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Thread: Flat tops, break angle and action height

  1. #1

    Default Flat tops, break angle and action height

    My recent Mexican flat top A, no great wonder of an instrument, but good tweaking material, has a tiny bridge, like a bowl back, a fretboard attached to the body, and a very small break angle as a result.
    There is essentially no way to lower the action by lowering the bridge; the downforce is already minimal. I had a go at warping the entire body - wet, clamps, internal jack, but no effect.
    Given that a neck reset is very difficult, is it more reasonable to detach the fretboard and put a wedge under, as sometimes done on violins?
    The general question is that of very small break angle regardless and low downforce at the bridge - permissible or just historically bad design?

  2. #2
    Registered User
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    Default Re: Flat tops, break angle and action height

    I don't think detaching the fretboard and wedging it would work. You would be creating an extreme amount of relief. By the time you have removed the fretboard from the body you have done most of the worst of a neck reset. It may be possible to fit it with a longer tailpiece that would create more break angle.

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  4. #3

    Default Re: Flat tops, break angle and action height

    Quote Originally Posted by Nevin View Post
    I don't think detaching the fretboard and wedging it would work. You would be creating an extreme amount of relief. By the time you have removed the fretboard from the body you have done most of the worst of a neck reset. It may be possible to fit it with a longer tailpiece that would create more break angle.
    I like the extended tailpiece idea. It would have to be mechanically fairly strong to take the torque upwards, but that’s doable. Or even just an add-on structure leaving the string attachment at the back corner, but pressing down aft of the bridge, say halfway. I’ve seen something similar on old mandolins, but just a chunk of wood screwed or glued to the top, which is not that strong right there. I thought it was just a damper, but maybe it served another purpose.

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    Registered User sunburst's Avatar
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    Default Re: Flat tops, break angle and action height

    Is the bridge unstable and does it move on the top when playing the mandolin?

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  7. #5

    Default Re: Flat tops, break angle and action height

    John, not unstable, and good contact, but can be pushed out of position easily. It’s maybe 3/8” high. If I lower the action it would need to be around 1/4”, as a rough guess. Looks like factory original, all wood. It sits above a substantial transverse brace, so fully supported. If I go in this direction, I’d make a temporary test bridge to work things out.
    I’m thinking this geometry comes from guitars, where the bridge is actually attached and can be barely above the fretboard thickness, and just mistranslated to mandolin.

  8. #6
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Flat tops, break angle and action height

    This is the fundamental problem with flat-top mandolins. The geometry of a 'flat', even arched over transverse bracing, soundboard just does not work with a standard 3/16"/5mm fretboard and any common tailpiece. A flat-top soundboard can work if there is a decent longitudinal arch as well as the transverse arch. A 15' longitudinal arch will give enough break angle over a bridge to drive the soundboard without too much down-bearing pressure and the resulting collapse. It is a delicate balance of forces but it can work well.

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  10. #7
    Mandolin & Mandola maker
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    Default Re: Flat tops, break angle and action height

    Iím thinking this geometry comes from guitars, where the bridge is actually attached and can be barely above the fretboard thickness, and just mistranslated to mandolin.
    The geometry of flat top mandolins does come from flat top guitars, and as Graham says, the top needs to be arched longitudinally (as well as transverse), and a 15ft arch is about right to give you sufficient bridge height to drive the top. That is assuming the fingerboard is not elevated and is glued to the top similar to guitars. Guitar tops usually have arching of about 25ft and the distance of the strings from the top at the saddle is usually 12-14mm. I aim for 12-14mm on my flat top mandolins, but have gone as high as 16mm. With the longitudinal arch it works well, but any tailpiece that reduces the break angle is to be avoided. 1/4inch (6mm) is way too low. What is needed is a neck reset and a wedge under the fingerboard where it is glued to the top.
    Peter Coombe - mandolins, mandolas and guitars
    http://www.petercoombe.com

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  12. #8
    Registered User DavidKOS's Avatar
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    Default Re: Flat tops, break angle and action height

    Is it possible to post a picture please?

    Some good ideas so far....

  13. #9

    Default Re: Flat tops, break angle and action height

    So this is what I did. Taking a cue from the antique bowl-backs that have a ‘tensioner’, I added one about halfway between the bridge and tail, which allowed a lower bridge and action. These components are rough pieces, but may be permanent. Since, on this mandolin, there’s one large transverse support directly under the bridge, and nothing behind, I added a short beam inside that bears on that and goes aft to the tail block, no glue. The screw on the tensioner bears against this, so there’s no worry about ripping the top apart, and it’s adjustable. Sorry, didn’t take an internal photo before it went together.
    Of course, this is a $30, flimsy south of the border machine, so liberties can be taken, but maybe the idea could salvage other instruments. Click image for larger version. 

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