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Main Boards => PowWow => Topic started by: Ssamac on February 23, 2009, 11:57:00 PM
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Can anyone (or everyone) provide a good definition of "stacking", when/how it happens, and problems encountered?
Thanks
sam
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Stacking: Disproportionate increase in bow weight during the last few inches of the draw.
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Joe,Havnt herd it put like that.Thats a awesome definition.
Mike
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It occurs as the bowstring approachs 90 degrees to the limb during the draw. The mechanical advantage is lost and the bow begins too gain more weight per inch of draw then before. Sometimes a lot more.
Mike
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What Mike said. Where the string angle at the tips approaches 90 degrees is determined by length, profile and tiller of the bow. Extreme examples:
1. 54" horsebow with extreme siyahs that take up nearly half the limb - the angle at the tips never comes close to 90 and there's no stacking out to at least 32"
2. 66" Hill style bow with too much taper in the limbs (imo most of 'em). String angle at the tips approaches 90 degrees and it starts to stack somewhere around 27".
A longbow will generally start to stack at an earlier point in the draw than a recurve of similar length. It's all about the mechanical advantage Mike brought up and simple geometry.
Note: "stacking" is often confused with "finger pinch" which is the string angle at your fingers. Finger pinch is determined by bow length. I can't comfortable shoot a bow under 62" for any length of time (30" draw and sensitive fingers ;) ) regardless of whether it stacks my my draw length or not. Finger pinch is subjective - stacking is measurable.
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Now I understand. So a well built and tillered bow should keep drawing evenly to at least 90 degrees string to limb or beyond?
sam
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None of them will draw evenly beyond 90 degrees angle. How "even" will depend on many design factors. Most well designed bows will store more energy (pounds) per inch of draw early than late in the draw. Studying some FD curves helps a lot in understanding this.
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I had a K-Mag that defined stacking. It began stacking at 28". My draw is 28.5" and when I hit 28" it was like running into a brick wall...
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The only way to find the stack point is by making a force/draw curve. A simple task using a scale and drawing the bow....measuring the draw at each inch out to where it begins to build. I've done force draws on Kodiak Mags and most don't start to stack until about 29". String angle will sometimes make it feel like stacking when it actually isn't. Force/draw will tell you.
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If you put a bow on a scale and measure poundage every inch, here is an example of what you might see:
25" 44#
26" 47# weight varies from 2-3 lbs per inch
27" 49#
28" 52#
29" 55#
30" 58#
31" 63# stacking here, gained 5# this inch
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So does a bow maker have a way to figure this out before making the bow or is it a trial and error type of thing until you get just the right design that will minimize stacking?
Do different woods make a difference?
Thanks
sam
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Sam,
different woods make no difference at all... only the limb profile, length and taper determine string angle at the tips (and therefore stacking).
"Standard" bow designs are normally good out past 28". After you make a few bows you get a feel for where you can push the design some and can adjust it accordingly. Sometimes it works well, sometimes you learn something else you really shouldn't do :)
I shot a bow at the JLMBH this last weekend that quite literally hit a wall at around 28". The enxt inch probably gained 5# and the inch after that I'm not even going to guess at.
I have an old 54" Shakespeare that doesn't start to stack until just before my 30" draw. My father's 55" Root though starts to stack at just over 27". The limb profile of the two bows aren't very much different, but the Root has a whole lot more taper in the limb laminations. It's tips are "softer" and uncurl early in the draw, raising the string angle at the tips quickly. The Shakespeare's tips are relatively stiff, forcing the inner limb to work more during the draw and keeping the string angle at the tips lower.
Bottom line in that comparison: if you want to keep stacking to a minimum, especially with short bows, stiffen up the tips. Ideally the tips would be stiffer because they are deeper, not wider. That saves weight right where you really don't want any extra.
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Riser design also plays a role in when stacking will begin to occur. A forward handle tends to stack much further back in draw due to the limb angle changes. This is why guys can draw Shrews, Griffins, Swans and some of the Bear MAgs further than one might think before stack and can also lessen finger pinch.
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Makes a lot of sense. I guess over a thousand years or so of making bows the bowmakers learned by trial and error and started to incorporate this knowledge into how they make a bow, just like laminates, fiberglass, reflex deflex design etc have made continual improvements. And the more we shoot the more we learn about what we need to do to use these bows better.
Thanks guys.
sam
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When a bow is tiller,is done to a certin draw leanth.It should'nt stack untill after that draw leanth is reached.
If the bow stacks while being drawed back to full draw then it's not tillered corectly.One limb is stronger than the other and is being drawed faster than the other and when released snaps back faster.Even if it looks even at full draw on the tillering board.This is where stacking comes from.It's all in the tillering process.
Why do you think 2 bows that are made the same,look the same draw leanth same weight.But one shoots noiseabily better than the other.This is why some bowyers make better bows than the others.Most bowyers don't even know why there bows shoot good or bad.
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Roy You just destroyed my faith in bowyers!! LOL> I guess that's the hazard of a handmade bow or anything else. I play guitar and had the rare opportunity to sample and select my new instrument from the batch of 26 guitars that was a luthier's output for the year. He was a good luthier and knew what he was doing, but believe it or not they all played a little different, some a lot different, and I picked the best. I still have it (40 years later) and it gets better every year (as I get worse)
Sam