Primitive Archer
Main Discussion Area => Bows => Topic started by: Allyn T on November 03, 2020, 07:35:05 am
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I was reading about Perry reflex in the archives and it seems like a back that is less strong than the belly works best so why do people use bamboo backings at all if it's so strong? Wouldn't it always overpower the belly and create more set even if you can't see the set?
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A lot depends on the belly wood used. Typically compression strong woods like osage, ipe or yew are used for boo backings with very good results.
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How could I not see the set?
Bamboo works great with strong woods like Pat said. It's always my first choice for backing, and osage is always my first choice for the belly, and core if a trilam.
If I make a bow with cherry, walnut, maple, or other wood that's weaker in compression resistance, THAT'S when I use a weaker backing piece, and make it correspondingly thinner... but honestly, I don't know whether I'll ever do that again. I have plenty of osage and bamboo and they make a much better bow in my preferred aestetic and design specs.
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I was reading about Perry reflex in the archives and it seems like a back that is less strong than the belly works best so why do people use bamboo backings at all if it's so strong? Wouldn't it always overpower the belly and create more set even if you can't see the set?
Because is it available, it is near bombproof when done properly (safety counts, back failures are bow explosions) and you can somewhat balance the back stiffness to the belly by making the bamboo much thinner than the belly lam. As Pat says, often lam bows are made with belly woods that are very similar in strength and stiffness to bamboo, which gives superb performance.
It is stiffness that matters for balancing the back and belly, not ultimate strength. The only natural material that is less stiff than wood but able to survive more strain is sinew and that is a lot of work to use. Gluing up a lam bow with bamboo is way less work and performs very well when done properly.
Mark
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How could I not see the set?
Because the backing pulls the bow back after unstrung even if the belly cells are mush.
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The thickness of the bamboo has a lot to do with it, I see a lot of bows backed with too thick bamboo. I make my bamboo 1/16" thick on every edge, this is great backing for questionable staves, heat treat the belly and you will have a rocket launcher.
I know a guy who makes bamboo backed ERC bows with flipped tips, they don't fail on him, he sells them commercially.
He is quite a bow maker, told me he made up tp 75 bows a month and worked 7-12s on them.
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Jesus that's a lot of bows
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Bamboo thickness (in bow making terms) has nothing to do with its resistance to being streched.....
It is the outer surface that is doing the vast majority of the work. Same as the belly.
Backings should be trapped to make them suitable for less compression resistant woods - not thinned. You can not make bamboo thin enough to lower its stretch resistance and still be suitable for a bow backing. This is one of the most commonly mentioned misconceptions in bow making :)
Why do you think bamboos outer fibers become somehow 'weaker' when the backing is 1/16th thick as oppossed to 1/8ths?
I've experimented with many different thicknesses for boo backings and it don't make any difference to set.
Has anybody made a bow pulled it too far to induce set, then cut the bow in half along the nuetral plane? Doing so will prove what i'm saying if anybody wants to experiment :)
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You can not make bamboo thin enough to lower its stretch resistance and still be suitable for a bow backing. This is one of the most commonly mentioned misconceptions in bow making
unsuitable in what way?
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Mike means that in order to make it thin enough to reduce its stretch resistance it would have to be reduced to dust.
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What happened if you reduce the bamboo on top, take off the hard putter layer? Then the tension would be carried by the slightly softer inner fibers, would that help?
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I have made a lot of bamboo backed bows, at least 50, back when I was selling bows that is what most people shooting tournaments wanted because of the increased performance. Back then they were allowed in the selfbow class, no so today.
If the bamboo was thick I would have what I called an osage belly bamboo bow, all of which were under performers because the bamboo was doing all the work and bamboo by itself is not a great bow wood/grass.
For me bamboo was merely a backing, a thin strip glued in a reflex and tillered to perfection made for a nice performer with "0" set if I glued in 2 1/2" of reflex on my form. I found a formula that worked for me, every one of my BBOs was exactly like all the the others.
One can argue about this and that as far different qualities of a bow components is concerned until you are blue in the face, neutral plane, compression resistance and all are not on my radar. I just make bows, and pretty good ones at that.
I do get a chuckle out of some of the "facts" put out here at times. The most recent one was a bunch of guys telling a newbie that his osage had too many run outs and even backed with bamboo it was sure to fail.
Well, well, I reserved my tight ring twisty osage for bamboo backed bows, and never had a failure, the bamboo did it's job.
Here is a typical example, this bow, or one just like it, won a national championship in the hands of a very skilled lady archer. It took about 10 years and hundreds of thousands of arrows before it lost tiller, got mushy and had to be was retired.
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To be perfectly honest I did have failures in my bamboo bows on occasion, it was always the bamboo that would raise splinters never a core wood problem.
I once bought a 25 slat bundle of beautiful bamboo from Frank's in California, every bow I mde with this bamboo failed, all bamboo failures. I had to replace 5 or 6 failed bows before I wised up and burned the rest of the slats.
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Bamboo thickness (in bow making terms) has nothing to do with its resistance to being streched.....
A piece of bamboo that is half as thick is also half as stiff, regardless of what you think of it. That is basic mechanics of materials stuff. In bending on a lam bow it will be a bit more than half as stiff. Yes, the outer fibres do more work, but the inner ones on a thin backing piece are also doing significant work as well. The strain/stress/energy storage does not go from everything to nothing in one very thin layer, it is linear from maximum at the outer surface to zero at the neutral axis.
The thinner boo also shifts the neutral axis less than a thicker piece does, reducing the extra load on the belly wood from the NA shift.
I believe your experience that says you can't thin the boo enough to completely balance it, but thicker is always going to be worse than thinner. Seems completely reasonable to me that trapping the back may also be necessary depending on the woods being used.
Mark
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I know bamboo isn't wood but does it's strength correlate, so half as thick would be 8x less stuff. Even if that is the case does that not matter since it is a backing and won't be under compression anyway
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My bow building experiences mirror Eric Krewson to a "T". I don't have as many bows under my belt, but I believe Eric is dead-on on every aspect mentioned. .
-Taylor
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I know bamboo isn't wood but does it's strength correlate, so half as thick would be 8x less stuff.
Good question. I will leave the math up to Mark. Another aspect of what is casually called "strength" is that bamboo may be able to stretch further than wood on a percentage basis. The percentage a materiel can stretch or compress before rupture (in tension) or set taking (in compression) is actually the technical definition of strain.
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Good grief, you guys are overthinking this stuff.
I try to keep things simple so as not to discourage new bowmakers who think they are incapable of doing the "math".
Fear of failure is the #1 thing that shuts prospective bow makers down, I have seen it over and over when was trying to teach bowmaking to newbies in my shop.
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I try to keep things simple so as not to discourage new bowmakers who think they are incapable of doing the "math".
I agree Eric. In this thread though, Allyn, the op is asking.
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Conversely though a better grain selected board for a belly might last a lifetime.
Bamboo is not like wood through its depth so it's not going to follow the same math as a more homogenous material
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I know bamboo isn't wood but does it's strength correlate, so half as thick would be 8x less stuff.
This is correct when it is a solid piece being bent, it is not the case when the bamboo is a thin layer (relative to the stack thickness) forming the back. As a backing it is primarily a tension member, so half the boo will be half as stiff in tension. Note this is only the boo backing, it does not reduce the stack stiffness by half.
Mark
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I have never cut two staves that were the same, even out of the same log so making precise calculations is a moot point.
I have supplied bows to some really serious tournament archers, they shoot every day, lots of arrows. I have found the best made self or bamboo backed bow is going to give up the ghost at somewhere over 200K shots. That would be 2 lifetimes worth of shooting from us normal folk.
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Eric I will say that fear of failure isn't holding me back. My problem is time constraints, I have a one year old and a wife that thinks I have too many hobbies so I don't have much bow building time. Getting on here and getting as much information as possible is what keeps my interest up and fires my imagination for future bows. I would love to pick someone's brain in person rather than on here but mojam was canceled so I was deprived of that experience. Don't be afraid to get too technical with me, I won't be scared off I promise.
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I have never cut two staves that were the same, even out of the same log so making precise calculations is a moot point.
I disagree (naturally... :o). You do have to measure the properties of each piece of wood you use and leave a bit of margin to account for the accuracy of the measurements, but it is very possible to do quite accurate design calculations based on measured wood properties and then build to the design dimensions. I think there will always be a bit of final tillering adjustment required due to the nature of wood, but a lot of the guesswork can be eliminated for those of us who haven't built hundreds of bows to get a feel for it.
Mark
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How are you measuring the properties? Few people have access to testing equipment.
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......but the math part of this IS why I love it so much. The science and math behind stress and strain and all of the other physical properties is probably what I love most about making bows. I don't see why anyone would be scared by it.....math is our friend and it helps us to understand what is going on with bows. I did my grad school research on strain, so I may be a little slanted, but I still I'm not the only one who likes the technical aspects. It's what has kept me making bows for so long after I had made all I really needed for hunting. Certainly natural materials have a lot of inhomogeneity that makes forecasting their behavior challenging, but it's still fun. I think the anachronism of it is a strong attractor.
Incidentally, bamboo is my favorite backing material :BB
Matt
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I used the term "math" as a generalization of how to over complicate bowmaking, not actual math.
My hats off to you guys who get into the intricacies of stress of this and that but my point is 99.5% of us cut a stave, make a bow and shoot it, period. We don't even think about the technical side and still make great bows.
My bows have won over a dozen national championships, I guess I did something right.
I understand your position though, I was once a duck decoy carver, I couldn't get enough of duck anatomy, feather detail, art classes and such. I just found it all very interesting much like you guys enjoy the technical side of bow making.
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It's the art versus the science.
Bowmakers are more artist than scientist and they arrive at the correct math through actually doing.
People approaching it from the science side rarely develop the skills to actually execute what the math tells them, unless they go to CNC cut glass lam bows.
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Well said Pat, I am a half fast artist, or was. I have an eye for detail and can see the slightest anomaly in a bows tiller. I can build pretty good flintlock rifle from looking at a picture as well.
One time a great friend and fledgling bow maker sent me a picture of his bow tiller and asked my opinion. I told him he was 1/8" too positive on the top limb. He measured and said "how did you see that, it was exactly 1/8" too positive"? I told him "I just can".
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We don't have to pick one side or another. Someone who is drawn to wooden bows and primitive archery because of science/engineering/interest in the materials and their properties may learn to appreciate the artistic side. Someone who is not so interested in these things still can benefit from the growth of community technical knowledge; the Bowyer's Bibles are a good example. Even though they aren't perfect, they changed the way the "average" self bow looks (and performs) quite a bit. I definitely agree that actually building bows is the best way to learn, no matter what your initial approach might be!
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To be perfectly honest I did have failures in my bamboo bows on occasion, it was always the bamboo that would raise splinters never a core wood problem.
I once bought a 25 slat bundle of beautiful bamboo from Frank's in California, every bow I mde with this bamboo failed, all bamboo failures. I had to replace 5 or 6 failed bows before I wised up and burned the rest of the slats.
I have had a lot of failures from Franks Bamboo as well. I used to buy 100 at a time but now if I do go there I hand select a few. He has trouble with mold. I only live a few miles away so it is easy for me to drive over there and hand pick.
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My last bundle arrived soaking wet and covered with heavy mold, I threw it all away. I found a place to cut locally and never looked back, wonderful stuff.
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How are you measuring the properties? Few people have access to testing equipment.
The only one that is really critical is the modulus of elasticity (Young's modulus, MOE). That I measure with a bend test, which gets me within ~5% accuracy. The TBB bend test offers good info in the modulus of rupture and set strain, but those are not nearly as critical if you are designing to minimize set and not for the absolute maximum you can squeeze out of the wood.
The 5% accuracy is good enough to design the bow and get a set of dimensions to work to, then I use the measured draw weight at short draw lengths (16-18") to tweak the MOE to get the calculations and measured numbers to match and fine tune the final tiller and draw weight.
My hats off to you guys who get into the intricacies of stress of this and that but my point is 99.5% of us cut a stave, make a bow and shoot it, period. We don't even think about the technical side and still make great bows.
Taking a stave and making it into a bow by eye has worked for 10,000 years. Some of us just like the more complicated way to the same end point. :D
We don't have to pick one side or another. Someone who is drawn to wooden bows and primitive archery because of science/engineering/interest in the materials and their properties may learn to appreciate the artistic side.
There is definitely room for everybody here (or should be). I appreciate the artistic side, I just have no artistic ability so I have to make it work from the technician side of the equation.
Mark
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How are you measuring the properties? Few people have access to testing equipment.
The only one that is really critical is the modulus of elasticity (Young's modulus, MOE). That I measure with a bend test, which gets me within ~5% accuracy. The TBB bend test offers good info in the modulus of rupture and set strain, but those are not nearly as critical if you are designing to minimize set and not for the absolute maximum you can squeeze out of the wood.
The 5% accuracy is good enough to design the bow and get a set of dimensions to work to, then I use the measured draw weight at short draw lengths (16-18") to tweak the MOE to get the calculations and measured numbers to match and fine tune the final tiller and draw weight.
Mark
Mark,
do you think a arrow spine tester is good enough to test bow wood for stiffness?
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That's only one variable which won't tell you much about the potential as bow material.
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My main interest is self bows, I really don't plan on even making a backed bow but I like the technical side for discussions. As far as artistry goes I enjoy carving wood and make wooden rings and other jewelry for my wife. I don't measure anything and just eyeball it. Besides measuring out my starting dimensions on my bow I haven't measured anything or done any bend test and I don't plan too. I really just like talking about the technical stuff, I hope I haven't offended any of you artist out there but it's just about kicking ideas around.
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Mark,
do you think a arrow spine tester is good enough to test bow wood for stiffness?
I have never used one, so I can't say. If it will get as close as my 5% accuracy with a home brewed bend tester then it probably will work. The biggest determinant of accuracy is being able to accurately cut/grind and then measure the thickness of the bend test sample. Bending stiffness is proportional to the third power of the thickness, so small errors in thickness magnify greatly. You also want fairly large deflections so that they are easy to measure accurately without lab quality instruments.
That's only one variable which won't tell you much about the potential as bow material.
I agree, it means almost nothing in assessing a new wood for bow use. I need the MOE to calculate limb thickness for the target draw weight, nothing else.
Mark
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I should have specified, I am a terrible artist. My bows won't ever win any contest on aesthetics. I just love making bows so I can study how they work and how different materials work in a dynamic manner. I totally appreciate those of you who make beautiful bows because I can make bow that shoots well, but it won't ever be pretty because that just isn't a skill set I have.
Matt
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Nothing wrong with that function is really the most important part, I started down this rabbit hole strictly to make a hunting bow and now I'm fully engrossed. I have a feeling my finish work will be subpar
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Making a nice looking bow is easy if you keep the flow in mind, no abrupt angles, make one part flow into another, plus you can never over sand a bow with 400 grit paper followed by steel wool. Ideally there should be no tool marks showing anywhere.
I use a small orbital sander for final tillering, this is great for getting rid of tool marks and any washboard on your limbs or in the fades from scraping.
Flow;
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Eric what is you finish routine, as far as sand grit and how many times you move up grit count?
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All I have is 220 and 400 grit. First I take the bow out in the light of the setting sun with a scraper, every tool mark on the bow will become visible in this light. I use a card scraper and remove any tool marks I find. I sand the whole bow with 220, give it few minutes with 400 and switch to a green scotch bright pad for that last push for perfection. Next I put on one coat of finish and inspect the bow for any tool marks I might have missed, If I find any I scrape them off, sand the spot and proceed with the multiple coats I always put on for a finish.
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So you like the green pad over 0000 wool steel wool
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Steel wool is OK, it works a little better than a scotch bright pad but leaves those little bits of steel on the surface.
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I use the automotive synthetic steel wool pads. They come in finer grades than just plain Scotch-Brite.
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Thank you Eric. PatM can you get those at an automotive store
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That may depend on the store. I just get them from a more general store with an automotive section. They are used for doing repair finish work.
You can probably order them pretty easily. They are made by 3M.