Does a straight truss rod channel really work?

Discussion in 'Luthier's Corner' started by Blackstar1099, Feb 24, 2018.

  1. Blackstar1099

    Blackstar1099 Junior Member

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    Hi all. My first post here and I need some advice.

    I've always used 2 way truss rods in the past but as I'm working on my first LP style build, I wanted to use a traditional one way compression rod I got from Stewmac. After doing a bit of researching on this forum, I've read that you don't really need the slot to be curved, so I routed a 1/2" straight channel with the same depth all the way through.

    I set up a little jig to test the rod to see if it works. I haven't glued the filler strip in yet just clamped. The bands are attached to the roof and are pulling the neck into about .012" of relief simulating string tension. Without the bands the neck is perfectly straight. The problem is when adjusting the nut nothing happens. The neck is around 3/4" thick at the moment so the rod is only a little more than 1/4 inch from the back of the neck. Should it straighten the neck? Would it work better with less meat on the back of the neck and a fretboard glued on? I really don't want to glue the already inlayed and fretted fretboard on yet without confirming that it will work.

    Any help would be greatly appreciated.

    Piero
     
    Last edited: Feb 24, 2018
  2. B. Howard

    B. Howard Premium Member

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    Position of the rod when using a flat slot is critical to function. It must set below the centerline of the neck to actually work.

    That said I must point out some flaws in your test procedure. You are only dealing with a neck that is partially there....The FB is missing so the final thickness is not correct. Next is the fact that 3 clamps will never simulate the cap strip being glued so the possibility exists that when you tighten the rod it just snakes and pushes up the strip a wee bit between the clamps rather than bend the neck.....path of least resistance.
     
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  3. pshupe

    pshupe Senior Member

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    I'm not sure why this comes up so often. :io:All the vintage Gibsons were made with 1 way rods. Literally thousands of guitars were built with one way rods. If it did not work they would not have built them this way at all. Is it the best way, absolutely not, but it works fine. In vintage Gibsons the slot is deeper at the heel than at the neck. I do not believe a straight flat channel will work, or at least not as well. They only could adjust one way, to resist backbow.

    As B.Howard comments your test really wouldn't tell you much as that is not an accurate representation of a guitar neck.

    Regards Peter.
     
    Last edited: Feb 24, 2018
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  4. nuance97

    nuance97 Premium Member

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    Yes they work very well
     
  5. nuance97

    nuance97 Premium Member

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    If you want to do a curved rod this is a cool method. I imagine you could modify this same concept to work with a router table as well.
     
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  6. pshupe

    pshupe Senior Member

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    Don't forget to make a curved filler strip as well. :)

    Cheers Peter.
     
  7. WhiteEpiLP

    WhiteEpiLP Senior Member

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    Typically speaking if a guitars neck is straight and stable under no string tension, when tension is added it will bow the neck up creating relief. It will be too much relief without a trussrod to pull it back, so a oneway truss rod is a perfectly fine solution.
    2 way are better but are really only needed in a neck thats not stable.
    But most important is what Peter mentioned, the channel needs to be 1/8" deeper at the heel then the nut. If its parallel to the fretboard its only going to compress the wood at the nut. The angle allows the tension on the rod to pull down on the neck.
    The majority of my guitars have 2 way rods and ive never had to use te rod to add relief, string tension does that. If string tension isnt enough to create relief then imo the neck is bad.
     
  8. cmjohnson

    cmjohnson Senior Member

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    I use a single rod double action PRS style truss rod. It works under the turnbuckle principle and I install it in a curved channel.

    It's effective. It works on forward and backbow.

    I've seen that straight channel single rods are not AS effective, and I've seen some examples that were not really effective enough to even call effective at all.

    I will never use a straight channel with a single rod type truss rod. I do not trust them.
     
  9. valvetoneman

    valvetoneman Senior Member

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    if you use a traditional one way rod then you've got to pre load it then flat the fretboard again before fretting, this is how I build mine and Freddy did the exact same thing, especially if you've got a big chunky neck imo, nothing to do with the wood is no good, if it's well seasoned it's stable and I'm never using a modern rod ever again
    I will have to do curved rods though for slimmer necks like a 60's one
     
  10. timfred

    timfred Senior Member

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    What you have should work fine as soon as you carve the neck and add a fretboard. The magic of the one way trussrod is that it needs to be well away from the cross-sectional center of mass to work.

    I know this is a leap of faith, especially if you are used to being able to test 2 way trussrods on the bench prior to install.

    In rod we truss.
     
    Last edited: Feb 25, 2018
  11. Blackstar1099

    Blackstar1099 Junior Member

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    Thanks for the help guys. I think I'll make the heel end a bit deeper to be safe.
     
  12. B. Howard

    B. Howard Premium Member

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    That is correct, I did not mention it but it is the same rule that the rod must stay completely below the center line. As the neck gets thicker toward the heel the channel must get deeper to stay below center. This is where detailed scale drawings become necessary to confirm that all sections of the rod are below the actual center line of the neck.

    The farther below center line of the neck to the center of the rod is the faster it will react and the more relief it will be able to take up.
     
  13. cmjohnson

    cmjohnson Senior Member

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    I just checked and verified against a genuine PRS neck blank that I got from a former PRS employee. The neck is a cast-off from assembly due to a small flaw in the headstock area, but clearly shows that the truss rod slot is deeper at the body end by about 3/16".
     
  14. Pesh

    Pesh Senior Member

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    So to ask a possibly obvious question, but for the sake of clarity...

    I have a traditional Gibson style TR for a LP build I'm doing. The neck is mahogany, so could I get away with doing straight slot with a slightly deeper heel, or should I go for the curved method? I think with the limited tools I have I could fashion the carved base to the channel, but I would like to make things as easy as possible for what is effectively my first full build.
     
  15. nuance97

    nuance97 Premium Member

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    I would go for a straight-tapered channel unless you just enjoy the extra challenge. It will definitely work
     
  16. fatdaddypreacher

    fatdaddypreacher V.I.P. Member

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    for what few i've built, i too have used the straight, but tapered rod channel. i cut mine on the table saw while the neck is wide and square, tacking two equally parallel strips of quarter inch plywood to the sides of the blank. I tack them with and 1/8" difference from the neck plane, so when make my plough, the channel gets deeper at the heel end. Have never had an issue with any of the adjustments i've had to make.
     
  17. bfcg

    bfcg Senior Member

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    That first center line response was almost there,
    The truss rod must cross the center line in order to be effective.
    The slot is sloped or ramped and goes from above the long center line to below. High at the nut to low at the heal.
     
  18. bfcg

    bfcg Senior Member

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    Yeah, what fat daddy said.
     
  19. Freddy G

    Freddy G V.I.P. Member

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    Good to see you around! Hope you've been well.
     
  20. pwhite

    pwhite Senior Member

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    Yeah Bill you have been MIA for a long time, did you every finish that V you were doing?
     

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