Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on September 20, 2020, 10:05:16 am
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I thought I would make up some Bamboo/Yew recurves(just the curved part) out of scraps and test them to destruction to see how small I can make them. The questions are how to test them and how much weight should they take.
I think the most strain would be at FD when the recurve has "opened up". I thought I would grab the recurve in the vice and hang a 5 gal bucket from it. Then slowly fill the bucket with water until the recurve failed. I considering grabbing the test piece at a bit of an angle to simulate a bit of twisting to the mix. I'll make them as thick as I do now and then shave them thinner as I test them.
I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that ;D). I'm wondering how much of a safety factor I should add. I've heard everything from double to six times the load for stuff but I don't know what is reasonable in this case. I pretty much make sure I have primo wood for bending the tips. So for a 50# bow how much should I hang off the tip and expect it to survive.
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You could probably hang a small vehicle from a piece of laminated wood of that shape.
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That had occurred to me but that would eliminate Simple Composite from the works. In this quest for speed I find myself drifting further away from the stickbows that got me into this. It's starting to weigh on me a bit. But then so is having 75 bows hanging on the wall. ;D
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I have tested tips to destructioin,, I put it in vise, and pulled with string that had a scale attached,,they usually didnt explode,, just started to fail,,and I knew about the weight they started to fail,,
also I remember a thread,where the brace weight of a bow was more than I thought,,on the string and tips,,test it to make sure,,
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I was thinking about using a scale but the shrapnel was scaring me off. I thought the water bucket also preserved the weight at the moment of breakage so I could look at the weight at my leisure rather than have to catch it right at the moment.
Did you find that your tips were way stronger than they had to be? I've always suspected that they were but I was too chicken to test my theory. ;D
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yes they had to get incredibly small to fail,,
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I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that
if you put a bow in the vice at the handle and pull on one limb with a scale, the readings will be very different as the angle changes. Finding a way to pull the same deflection with 1/2 the draw weight is not as easy as is seems.
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I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that
if you put a bow in the vice at the handle and pull on one limb with a scale, the readings will be very different as the angle changes. Finding a way to pull the same deflection with 1/2 the draw weight is not as easy as is seems.
So you're saying it's not 1/2 the DW? What does science say? My gut says it's half ;D
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I was thinking about using a scale but the shrapnel was scaring me off. I thought the water bucket also preserved the weight at the moment of breakage so I could look at the weight at my leisure rather than have to catch it right at the moment.
Did you find that your tips were way stronger than they had to be? I've always suspected that they were but I was too chicken to test my theory. ;D
You need something like this DC ... http://www.billfishtacklesupply.com/Scales/scales.htm. They use them to test drag on heavy fishing reels. There is a slide that remains at whatever point the drag got to so you can read the weight. These only go to 55lbs... but maybe 2 side by side would be enough - I am assuming that gives you 110lbs...
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This is a one off test, I don't want to spend any money. And I'm cheap, you were thinking that anyway ;D ;D
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A smart man like you could fit a “one off” sliding bit of scrap yew to a cheap scale that I am sure you already have! ;) ;)
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So you're saying it's not 1/2 the DW? What does science say? My gut says it's half ;D
You are correct that each half of the string on either side of the arrow nock carries half the draw weight. But the string tension is not at all equal to 1/2 the draw weight because it is puling at an angle to the arrow shaft. Simple geometry can be used to calculate the string tension if you know what the angle is at the arrow nock. Even just measuring the angle from a full draw picture will get you in the ballpark.
For instance, on a recent R/D recurve design I have been working on the string angle from the arrow shaft to the string is 65 degrees at full draw. Using trig you can calculate what the string tension will be.
1/(cos 65) = 2.366
So the string tension in my case is 2.366 x half the draw weight.
I would say figure on the string tension being at least twice 1/2 the draw weight, maybe closer to 2.5x.
Also realize that the string tension is lowest at full draw, so you need to test your recurves with several string angles and higher loads to simulate the loads the recurve sees as the bow is drawn. It is very possible the worst loading condition is somewhere closer to where the string lifts off the recurve than at full draw.
Mark
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I just pulled a scrap tip,,cut from a bow,,at about 90 degress,, not scientific at all,, but it gave me a general idea,, I just kept reducing the size of it, till I could pull it and make it fail,, I had the vise on the floor and put my foot on it and pulled up till it would start to break,,,,
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I thought I read it was a little more like that,,
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I never even thought about string tension. I've measured it on some of my RD's and it was at least double DW at brace but it dropped steadily until FD. Now, at brace, if I'm right, when the string is laying on the recurve there is no, or minimal, bending force on the recurve. It's only when the string lifts off that bending starts to happen. So by the time enough of the recurve is under bending pressure the string tension has dropped way off, probably to near DW. Am I close :D
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So by the time enough of the recurve is under bending pressure the string tension has dropped way off, probably to near DW. Am I close :D
It is around draw weight at full draw, but that is a very crude approximation as it changes a lot through small changes in string angle.
I realized I should have put the string angle numbers up for the string tension range I mentioned above. A string angle (as measured from the arrow shaft up to the string at full draw) of 60 degrees results in string tension equal to draw weight. A string angle of 66 degrees results in string tension of 1.25x draw weight. This means you have a 25% change in tension for a 10% change in angle.
Mark
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So you're saying it's not 1/2 the DW? What does science say? My gut says it's half
Marks got you covered with the science. :)
I put a bow in the fixture and drew it to full draw and marked where the limb was drawn back to. then I unbraced and pulled only one tip to the same mark with the scale attached to the string. I am still confused by my results. try it.
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DC seems to me a less scientific but more practical way to achieve what you are trying to figure out is to sacrifice one of the bows you have built and basically just keep reducing the tips/ recurves until they fail. This will also show you the string angle that they fail at as well.
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I've already glued up a pair. They have to cure til tomorrow. I didn't realise it was going to get so complicated. I'll read back over this tonight and see if I can come up with something that will satisfy me. I have a feeling that these things are strong enough that string alignment and having they break off sideways is more of a concern. That and slamming them in car doors.
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are strong enough that string alignment and having they break off sideways
torsional stability just might be the controlling factor. what cross section shape is best for that?
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yes breaking off sideways is a concern,, :NN
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are strong enough that string alignment and having they break off sideways
torsional stability just might be the controlling factor. what cross section shape is best for that?
Well, if it's steel tubing round is best but solids may be different. I think I've heard that wide is best. They always say to keep the limb wide up to the base of the recurve but there may be a difference between torsion which could involve the whole limb and just the recurve bending sideways.
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I just worked on a recurve,, and the tips wanted to bend off sideways, I couldnt get to line up,,I cut them a bit shorter and they were stable at the shorter length,,the bow still holds 6 1/4 reflex,, so I think I was just being overly optimistic bout the longer curves,,the bow is 46 inches long, I have been putting off narrowing the tips, ,Im afraid they wont be stable again,,
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I like that. Is it backed?
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yes about 1000 grains of sinew,,the tips literally tried to twist off when I pulled it on the tiller tree,, I thought I ruined it,, but was able to get it to line up and shoot,, it was tricky to tiller,,
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Well, if it's steel tubing round is best but solids may be different. I think I've heard that wide is best.
I did some reading , and round and smooth seems to make sense for most torsion bar applications, as they are expected to flex millions of times, and cracks tend to start at corners. found nothing on tubes versus solids. I suspect more squareish might be better than flatish for rectangular.
here is a illustration of a horn bow that may be informative. I read often the fg guys issues with stability. their limbs are typically flatish retangular, and they often are trying for more quite radical reflexes though.
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I was just about to say......Turkish hornbow tips :) Triangular is best from a weight versus stability point of view. Flat back, slightly rounded sides to the tip. Nock cut into the back.
Their tips were 10mm wide at the base of the tip, 17mm thick (average) but these dimensions were for 100# + bows with radical reflex.
More than half the problem with reflex/recurves/stability is laying out your centre line and width profile absolutely perfectly. Get this dead right and you can safely push the limits.
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My precision test rig.
0 pounds
40(approx)pounds. I had a 4.5 litre bucket(9.9 lbs) that I filled from the pond so there was a little spillage Edit-- it's 47#< I weighed it.
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rocks work good too, i think you gonna need to get to 90# to see any failure,, maybe more,, I love the test rig,, looks very effecient and safe,,, (-S
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So you're saying it's not 1/2 the DW? What does science say? My gut says it's half
Marks got you covered with the science. :)
I put a bow in the fixture and drew it to full draw and marked where the limb was drawn back to. then I unbraced and pulled only one tip to the same mark with the scale attached to the string. I am still confused by my results. try it.
I did roughly the same thing. I put a 50# bow in the vice, put a scale on one tip and pulled it in the right general direction to the FD mark. I got around 40#. My vice rotated a bit so this is a little low. I didn't start off in the braced position because there was too much junk in the way. But to me it's looking like a 50# is 50# for one limb at FD. I know the braced string tension is way up but because of the angle it's pulling at I don't think that's an issue. Is that the surprise you got Willie?
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rocks work good too, i think you gonna need to get to 90# to see any failure,, maybe more,, I love the test rig,, looks very effecient and safe,,, (-S
My wife had me throw away all the rocks. I do have a bucket of lead. I hate to even think of lifting it. My back started to hurt just mentioning it.
I just weighed it 99.7#
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I hung the lead on it and the bucket hit the floor. Nothing broke. No pic.
I reduced the yew thickness to just over 1/4"(approx) First pic.
I trapped the back so the back is 5/16" wide and the belly is 1/2" wide Yew thickness is still just over 1/4" Boo is about 1/16" thick. Bucket hit the floor. I have to modify my rig. Still everything is sound, no chrysals. Second pic
Edit changed to just over 1/4"
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I partially emptied the lead bucket to get it down to 50#. Then I hung it on the recurve. Then I started filling it back up. It's tire weights mostly. I got it up to 67# and called it quits. It was about halfway straight. Way, way more than I would ever want on a bow I was pulling. The recurve was also leaning a tad so it was seeing torsion. I lifted the weight off and it went back to normal. The picture is comparing one straight off the caul and the test subject. No sense wrecking them, I may use them on a bow. I figure that I can take a bunch off my bows. There's probably 5 fps in there.
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torsional stability just might be the controlling factor. what cross section shape is best for that?
Round is the best in terms of carrying torsional loads and for torsional stability. What bowyer's call a torsional failure mostly looks like lateral buckling to me, which is a different thing.
I did some reading , and round and smooth seems to make sense for most torsion bar applications, as they are expected to flex millions of times, and cracks tend to start at corners. found nothing on tubes versus solids. I suspect more squareish might be better than flatish for rectangular.
Square will be better than flat. The closer you get to a circular section, the better for torsional properties. Solid versus hollow makes no difference, it is the section shape of the outer surface that matters.
Triangular is best from a weight versus stability point of view.
That would be for lateral stability, which I think is the real issue.
I partially emptied the lead bucket to get it down to 50#. Then I hung it on the recurve. Then I started filling it back up. It's tire weights mostly. I got it up to 67# and called it quits.
In that case you could probably still take more material off, but it does get sketchy when you are down to those dimensions. My last bow was 5/16" wide at the nocks and looks impossibly delicate and thin to my eye, but works fine.
Mark
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Best to decide if you want a belly groove style or a longer loop straddling a teardrop cross section.
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Triangular is best from a weight versus stability point of view.
That would be for lateral stability, which I think is the real issue.
Mark
Mark,
Yes, after some more reading, I believe lateral stability seems to be the controlling factor. thanks. My google-fu must not be very good, as finding some resource that confirmed triangular as the best cross section was not to be. Can you link to a reference useful for doing some calcs?
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My google-fu must not be very good, as finding some resource that confirmed triangular as the best cross section was not to be.
I don't know that you would find anything on that sort of reference, it is pretty specific to our discussion on bow making. The best cross sections for bending strength and lateral stability are wide flange beams and if you wanted to maximize the structural efficiency would design the tension flange to be near yield at max load (tension is inherently stable, so there are no buckling considerations) and the compression flange to be near buckling failure at max load. That is impractical in wood, so we usually make a trapezoidal cross section that balances the tension and compression sides in a similar manner.
Can you link to a reference useful for doing some calcs?
I would have to do some looking for something like that. When I am worried about lateral buckling it is usually in regards to a steel structure I am designing and I use the steel design codes as specified in the applicable building code or industry regulations, I seldom do it from first principles. I suspect any mechanics of materials or structural design text would have a section on lateral buckling of columns or beams. All of that should be online these days. Maybe a search on 'lateral buckling of beams in bending' or similar would find some class notes from one of the many universities that put all their resources online.
Mark
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As I stated earlier stability is the issue. Just check out Turkish hornbows :) Nothing new under the sun :)
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if you wanted to maximize the structural efficiency would design the tension flange to be near yield at max load (tension is inherently stable, so there are no buckling considerations) and the compression flange to be near buckling failure at max load. That is impractical in wood
thanks Mark, the sinew build up in sections 9 and 10 of the horn bow illustration above, seems to verify your analysis.
Nothing new under the sun
mike, I believe if one of the designers of this bridge had been a turkish hornbow shooter, things might have turned out differently.
https://www.youtube.com/watch?v=lXyG68_caV4 (https://www.youtube.com/watch?v=lXyG68_caV4)
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very great info guys, thanks for posting,, :)