Determining guitar balance through simple visualisation

Discussion in 'Luthiery, Modifications & Customizations' started by ixlramp, Dec 8, 2018 at 8:37 PM.

  1. ixlramp

    ixlramp SS.org Regular

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    I worked through the mechanics of the forces and torques and have derived a simple way to consider and perfect guitar balance through visualisation.

    This is very useful to know as a large majority of guitars have poor balance, even at fairly horizontal playing angles, and balance that becomes rapidly worse at higher playing angles. On an existing guitar balance can often be perfected by moving the strap buttons.

    The intention here is to equalise the forces pulling on each end of the strap while the guitar is at a particular playing angle, so that there is zero tendency for the instrument to rotate. This stops the strap pulling forward on your shoulder, which is unhealthy, with only friction stopping the guitar neckdiving.

    When designing a guitar this knowledge is very helpful to create a design with good balance, it's also possible to create a design that balances perfectly over a wide range of playing angles.

    First, find the 'centre of mass' (COM)
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    This is often called the 'centre of gravity'.
    In mechanics it is very often possible to simplify analysis by considering all the mass of an object to be concentrated at one point, the COM.

    The method used is balancing the guitar on and above one hand by pinching the lower body edge between thumb and first finger, in a way that allows the guitar to pivot at that point (pivot point).
    This hand is probably best resting on your knee to avoid fatigue.
    The other hand shifts and stabilises the guitar until it balances, at which point it is barely touching the guitar.

    1. Balance the guitar on your hand with the guitar perfectly vertical, and shift the pivot point left and right until the guitar balances vertically without falling left or right.
    Now we know the COM is somewere directly above the pivot point, you could mark the pivot point or just remember where on the instrument the vertical line above the pivot point is. It will often be near the centreline of the guitar:

    COMvert.png
    2. Now do the same with the guitar perfectly horizontal.

    3. Where the 2 lines cross is the position of the COM:

    COMhori.png
    (Continued below)
     
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  2. ixlramp

    ixlramp SS.org Regular

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    Balance visualisation
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    1. With the guitar at the playing angle you want to analyse, visualise 2 vertical lines passing through the strap button points B1, B2.
    2. Visualise a horizontal line passing through the COM point and visualise the points E1, E2, where it crosses the vertical lines.

    RGbalance_points.png
    3. The magnitudes of the forces pulling on the strap ends will have a ratio equal to the ratio of the distances E1 - COM and E2 - COM.
    The shorter the E - COM distance is, the larger the force is on that end of the strap.
    If the distances E1 - COM and E2 - COM are equal the forces will be equal and the guitar perfectly balanced.

    Above you can see the distance E2 - COM is roughly twice the distance E1 - COM, so the force on the horn end of the strap will be roughly twice that of the force on the rear end of the strap.
    (Note i have probably incorrectly guessed the COM point for this Ibanez RG, the diagram is meant to be general not specific to this guitar.)
     
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  3. crackout

    crackout SS.org Regular

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    I would not call this 'perfectly balanced' in the understanding of a nicely hanging guitar.
    If both forces are equal, you will have no 'restoring force' into the desired position, the guitar will always stay the way you put it. While technically speaking this is well-balanced, at least to me in addition to a nice hanging position a guitar should also have a tendency to move there.
     

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