Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on April 12, 2019, 12:04:27 pm
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I had a small piece of boo left when I cut my new boo so I thought I would see if it was the right size for a bow for my grandson. I roughed out a bow shape and braced it with the outside of the boo for the back. I pulled it to 20" and it was 10# and it took some set. I remembered that the Bhutanese do their bows with the inside for the back so I braced it that way and pulled it to 20" again. I got 14# and no set. I was surprised. Almost 50% more draw weight by turning it over. This must say quite a bit about bamboo and a bit about bows in general. We've been told that the outside of boo is the strong part and I don't doubt that but the inside must be almost as strong because it didn't break when I bent it "backwards". The inside must not be very good in compression because it took quite a bit of set when the inside was the belly. The outside must be good in compression because it took no set when it was the belly. So the outside of the boo is good in compression and tension and the inside of the boo is not so good in compression but is good in tension. So build your boo bows backward or use the outside for belly and back.
I'm also thinking that this says something about bows in general. It seems to me that this shows that the belly is the controlling factor in set and draw weight. I guess as long as the back holds together the belly does the rest.
Thoughts?
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it seems a sinew back keeps the belly from taking set,, my backing came off of a bow,, and the belly had not taken any set,, if it had been a self bow at that length and draw,,, it would have,, not sure how that goes with what your saying,, :NN
I waiting for you to delve into the sinew back,,, :D
I am thinking because the sinew stretches more than say a boo back,, it does not compress the belly so much,,???? but sometimes I think too much,, (--)
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I'm also thinking that this says something about bows in general. It seems to me that this shows that the belly is the controlling factor in set and draw weight. I guess as long as the back holds together the belly does the rest.
Thoughts?
Wouldn’t both the back and the belly try to return to its original shape? Like how if you stretch a spring it tries to shorten, and if you squeeze it, it tries to bounce back. I don’t think the back is just there to hold it together but I’m most probably wrong.
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it seems a sinew back keeps the belly from taking set,, my backing came off of a bow,, and the belly had not taken any set,, if it had been a self bow at that length and draw,,, it would have,, not sure how that goes with what your saying,, :NN
I waiting for you to delve into the sinew back,,, :D
I am thinking because the sinew stretches more than say a boo back,, it does not compress the belly so much,,???? but sometimes I think too much,, (--)
No set, that's very interesting. I wouldn't have expected that. I think sinewed bows are a whole different field. We've had two or three long threads on here about sinew and I still don't think I'm even close to understanding how it works. The best I can do is speculate that somehow the stretchiness of the sinew transfers the compression to the whole bow rather than just the surface of the belly. How it would do that is beyond me.
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I'm also thinking that this says something about bows in general. It seems to me that this shows that the belly is the controlling factor in set and draw weight. I guess as long as the back holds together the belly does the rest.
Thoughts?
Wouldn’t both the back and the belly try to return to its original shape? Like how if you stretch a spring it tries to shorten, and if you squeeze it, it tries to bounce back. I don’t think the back is just there to hold it together but I’m most probably wrong.
Yes, the back will stretch but the fact that I got a higher draw weight with the "weaker' back and the stronger belly seem to show that it's not as important as I first thought.
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Bamboo is also excellent in compression but it's a bit like Hickory in that it needs to be drier than normal. The best solution is to use two pieces of bamboo so that you are getting the full tensile strength for the back rather than the more dubious interior.
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Yes, the dryness is suspect here. I've only had it for a couple of days and the weight is dropping, very slowly, but it's dropping. Good catch.
PS I just weighted it and it roughly averages 1% weight loss in two days so It is damp :D I'll leave it for a while.
That makes the outside drier than the inside. Could explain some of this.
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I'm reminded of something Tim Baker wrote about. He took a bow with some set and cut it in-half, back and belly. The belly cut took all of the set. The back portion sprung back to it's original shape. I think you are right.
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Is everything wrote and done by Tim Baker written in stone. I mean something different and new comes along once in a while discovered by even amateur Bowyers. :BB
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Wow...Outside of Moses, I don't consider much of anything written by anyone as written in stone. Tim Baker included. But I will give the man his props when it's due. His approach to bow making was fresh and full of insight on why something worked, or at least should work a certain way. He was often right on the money. Sometimes not. He didn't believe too much in the idea of "seasoned Osage". I believe he was wrong about that. On wood mechanics as it applies to bow making, he was more often right I think. Seems that DC theory here is somewhat shared by Baker. That was the only point I was making.
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He was wrong about perry reflex with a hard backing relieving the belly of compression. That can also be tested by delaminating a backed bow. Sinew on the other hand seems to work a bit differently as Brad notes.
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From memory, his take on perry reflex was in dispute early on.
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Its not really about the properties of compression or tension of the back or belly of boo, but the fibers. There are more of them as you move toward the outside layer of boo and they are smaller. I've told people multiple times about doing boo bows in reverse and tillering the back rather than belly and a lot of people can't handle the concept, but it isn't a new concept. That's the way many traditional cultures handle boo bows. The grains are 100% straight, so it just doesn't matter much tillering the back. Boo takes set regardless. I think the fact you tried it one way first then flipped it is why that didn't show up...yet. It will. The best boo bow to make is using the outer material for both the belly and back. You need to plan your design well and preplane the boo like you'd do in any other laminate, but this makes a good bow. If you roast the belly lamination good and well before glue up and then glue in a recurve or reflex, you're looking at a good bow.
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what do you do with the nodes,,
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Remove and tiller normally if they are throwing the tiller off too much. You can sometimes tiller in-between them, but usually, you jut need to sand them down and leave the belly with the natural round of the boo. Use a razor blade or cabinet scraper to tiller. Works best on boo. Don't try and flatten it or anything like that because you want to leave the power fibers in tact. Point from before, pretaper the boo before glue up like any other laminate so the bend doesn't need too much correcting. I can't remember how much taper I used in the past or how I got the boo to taper correctly. I am guessing with a belt sander and a lot of patience. My theory (don't know if it is true at all or not) is that if you line up the backing nodes in-between the nodes on the belly, they work the forces out better. Could be totally wrong on that, but it at least seems logical. I believe I came on that after one of them developed a hinge when I had aligned the back and belly nodes. If the boo is smaller diameter, it is necessary to add some sort of a core wood. I've used a few woods for this and also boo sheets. Hickory looked exactly like boo fiber from the sides, so I settled on using it. Blended right in like no wood was used at all. The only issue I had with making a few of these bows was the handle. The boo is round, so you are pretty much stuck using a soft material to build the area up. If you insert a core wood, you can build one though. I'd just laminate pieces of leather and then wrap them in reed. Matched the boo well. Give it a go. Boo is cheap and fast working with.
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Let's assume there are more fibres on the outer layer of bambo. The bamboo might be stronger on that side as a result. As a result, the neutral plane is closer to the outer layer of the banbo. When strung the "wrong way around", the neutral plane is very close to the belly and there is not as much set!
lonbow
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I have made bamboo belly and back bow. Tiller was good, but its speed: 210 fps is not good for this draw weight..- some 30+ kg. I made outerlimbs to heavy.