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Thread: Top strength/Tuning stability

  1. #1

    Default Top strength/Tuning stability

    I've made myself a little flat top, and I reckon I've got a problem with the top flexing too much. It's still early days, but it needs to go through the strings twice to get them back in tune from overnight, and it seems to require more movement on the keys than I expect. The construction is unconventional, but I'm pretty secure about the neck and body, which involves a lot of carbon fibre, but less so about the top. It's spruce with a small brace under the bridge right across the body, and due to a disaster with stains I sanded it down, so its relatively thin.

    What principally disturbs me is that the pitch changes if I press down with a single finger on the bridge. Im away from home and haven't anything else to compare. It wouldn't be a disaster to put a replacement top with more bracing on, but equally I'd like to be sure I'm looking at the correct problem. Any thoughts?
    Last edited by JimCh; Sep-13-2020 at 6:57am.

  2. #2

    Default Re: Top strength/Tuning stability

    I'd give it some time before deciding. It takes a week or two for a new build to settle to its "working" shape.

    I'd guess your main risk is that it settles into a dip on the top (might still sound good though). I'm building a tenor guitar which will have light ladder bracing, and I'll arch the top from side to side, to add stiffness and help it survive humidity changes.

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  4. #3
    Registered User John Kelly's Avatar
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    Default Re: Top strength/Tuning stability

    If you have only a single transverse brace across the top under the bridge then it may well be that you will have to add further bracing, especially if you have sanded the top down after the staining mishap. Carved tops with their longitudinal brace rely mainly on their carving to give them the strength required to counteract the string tension, and as ProfChris says, arching the flat top can give some of this strength, but flats tops need more than just one brace. Wait a few days and see how it settles, but I would say you need to be ready to get the top off and add braces, either cross or ladder, and add the slight curve into your bracing profile.
    I'm playing all the right notes, but not necessarily in the right order. - Eric Morecambe

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  6. #4

    Default Re: Top strength/Tuning stability

    Thanks folks. I'll see how it settles down then. It has got a gentle arch to the top, but only two or three mm.

    In my head I can imagine having a tiny dart in the centreline join to give a bit of 3d shape to a flat top, but the practical issues of getting a first class join seem almost insuperable without extraordinarily complex tooling.

  7. #5

    Default Re: Top strength/Tuning stability

    In my head I can imagine having a tiny dart in the centreline join to give a bit of 3d shape to a flat top, but the practical issues of getting a first class join seem almost insuperable without extraordinarily complex tooling.
    An arch or domed shape, with smooth contours, is always stronger than a peaked or pointed shape with the same height, if that is what you mean by a dart. Roofs are built with peaks instead of domes or arches for convenience of building. Bridges are built with arches because the need for strength overrides the building difficulties.

    I have done a lot of engineering analysis and computerized structural modeling on thin walled structures with both internal and external loads. The structural properties of domes and arches have been known for several thousand years and I have had to confirm that with the structural models a few times when people did not want to believe it.

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  9. #6
    Mandolin & Mandola maker
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    Default Re: Top strength/Tuning stability

    The old Gibson army navy flattops only have two cross braces on the top. One is under the bridge and the other is in front of the sound hole under the fingerboard. So, that one brace under the bridge is ok, but it would be advisable to only use light strings. Medium strings are too heavy, those old Gibsons are notorious for sunken tops, but heck, they are 100 years old.
    Peter Coombe - mandolins, mandolas and guitars
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  10. #7

    Default Re: Top strength/Tuning stability

    Quote Originally Posted by JimCh View Post
    Thanks folks. I'll see how it settles down then. It has got a gentle arch to the top, but only two or three mm.

    In my head I can imagine having a tiny dart in the centreline join to give a bit of 3d shape to a flat top, but the practical issues of getting a first class join seem almost insuperable without extraordinarily complex tooling.
    It's easier than that! Just glue the plates together flat, and then shape the braces (my tenor guitar will have two on the lower bout, one above the sound hole) so they are straight on the underside, but on the face which glues to the top are curved a little more than you desire once the body is assembled. Your soundboard will happily bend to conform to those curves. When you put strings on, it will sink a little.

    So for my tenor guitar which will be around 300mm wide at the lower bout, I want to achieve an arch of 4-6mm. I'll shape the braces to an 8mm arch, and that will get me down to around 6mm under string tension.

  11. #8

    Default Re: Top strength/Tuning stability

    Well, update. After an interval spent mainly making furniture (don't ask) I was able to get back to it. The instrument certainly stabilised, and stays in tune, but there was still a vague soggy feeling to the tuning... So today I was tidying up the bridge, thinning it down, and adding holes as per the Red Henry recommendations. But while I was at it I put a little wedge between the brace and the centre block, which means the brace is now rigidly supported in the centre. Think violin sound post, but instead of connecting to the active back of the instrument it's rigidly supported.
    What a transformation!
    It was first immediately obvious that the tuning had changed dramatically, and the strings were changing pitch as one would expect without the 'soggy' feeling.
    The second obvious change was that the instrument was far brighter in tone. Except that I'm not sure it is brighter so much as lost the bottom end. Obviously the top must be vibrating utterly differently, as one would expect. I'm not at all sure that I prefer it. Well, live with it for a while and see. Wouldn't be hard to pull the wedge out, I deliberately didn't glue it.

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