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  1. #35
    Administrator kwun's Avatar
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    Originally posted by bigredlemon
    You said it was a real life comparison, when in fact it was a lab comparison. In the real world, you only experience dynamic stiffness.

    Dynamic stiffness can be tested very cheaply. A small weight, pen, and paper is all you need. Since dynamic stiffness (not static stiffness) is the only number that matters and can be very cheaply measured, I don't see why it's unreasonable to ask for it.
    i see. so are you volunteering?

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    Originally posted by cooler
    i see i see However, just for completeness, the rod example u have highlighted is straight forward, linearity. For an objects with irregular shape, moment of inertia would still be different even when these irregular shaped and density profile object have similar CG location and weigh similarly. If we assume the rackets to the ideal case of the rod (linear system), then the cg * mass comparison is pretty straight forward. It was this issue of irregular frame shape that held me back of not doing the moment calculation

    Before i make those calculation, please note the limitation of calculated moment of inertia ...
    understood. but the static moment is closer an indicator than just balance point alone. if we have equipment limitations that disallow us to get to the true value, we just have to settle for the closest approximation within our constraints.

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    Originally posted by cooler
    i see i see However, just for completeness, the rod example u have highlighted is straight forward, linearity. For an objects with irregular shape, moment of inertia would still be different even when these irregular shaped and density profile object have similar CG location and weigh similarly. If we assume the rackets to the ideal case of the rod (linear system), then the cg * mass comparison is pretty straight forward. It was this issue of irregular frame shape that held me back of not doing the moment calculation
    Irregular shapes can be approximated to a rod if there is no angular rotation along the axis perpendicular to the rod. This is true for all strokes except a smash, in which there is a small 90 degree rotation that is negligible. Thus it's perfectly fine to approximate a racquet as a linear rod of non-uniform density. If you presume the hitter always hits in the same spot on the string bed, you can further approximate it as a point-mass or ball at some distance from the axis of rotation.

    With either approximation, MOI is easy to calculate.

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    brl, obviously you had taken my words out of context. I could have reported stiffness in term of deflection per unit length of racket shaft. Real life meant stiffness in relation to a known or common reference point like a cab 20, in my case i didnt have a cab 20 so i used cab20M. If you are so adamant about knowing real life dynamic, just ignore this thread and go read the racket review in the BC section. I can also tell you how to experience real life dynamic stiffness but i doubt kwun would let me elaborate in BF


    Dynamic stiffness can be tested very cheaply. A small weight, pen, and paper is all you need. Since dynamic stiffness (not static stiffness) is the only number that matters and can be very cheaply measured, I don't see why it's unreasonable to ask for it.

    Well, let see you run some dynamic stiffness test on your rackets. I don't see why it's unreasonable for me to ask for it.

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    Originally posted by bigredlemon
    Irregular shapes can be approximated to a rod if there is no angular rotation along the axis perpendicular to the rod. This is true for all strokes except a smash, in which there is a small 90 degree rotation that is negligible. Thus it's perfectly fine to approximate a racquet as a linear rod of non-uniform density. If you presume the hitter always hits in the same spot on the string bed, you can further approximate it as a point-mass or ball at some distance from the axis of rotation.

    With either approximation, MOI is easy to calculate.
    kwun, brl, i agreed.
    I just want people here to understand the limitation of calculated MOI although it's very small in this case.

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    if i'm going to systematically report real world stiffness, i'd be sure to do that.

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    Originally posted by cooler
    I can also tell you how to experience real life dynamic stiffness but i doubt kwun would let me elaborate in BF
    Don't take comments on the limitations of your measurements too personally.

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    Default Part 3 - Relative Moment

    This is part three of the 2003 (yonex) racket measurment focused on moment or the twisting force needed to put the racket into motion from the rest position. It doesnít reflect perfectly the amount of totally energy to fully swing the racket but the relationship between moment to angular momentum should be reasonably close.

    Again, the result presented is referenced to the 3U cab20m. Less than 1 mean the racket take less effort to move it than a 3U cab 20m. Relative moment greater than 1 means it would take more effort to move it than a 3U cab20m.


    Iím sure BF members could draw many conclusion from these data.
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    Last edited by cooler; 12-02-2003 at 12:59 AM.

  9. #43
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    Originally posted by cooler
    The old fashion way, I bend them
    I'm sure several of us would still like to know how you measured the stiffness.
    i.e. what equipment etc. If you clamped the handle in something did you clamp it right at the butt, in the middle, by the cone. Some of us just need to know

    Also, when you normalise results for different length racquets, are you using the total length of the racquet, or just the length of the shaft, or shaft+head ?

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    the definition of relative movement is vague. How did you measure this?

    Since there is no static force against angular movement, the value for every racquet should be about 0. (1/infinity to be exact.)

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    Originally posted by bigredlemon
    the definition of relative movement is vague. How did you measure this?

    Since there is no static force against angular movement, the value for every racquet should be about 0. (1/infinity to be exact.)
    It's obvious you are too quick trying to be overly precise and critical with my data while you haven't understood the objective of the matter.

    The (Part 3) data presented relates to relative moment, NOT relative movement. How it was calculated? check kwun post not too far up from here.

    http://www.badmintonforum.com/vb/sho...0&pagenumber=4

    Moment does not necessarily has to be associated with movement or motion. For example, it's like Arnold Schwarzenegger twisting your arm behind your back. You feel pain but you ain't moving. Of course Arnold can apply lock-arm force + some additional moment (N-m) and you would be flying somersaut and experiencing lots of angular movement or momentum. (hasta la vista, baby)
    Last edited by cooler; 12-02-2003 at 12:15 PM.

  12. #46
    Administrator kwun's Avatar
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    agree with cooler. there can be lots of interacting forces / moments in a static system.

    BRL, time to go dig out those high school textbooks...

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    Originally posted by Neil Nicholls
    I'm sure several of us would still like to know how you measured the stiffness.
    i.e. what equipment etc. If you clamped the handle in something did you clamp it right at the butt, in the middle, by the cone. Some of us just need to know

    Also, when you normalise results for different length racquets, are you using the total length of the racquet, or just the length of the shaft, or shaft+head ?
    Neil, I clamped the handle, not the cone because some shaft flexing does occur inside the cone. I normalized it to each racket total length.

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    eck... i misread that word.

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    Make my day and add a Ti-10 Long to your tests...

  16. #50
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    Originally posted by Framerate
    Make my day and add a Ti-10 Long to your tests...
    I don't have a ti-10 with me now. Maybe later when someone let me borrow his/her ti-10. Ahhhh, come to think of it now, there is one person near new condition too

  17. #51
    Administrator kwun's Avatar
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    need to make a note. next time when i see cooler (whenever that might be...) i should donate him my broken rackets for testing. that'd include my Iso900 SX, SS(CN,SP), Ti-10, Aerotus 77/110...

    it is a good cause, afterall.

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