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Thread: Power stroke from release to launch.

  1. #1
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    Power stroke from release to launch.

    One aspect of the power stroke that I know I don't understand but would like to, is the part where the string goes tight or straight; somewhere around brace height I suspect.
    Quite often the limbs seem to wobble at the end; sometimes that is quite violent and noisy and we try to correct it with brace height changes etc.
    If I could show a really slomo version of this, I think we would see one limb carrying on a bit further than the other, and then rebounding, letting that other limb go forwards, and both end up see sawing for a while.
    I have read that the limb that carries on further still has more stored energy in it than the other. I can understand that up to a point. With more energy still in it, that limb will over power the other when the string goes tight. The energetic limb will carry on and work on the weaker limb; pulling that limb back slightly. We can mimic this with the bow braced, by pulling one limb tip back and seeing the other limb move the other way.
    The bit I don't understand is why the more energetic limb has more energy still it and how that energy gets there. Put another way, when the limbs move forwards on the power stroke, they will both start losing energy(?? )and gaining speed. Or are they gaining energy by moving faster??
    At full draw there is a point where the forces are in equilibrium and things settle to a sort of "at rest" state; bow balancing archer's draw force. Are the two limbs storing equal amounts of energy at that stage?
    I understand that on release, things change and the arrow has to be moved, and that takes energy.

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    Well, no one else has answered so I will try. But on the basis that, while I am an engineer so think I understand the basic physics, I don't really know how bows work!

    The way I see it is that when you draw back you are putting energy into the limbs (so potential energy). Each will store roughly the same but how balanced they are will probably depend on quite a lot of factors - the limbs, tiller, nock point, that sort of stuff? I suspect they are never truly balanced.

    On release, the key thing to remember is energy is always being lost from the system, never added (until you pull the string back again). So you are talking about transferring energy elsewhere, from stored (potential) energy to movement (kinetic) energy or even heat. So either to the arrow or to air movement, noise, etc.
    When you release that energy is converted to movement. ie. moves the limbs and string and arrow forward. By the time it gets to the "at rest" position we would like all the all the energy to have been transferred to movement in the arrow (so the arrow goes as fast as possible) but it will not be, some will be left in the limbs as they continue to move forward. So the limbs oscillate until they have dissipated that energy and are at rest again.
    I assume this is why people tune their brace height for minimum noise, that being where minimum energy is lost in oscillation and so max energy is transferred to the arrow?

    So to answer you question: I think there are likely to be a number of factors. eg:
    - Tiller, nocking point, how you draw the bow, manufacturer of the limbs etc will mean that there is not going to be an equal amount of energy in each limb at full draw.
    - position of the nocking point,how it is released, maybe even arrow flex and probably other factors will mean that the energy is not transferred evenly from each limb to the arrow
    End result, when the bow reaches the "at rest" position one limb has more energy than the other, causing them to oscillate unevenly, pulling against each other.
    Again, I suspect that if you could tune the bow (and your form?) to minimise this imbalance then you would maximise the arrow speed but I am speculating here!

    Am I making any sense?

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    Hi Brman, Thanks a lot for posting, Yes, you are making very clear sense.
    Can I just add a few ideas that you might be able to help me with?
    If we use a crossbow instead of a recurve or longbow; the symmetry will allow almost perfect balance between the two limbs and the string, and the arrow will be almost on dead centre of the system, too.
    When fully drawn and locked back at the trigger mechanism, I am assuming the energy in each limb is very similar to each other. Tension in each part of the string is equal, the system is in balance,( forces at a point??? I seem to remember from school,)
    With the ordinary bow, the asymmetry changes the forces acting around the nocking point at full draw. But are they not in a sort of balance? Equilibrium? I don't mean all forces are the same size, they are at slightly different angles( like a badly fletched arrow) and slightly different size forces; but they cancel each other to create stillness of sorts at the draw fingers.
    On release, those differences work to a pattern, and the hope is that the nocking point will be moved forwards in an almost straight line, horizontally.( ignore the string waving side to side.)
    Here are some details that I believe are correct. The bow hand, being nearer the bottom limb, makes that limb bend more easily than it would if the hand was on centre. The draw hand is almost on centre so the top limb bends less than the bottom one. Experience shows us that a stiffer bottom limb makes a bow that shoots better. The slightly stiffer bottom limb, is now not so easily bent as it would have been. At full draw, the bends in each limb look about equal. If the bottom limb had been equally stiff we may have noticed an extra bend in that bottom limb.
    Is there a simple formula that says how much energy is stored in a spring, when it is bent by a certain amount?
    Could we make a rough guess at what the numbers might be? they only need to be rough so we can see that the bottom one stores more energy; or stores the same if that is the likely case.

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    It's an X Del the Cat's Avatar
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    Wot brman said....
    Potential energy becomes kinetic energy... the chances of everything being perfectly balanced are zero.
    All systems oscillate...
    If you factor in the geometry of small angles (different for each limb).... the difference in string length between a 6" brace and a 7" inch brace is very small, less than the amount of stretch as the limbs slam home... thus the limbs can easily overshoot by that inch and oscillate, the string will also oscillate.
    You have two stable states... full draw and brace...going from one to t'other will always give rise to some oscillation, exacerbated by the search for ever increasing arrow speeds which leave more energy in bow.
    Try shooting a 1/2" diameter 32" Oak shaft with a forged head... that'll dampen it down a tad
    Del
    Health Warning:- These posts may contain traces of nut.

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    I don't know if I'm right, but I have always seen it as a timing/geometry problem in that one limb reaches its 'at rest' position before the other limb and hence overshoots it slightly. Thus when the string straightens and the system comes to rest one limb is slightly past it's at rest position and the other hasn't quite reached its. The system then vibrates as it stabilizes.

    So, if you can adjust the limb preloads so that the two limbs arrive at their equilibrium point at the same time then that will minimise vibration? Well, maybe not as to get that to happen maybe one limb would need to move a little faster than the other and then the momentum of the two limbs would be different and that in itself would cause vibration as it all equalised.

    So I think that all you can reasonably do is to experimentally adjust the tiller until vibration is minimised.

    But maybe I'm completely wrong?
    Where are my dancing penguins? I was promised penguins ............

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    Thanks Del and dvd8n,
    I understand that there is no perfect "stop" with no oscillation, but we do try to reduce the really bad ones.
    If I alter the preload on one limb; then I also alter the preload on the other. That is not the same as having one limb deliberately made stiffer than its partner, is it?
    If we alter preload at the bottom for example, that alters the angles of the limbs to the string. Is that new difference in string angles part of why the bow reacts differently? Or is it the nocking point being moved down( or adjusted back to compensate) or both?

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