build my necks with 3 pieces, inverting the middle to hopefully reduce the likelihood of warping and using two-way truss rods. My ear can't hear the difference and I'm just happy to hit the right notes.
     
  8. Freeman Keller

    Freeman Keller Friend of Leo's

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    Yes, that would correct a backwards bow to the neck but lets be honest, I have never seen a well made neck that had a backwards bow. It should be dead flat when you are done building and have some relief when the strings are at tension. But lets say it doesn't have enough relief for your situation, you could add it that way.

    Multiple piece necks are probably stronger and more stable (and they certainly look cool) and if you are using riff sawn wood its probably a good idea. I build all my necks from well quartered mahogany and since I do scarfed heads and heels its pretty hard to make a 3 piece neck. The one time I did it was strictly for looks.
     
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  9. Yonatan

    Yonatan Tele-Meister

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    @Freeman Keller
    You inspired me to do my own test:

    • You can see that when the TR is in a back bow, there is less than a mm between the moving block and that piece of bamboo.
    • And when the TR is in an up bow, there is over a mm.

    BTW I'm thinking of gluing that piece of bamboo in (or a similar piece of wood from the neck blank cutoffs), to prevent the TR from sliding in its neutral position, honestly not sure it's needed, and based on this test I wouldn't want to make it completely snug against the block of the truss rod, which sort of defeats the purpose of doing this at all. Anyway I plan to wrap the end blocks of the TR in teflon tape to give it a snug fit.

    TRbackbow.jpeg TRupbow.jpeg
     
    Last edited: Nov 1, 2020
  10. Freeman Keller

    Freeman Keller Friend of Leo's

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    I never expect people to just take what I say as gospel, you should always do your own test. But yes, that is a good demonstration of how the DA rod works. Something to remember about a DA rod that is different from all the compression rods that Fender and Gibson uses is that the rod actually has to turn in the channel. With a compression rod it is only the nut on the end that turns.

    Depending on how I route the slot (angled head or not) I either leave a little bit of wood at the end or glue a 1/4x3/8 block in the end. I don't think the rod will ever move but it just seems like a good thing to do.
     
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