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Thread: The Issue of Arrow Spine Using a Compound Bow

  1. #1
    timujin
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    The Issue of Arrow Spine Using a Compound Bow

    I know this is an issue that has raised its head before but I am still personally dissatisfied with explanations given to support one side of the argument or the other.

    I do understand the issue of why arrows bend in flight and the importance of selecting correctly spined arrows for a bow - other than a compound bow.

    It's this latter that has exercised my mind constantly over many years and still does.

    Here's what I don't understand. Firstly, let's now forget about archer's paradox and the necessity for correctly spined arrows to shoot correctly with recurves and longbows.

    I just wish to emphasise right from the outset that I am concerned only with the behaviour of arrows out of a compound bow, being shot over a properly set up arrow rest using a mechanical release aid. I'm emphasising this so that readers don't misunderstand my points. OK?

    Right - clearly one can never get an arrow that won't bend. Even an inch thick arrow would bend by a measurable amount, if you could shoot such a thing out of a bow. The bend may not be obvious but it would be there. So that is a given for all arrows.

    The issue for me is simply this. I have shot many different types of arrows over the years I have been involved in FITA style archery and the one thing that has imprinted itself clearly on my mind in relation to the behaviour of arrows out of a compound bow has been that the weaker spined arrow never seems to perform as well as a very stiff arrow - and by this I mean a stiff arrow that is way over the recommended spines for the bow weight, cam type, release aid etc.

    I've thought a lot about why this appears to be so and all I can conclude is this. With a compound bow, the things that appear to eliminate the "bad" things associated with archer's paradox are the mechanical release aid and the properly set up rest.

    The release aid, in particular the really good quality caliper releases, allows the bow string, with the nocked arrow, to travel forward with none of the sideways movement caused by using one's fingers for release, whatsoever.
    The arrow still bends but as far as I have been able to ascertain that bending occurs mainly in the vertical plane in a porpoising action.

    I have seen high speed videos of this which show some arrows with very little such bending at all. My theory is that the sprung arrow rest - regardless of type - acts like a shock absorber and tends to dampen out fairly quickly these vertical oscillations - not completely, because there isn't enough time for that - but to a certain extent. It has always seemed to me that on a compound bow, the thing to get right is the "springiness" of the arrow rest.

    If it is totally rigid, then the arrow will behave, in the vertical plane, just like an arrows does, when it travels around a traditional long bow. As you make the rest springier, the arrow does not need to bend as much to overcome the resistance of the rest and some of its bending is absorbed by the rest. This can be taken to excess, I think (?), to the point where the rest is providing no absorbtion of arrow oscillation at all and I'm not totally convinced (but I'm not sure) that this is a good thing.

    Even the drop away rest provides a minor degree of absorbtion for a little while. The rest that really makes me ponder about this is the Air Rest, which, by its very nature provides no cushioning whatsoever. Whether that is a good thing or not I can't say but I would love to see some high speed video of an arrow passing through such a rest and discover how well or otherwise arrows group out of such a rest.

    Anyway, it is my belief that the sprung rest acts to provide a degree of absorbtion of the natural oscillations of the arrow caused by the thrust of the string onto its nock. Those oscillations are going to occur regardless of how stiff the arrow is, because that is how long cylindrical objects behave when given a hard and sudden kick at one end.

    Now, in a recurve or longbow, which do not have centreshot cutouts in them, it is not only desirable for the arrow to bend in the horizontal plane around the bow structure itself, it is essential that it does so, in a controlled manner so that the arrow won't go flying off to the left, but will, instead, bend around the bow and eventually straighten up along a path which will hopefully carry it on its way to the desired target. so the bending of the arrow with this type of bow is absolutely essential for good operation, particularly when a finger release is used.

    But, in the case of the compound, there is no "need" for the arrow to bend so that it can get around an obstruction and continue on its way to the target. The bending that occurs will result from the natural action of a long cylindrical object being given a hard sudden shove at one end. How much it will bend will depend on the stiffness of the arrow itself.

    Given that the mechanical release aid will, under most circumstances, release the string with the nocked arrow attached, without imparting any unwanted movement to the string and thereby transmitting that to the arrow AND given that the arrow rest, if it has been properly set up, will do nothing more than provide a degree of shock absorbing support for the moving, bending arrow, the question still remains - other than the normal bending that is going to take place in the arrow, why would anyone choose an arrow that has more bend in it than another that has less, to shoot out of a compound bow?
    As I picture the whole process in my mind I cannot help but come to the conclusion that the more rigid arrow has to be a better performer out of a compound bow because the bending is dampened out far more quickly in a stiff arrow and the arrow is able to continue on its way with the minimum loss of energy and speed caused by the loss of energy through bending and the increased drag that this causes.


    I don't see how a bendier arrow is going to be more "forgiving" of a poor release (in fact, using a release aid, I find it hard to envision how a poor release occurs). It seems to me to be more logical to choose the stiffer arrow for the reasons mentioned above and practise perfecting one's form so that induced errors are minimised.

    So, I'm yet to be convinced that a compound bow should NOT use the stiffest arrow available as opposed to one which has a more "correct" spine (ie a bendier arrow) for that bow weight. Having said that, I have to state that in all honesty, I really do not understand this issue of "correct" spine in a compound bow at all.

    I currently shoot the stiffest X10s that are made out of my compound and they perform beautifully. They are about two spines stiffer than the charts recommend but that doesn't seem to affect them adversely at all. Quite the contrary. So if I was ever asked for advice as to which spine to go for I would always say go for the stiffest made - only because it seems to be the logical thing to do. But I could be hopelessly wrong in this. I really couldn't explain to anyone in a scientifically accurate manner, what the "correct" behaviour for an arrow being shot from a compound bow ought to be and whether there is a "correct" degree of bendability for such an arrow.

    I have heard people say that arrows which have the "correct" spine are more "forgiveable" or "more easily tuned", but I really have no idea of what these expressions really mean or whether they are true or not.

    If it is at all possible I really would appreciate it if someone out there could explain this whole business to me in an understandable, and logical way, because it drives me around the bend not knowing.

    Please help!

  2. #2
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    Hi Tom,
    I'm not sure I can help to set your mind at rest, but perhaps I can add a few ideas to help in some small way. We see the arrow rest slightly differently I feel. You see it as absorbing some of the vibrations or bends. I can see that too but I think there is also an element of going with the bends while maintaining some contact or even control. If it was rigid and a downward bend went over that rest, the arrow would bounce off I think. The flex is there to "go with the flow" so to speak. On my golden key I could use it stiff or loose or as drop away. The groups moved left /right when the changes were made. This I think indicates some sort of control over the arrow. I don't shoot well enough to know for sure, but I believe the grouping was better with some settings ( stiffer If I remember correctly)
    As you make the rest springier, the arrow does not need to bend as much to overcome the resistance of the rest and some of its bending is absorbed by the rest.
    The above is the part where I get a bit confused ,Tom. I'm not saying you are wrong, but my thinking about that would be that the arrow will bend as much as it wants to bend regardless of the springiness. If it is springy the rest will move down as the bend goes past. If it is rigid it will push the arrow upwards if there is a downward bend.
    I have contacted manufacturers of rests as there are two in particular that go down different routes. One drops as soon as posible the other drops as late as possible, just before the fletchings arrive. I heard back from only one so the reasoning is not complete.

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    Hi Tom,
    I have just had another thought that may help.
    I think I'm right, that shooting an arrow through a small hole in a rigid sheet(steel) would be difficult even with a shooting machine. The arrow will not pass through cleanly because of the bends. They will strike the edges of the hole and bring the arrow to rest, probably in pieces. The air rest is a case in point. It supports the arrow, but from a distance, so the flexing shaft can carry on through unhindered. A really weak arrow would probably hit the air rest as the bends would be too large for the air rest to accommodate. Stiffer arrows will bend less and as a result can use a rest that is less likely to be pushed beyond its limits. Even compound arrows will have some slight bends to left and right I feel. The rest is not set up for left /right bends so the smaller these are the better.

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    It's an X Rik's Avatar
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    My understanding of the situation is that the main cause of arrow bending on a recurve, is the archer's fingers...

    On a compound/release you don't have that. The sources of bending motions are much smaller and come from the mechnical properties of the bow. That is to say, whether the setup is such that the string is pushing directly behind the shaft or not.

    With D-loops and properly designed bows, even those forces are being minimized. I suspect that we are close to a situation where a properly setup bow will only minimally bend an arrow shaft. Note the caveat. If the bow setup is wrong...

    Historically, I was told that the main sources of bending on a compound setup were in the vertical plane, but that was back when the majority of archers used a release attached to the string, under the shaft. This might explain the source of 'springy' rests - to take out that little bit of vertical bend.

    I'm not a compound guru though, so I'd be interested to see what the experts say...

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    Magnok people do a video to show how little bend there is in an arrow after the string/arrow is fitted with magnoks. I know it is an advertising video but it does show how little the arrows bend. Even the non magnok arrow bends very little.
    As timujin says, there seems to be no reason to want bendy arrows.

  6. #6
    timujin
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    Geoff and Rik

    Many thanks for your thoughts on this matter. For Geoff - in relation to what I wrote and what puzzled you eg:

    As you make the rest springier, the arrow does not need to bend as much to overcome the resistance of the rest and some of its bending is absorbed by the rest.
    That was badly expressed. What I really meant to say was that the rest , by virtue of the fact that it has some built in springiness, is able to dampen out some of the oscillations of the arrow, so my thought was that a properly set up rest was perhaps the more important item in this equation.

    I actually feel a little bit better about this issue now - not because I am any the wiser about the science involved but by the fact that Vittorio Frangilli posted this in answer to my queries on the Sagittarius site:

    From Vittorio Frangilli


    Intersting topic... I remember some discussion already done about same subject but not were and when..
    Anyhow, my bit of basic available infos are:
    - Bob Ragsdale web site ... "there are no arrows stiff enough for a compound bow" (or a very similar sentence ..)
    - All top compound men using X10 are using 410 or 450 sizes heavily cutted on the back (up to 2 inches and even more) and they do not use 380 because "too heavy".

    While it is not completely true that the arrow oscillates on the vertical plane, only (many high speed videos are showing also horizontal oscillation), surely compound is accepting a very wide range of spines. Basically, the maximum of the force is applied to a short segment of the arrow, so any kind of bending tends to happen on the second half of the arrow, while it is travelling out of the bow. This increases the tolerance of the acceptable spine, and also increses the need to have arrows stiffer at the end (cutted on back), opposite requirement from the recurve bow. The dynamic of the power stroke also changes the way an heavy point influences the grouping. It does, of course, but only in relationship to the flight of the arrow and the downrange part of it, not so much in terms of dynamic spine at the beginning of the flight.
    Same situation for the size of the vanes. Compound arrows need smaller vanes than recurve arrows to stabilize. Usually Compound archers use vanes bigger than they need, and recurve archers smaller than they should use.. (sentence by Don Rabska, not by me).

    Summarizing, ideal arrow for compound should be as stiff as possible at least in its second half, have small vanes and then, as for recurve, have an heavy point and be as light as possible...
    Only problem, present arrows aren't stiff enough for modern compound bows, so proper arrows for 60 pounds and 30" does not exist, yet.
    Fascinating commentary. and Ragsdale himself no less, made the comment that "there is no arrow stiff enough for a compound bow"

    This is what I have always felt intuitively but am unable to say why, because I don't understand the science behind it, except that it seems to me that a compound bow treats the arrow pretty much the way a rilfe treats a projectile and why highly accurate rifles have very large diameter, stiff barrels.

    I then went to the Easton 2006 shaft selector and noticed something I hadn't noticed earlier. If I wish to shoot a 60lb compound bow with a draw length of 29 inches I can't get a stiff enough X10 if I choose to use 110 grn points and if my draw length is 30 inches, I can't get a stiff enough X10 even with the lowest weight point.

    So to get the X10 that suits my set up with a 29 inch draw weight I have to reduce my draw weight to 57lbs or use the lightest points. What I intend to do with my X10s is to cut them down to the bare minimum to enable them to sit safely on the launcher and this should stiffen them up considerably.

    What started my thoughts along these paths was the fact that my new X10s are way too long for my rest set up but being the stiffest available, I didn't want to touch them without knowing a bit more about the issue and now I feel confident enough to go ahead and trim them as I desire (within the laid down limits of course)

    I'd still like to understand the scince of this issue though.

    Thanks for your advice Geoff and Rik. Much appreciated.

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