Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on February 25, 2015, 04:51:06 pm
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Just curious. Is there an ideal time for the string to lift off when drawing a recurve or does it vary with every recurve?
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Varies with every recurve. Bow length and draw length, size/diameter of curve, net angle of the recurve, limb length, bending limb length, amount of deflex if any, and width of the bow will all matter in this decision.
Basically, late enough to do some good for early draw weight, early enough to not make your limbs want to twist.
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The string can't be touching the belly at full draw.
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The string can't be touching the belly at full draw.
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....well it can...but prolly shouldnt 8)... Brian
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The string can't be touching the belly at full draw.
So could you say that the string should lift of just before full draw, or would that be putting words in your mouth :D I can see that any later than that would just be wasted weight on the tips.
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Post a couple pics of brace and full draw maybe.
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I would say that for a static recurve the string should lift off 3" before full draw.
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Post a couple pics of brace and full draw maybe.
Pictures here http://www.primitivearcher.com/smf/index.php/topic,50883.0.html
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What would be the downsides of the string not lifting off? The biggy I can think of would be the string slipping off the limb if you twist wierd at full draw. Is that a big part?
Kyle
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I would think that anything past the string contact point would not be doing anything and would just be excess weight.
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I would think that anything past the string contact point would not be doing anything and would just be excess weight.
+1
Del
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Short and tight statics are the only way to fly in my opinion. Big, long sweeping hooks don't do anything for me.
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Just for the halibut I did an F/D graph of the bow in the pictures. You can see where the string lifts off at 9" or 10". This must tell us something but I'm not sure what yet :D
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It tells us your statics aren't near 90 degrees at start. The sharper that angle the longer the string stays in contact.
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At lift off the bow effectively becomes longer which causes that deflection in the graph line,
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PatM That's the object of recurves, isn't it? You want the f/d curve to rise sharply and then level off to get more grunt under the curve?
PD So would a long slow curve to 90 degrees produce a gradual change in the f/d and an abrupt right angle would produce an abrupt change. Gee that follows, even I can wrap my head around that :D
So the objective is to get the f/d to go straight up and then level off and to do it without wheels. Wouldn't that take big hooks that don't lift off until the last minute?
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Either big hooks or long sharply angled siyahs.
The two bows that people often mention feeling let-off with are the Manchu style Chinese bows or the modern Border Covert The Covert Hunter has hooks that look like whole sheep horns. It only rises a pound or so per inch in the last part of the draw.
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That Covert Hunter would be hard to do in a self bow ::) ::)
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I should post a picture of one I'm working on. ;D
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So how much affect would something with curves like this show?
Kyle
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It's easy to do. Just write down the weight for each inch of draw and make a graph. I'm finding this very interesting. It may not affect the bows I make much but it's very cool to know :D :D
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At lift off the bow effectively becomes longer which causes that deflection in the graph line,
In a recurve with string contact or bridges, the bow acts like a short, stout bow headed for a high, stacky draw weight with poor leverage. It acts like a short, overdrawn straight limb bow. At some point of the draw, the string lifts and begins exercising its leverage against the tip of the bow, instead of where it was touching the bow. The bow is now 8" longer than it was, or whatever. Suddenly, it is headed for a much lower draw weight, has tons of leverage, and is already AT much of it's total draw weight.
I DON'T prefer tiny, tight radius static recurves that go to 90 degrees, but they work great on wooden bows. They lift off slowly, just creeping the string up the curves I like deflexed bows (for stability and to reduce belly strain) with bigger recurves that hit 60 degrees or so. They have really high string tension before you even start to draw, and the bow gets longer and the leverage increases, starting after just a few inches of draw. I often add string bridges because I can lower strain on the wood by using them, and they make the recurves act like they have more angle than they do.
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I'm thinking of pursuing this a bit with OS but I don't have a bunch of seasoned staves. What I do have is a bunch of seasoned off cuts that would make one limb of a three piece take down. Could I make up limbs with different recurves and just grab the thick end in the vice and bend a single limb to see how it behaves? That way I wouldn't have to worry about getting both sides the same. I could use a stiff board to simulate the other limb. I hope you can follow my reasoning
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(http://i399.photobucket.com/albums/pp78/pat_05/IMG_2282_zpskdssylnk.jpg)
(http://i399.photobucket.com/albums/pp78/pat_05/IMG_2281_zpsxcwqofra.jpg)
(http://i399.photobucket.com/albums/pp78/pat_05/IMG_2283_zpspyxczxlw.jpg)
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Woweee. How wide is that Pat? Wood type?
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Over 2 inches wide. Elm of course. Wish the handle was a bit deeper but it split out a bit shallow. I don't really want to glue any pieces on so I think I'll heat treat the handle to stiffen it and then build it up all around with cork or similar.
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Why so thin at the apex Pat? Adding a stiffener?
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Why so thin at the apex Pat? Adding a stiffener?
It's actually not that thin and it bent with no slivers that require reduction. I already toasted them to set them further.
I might just add stiffeners in the form of brush nocks to show a struggling newbie the fine art. >:D
(http://i399.photobucket.com/albums/pp78/pat_05/IMG_2293_zpsrrf3g2vy.jpg)
(http://i399.photobucket.com/albums/pp78/pat_05/IMG_2294_zps0ggbnrmm.jpg)