Compound radius fretboard and bridge saddle height

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by finnster, Dec 29, 2017.

  1. finnster

    finnster Supporting Member

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    When adjusting saddles on a bridge with a guitar that has a compound 10-14” radius board should I match the 10 or 14?

    My initial thought would be to match the 14” but I wanted some opinions of others.
     
  2. Jack Daniels

    Jack Daniels Supporting Member

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    Think of the compound radius like a cone or lets say a styraphone coffee cup. The small end is the 10" radius. The large end is 14" radius. What do you think the radius will be 10" past the end of the cup? It will be larger yet. So in your case, it may end up as 16" radius at the bridge. You can start with a 14" under-string radius, but I think that will be a bit high on the middle strings. The other option is to measure the string height at lets say the 20th fret. Make them all match to start and you will be close to the proper radius on the saddles
     
  3. walterw

    walterw Gold Supporting Member

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    that's all i ever do.
     
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  4. trap

    trap Supporting Member

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    Suhr measures at the last fret.
     
  5. finnster

    finnster Supporting Member

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    Good to know....it is Suhr guitar :)
     
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  6. Mark Robinson

    Mark Robinson Member

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    I treat each string as its own puzzle to be solved. I set it too low, so that it is buzzing, in various locations, then I raise the saddle screws until it plays clean. I do this in playing position. I also very carefully get the relief just right first, before either intonation or action setting.

    I never measure anything on my guitars, either it plays clean under my normal touch, or I manage the relief, and adjust the saddles.
    For me, with a guitar that is a perennial, I settle on gauge and maker of strings, and don't change around willy-nilly.
     
  7. Fireball XL5

    Fireball XL5 Supporting Member

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    So my question is:

    With a compound radius, and the bridge saddles set at the last fret to match the flatter radius - do you need to run more relief in the neck to avoid fret rattle in the lower part of the neck?

    To my way of thinking, the radius of the fretboard is greater with more hump in the middle in the lower positions, yet the strings are set lower/flatter in the middle to match flatter radius up high. Seems like you'd need more relief to keep the lower middle strings from buzzing?

    Am I off base on this?
     
  8. Mark Robinson

    Mark Robinson Member

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    My opinion is that the compound radius doesn’t increase relief requirement necessarily. Maybe it even reduces the need a bit. I find that 7.25 straight radius necks will often need a bit to avoid buzzing. One main point of compound radius neck is to allow lower action without fretting/buzzing up in the higher regions. I have two compound radius necks and I keep them pretty close to flat, with action high enough to not buzz anywhere. I don’t like buzzing, I can hear it through amps easily and it sucks energy out of the instrument. I don’t have rad speed skills that require ultra low action though.
     
  9. Mark Robinson

    Mark Robinson Member

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    The tighter radius down low shares tangent points in the center of the neck, up and down. So the smaller, tighter radius parts , lower, are by geometry, beneath the higher frets, it’s a cone. You can sight a pure straight line from tip to base of a cone at any given degree. Relief basically raises action.
     
  10. walterw

    walterw Gold Supporting Member

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    not at all.

    indexing each string to a given height off one fret incorporates the compound into the result, the strings match the contour of the neck.
     
  11. Husky

    Husky Gold Supporting Member

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    About 1/16" between the top of the last fret and the string (distance in-between) is the best place to start and tweak to taste, there is no rule.
     
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  12. KGWagner

    KGWagner Member

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    I frequently deal with necks with a 10" to 16" progressive radius, and on a 25 1/2" scale the bridge radius ends up needing to be around 18". As has been mentioned, the cone formed by the compound radius of the neck continues to expand the farther down you go, so you can't go by any of the fretboard radii.

    With bridges that have individually adjustable saddles, it's not an issue. With fixed-radius bridges, you have a longer row to hoe as nobody offers a bridge with that radius out of the box, so you end up shimming or filing saddle slots, depending on the bridge design.
     
  13. Tone_Terrific

    Tone_Terrific Member

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    If the string spread at the bridge is wider that the spread at the nut and you have any radius at all you WILL have a conical or compound radius to deal with.
    Basically all guitars. The conical fretboard is a way to bring the fretboard into better compliance with the shape defined by the strings.
    In practice, it doesn't really matter all that much.
    Set up to the best possible and it is good enough. The micro errors will remain.
     
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