Please explain about compund radius fingerboard...

Discussion in 'Guitars in General' started by Oakley, Aug 2, 2008.

  1. Oakley

    Oakley Silver Supporting Member

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    Does the action change on the outside strings so that it's farther away toward the nut end? Or does this do something more like even it out, given that the action is higher at the higher end? But even in that case - how does the action on the outside strings relate to the inside strings?

    Or do I not get the compound radius at all? :confused:
     
  2. kodecar

    kodecar Member

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    I would say it really depends on how you have it set up. The string height will be set by the nut and bridge. You can tweak the action to reflect what you want.
     
  3. VaughnC

    VaughnC Supporting Member

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    I don't think you "get" what a compound fretboard radius actually is. Due to the fact that the strings are farther apart at their bridge end than their nut end, this means that the strings naturally form a conical profile over their length. So, a compound fretboard radius is an attempt to have the fretboard match the strings natural conical profile by also being conically shaped (having a larger radius at the body end of the fretboard that tapers to a smaller radius at its nut end). Theoretically, if the fretboard's conical radii matches the strings natural conical profile, then the action should be able to be as low as possible.
     
  4. TwoGeez

    TwoGeez Member

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    The change in the radius of a compound board (it flattens out towards the bridge, just in case you're confused) is relatively too small to have any affect on the overall action or setup. The action already naturally gets lower towards the nut anyways, and since a compound board typically starts out at a more standard radius, you're going to set it up in reference to that, not the opposite end where the board flattens out. Does that make sense?
     
  5. bluesjuke

    bluesjuke Disrespected Elder

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    A compound radius neck refers to a neck on which the radius changes as you go up the neck.
    It may start at a radius of 7.5" gradually changing to a radius of 10" in the upper frets.
    This allows for less fretting out/ buzzing in the upper registers.
     
  6. Oakley

    Oakley Silver Supporting Member

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    Thanks, now I get it. What I was overlooking was the way the strings fan out creating the conical aspect.

    What a great idea!

    Does it work? I mean, to improve the action.
     
  7. MartinC

    MartinC Member

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    I've got a Suhr Classic with a compound 10"-14" radius. I don't think it improves the action ... I'm assuming that by 'improves' you mean allows a lower action. I think it's more about improving playability. Down the nut end, where there's probably a tendency to do more chordal work, the smaller radius is more comfortable than a totally flat fretboard. Up the other end, the flatter radius makes playing single note runs easier, and reduces the tendency for strings to choke on higher frets when bending strings.

    I wasn't able to get a lower action on my Suhr than on my Fender Strat (9.5" radius). If I take the action lower on either guitar, then I get buzzing around the 5th fret on the A and D strings. Both guitars have been plekked by the way ... so I think I'm probably making a fair comparison.
     
  8. Janne.M

    Janne.M Member

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    But it solves the problem with string bending "fretting out".
    On my previous strat I could get the action as low as on my Suhr, but if did large bends above the 14'th fret the strings would choke. The only solution were to raise the action until the choking were gone, resulting in a higher action that I would like to have...

    So this is why I can get lower action with a compound radius fingerboard.

    Janne
     
  9. darkmonohue

    darkmonohue Member

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    The purpose is not solely to get lower action. The rounder radius toward the nut is supposed to make it easier to finger open-string chords, while the flatter radius around the 12th fret and higher works well for playing single-note lead lines. While the effect isn't dramatic, I definitely think the one conical-radius guitar I have is the easiest guitar to play I've ever tried.
     
  10. Dana Olsen

    Dana Olsen Gold Supporting Member

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    Vaughn, that's a great explanation.

    Let's archive this one - very clear.

    thanks, Dana O.
     
  11. kodecar

    kodecar Member

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    Not trying to be a jerk here or come off as a know-it-all but, it is not the difference in string spacing between the nut and the bridge that forms the conical profile but the radius of the nut and the height adjustment at the bridge. If the nut and bridge were set "flat", the strings would form a trapezoid. It is only the curvature imparted by the radii of the bridge and nut that form the radius of the strings themselves.
     
  12. David Collins

    David Collins Member

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    The geometry of the fretboard is often quite misunderstood, and can get pretty complicated mathematically. Also, many times people refer to a compound radius board as conical, and while it can be a perfect cone it is by no means bound to this. After a proper dressing, you can end up with more of a parabolic shape depending on the start and end radii.

    Here's one way to look at it. Take a paper towel roll and stretch a string from one end to the other, parallel with the tube. Obviously the string will follow along a straight line even with the tube. Now take one end of the string and move it around to a different point of the circumference at one end. Obviously you see that as you shift across planes of radius of an arc, you end up with a curve rather than a straight line.

    This is exactly what happens as you spread the string spacing out along a constant radius board. When the neck is straight in the center, it would by nature of geometry be in slight backbow under the outer strings.

    If you shape the board as a cone, with the change in radius directly proportional to the change in string spacing, then the line on the frets directly below each string would fall within the same radius plane. The surface of the frets would be perfectly straight below each string. This formula limits you to a very specific ratio of radii. Given the string spacing at the nut and at the bridge, once you decide on the nut radius, the end radius would be locked in by the formula. Change it steeper or shallower, and you go back to the same backbow on the outer strings situation.

    Thing is, there is really no reason that you have to follow the straight cone formula. In practice, you can start with any radius, end at any one you choose, then simply connect the two with a straight line beneath each string. Of course this means the frets must be leveled in the plane of each string as you move from one side to the other, and not parallel to the center of the neck. Frets can be leveled this way with a single radius board, or any combination of compound radius. Only the directly proportional change mentioned above would for a true straight cone though. Any other layout would result in some parabolic form

    This philosophy allows much more freedom to design the board around ergonomics and playability issues. Remember that unless you are bending, the string only sees a series of points in a straight line directly below it - it knows nothing of any radius. This leaves the ergonomics a more important influence to deciding a radius, both in forming chords with the left hand and picking with the right.

    When you bend, all rules change though. Bending the string means you are now crossing radius planes. Just like in the paper towel roll example, you now have to cross over the radius, and can buzz out with a steep radius. The closer you get the end, the more sharply you will be crossing the radius and the more buzzing will be an issue. So with a compound radius you can keep it steep and comfortable on the lower frets, and by the time you get up to where bending/buzzing can be an issue you can flatten in out a bit. Best of both worlds.
     
  13. VaughnC

    VaughnC Supporting Member

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    Well...we'll have to agree to disagree on this one ;).

    The height of the bridge, in and of itself, has nothing to do with the strings actual profile. All adjusting the bridge height does is change the angle between the strings and the fretboard. The strings profile itself is not changed no matter where you set the bridge height.

    All electric guitars I've played have had some radius at the nut and the bridge. If you start with equal string spacing (and equal radii) on both ends of the strings (cylindrical profile), then you increase the strings spacing at the bridge end, you then form a conical string profile as the string fan out. Same thing happens if you lessen the strings profile radius at the bridge. So, it's changing the string spacing at the bridge and/or a difference in radii that forms the strings conical profile...not the bridge height.

    I do agree however that a flat nut and a flat bridge would form a trapezoidal string profile...but in my 40+ years at this stuff I've yet to see an electric guitar like that, so its a very special case.
     
  14. kodecar

    kodecar Member

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    I think we are on the same page here. When I mentioned hight adjustment on the bridge, I was really referring to individual saddle height adjustment than the overall bridge height.

    Same here. I was only using this to illustrate a point rather than as a specific or actual example.
     
  15. Tone_Terrific

    Tone_Terrific Member

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    There is no downside to compound radii except a slightly more complicated building process that I can see. It really should be universal by now, even if the trad method is good enough.
     

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