Primitive Archer
Main Discussion Area => Bows => Topic started by: Acheulean on November 27, 2017, 06:08:30 pm
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If I use a 1/8" strip of straight-grained wood as a backing, how bad can the grain be on the wood that I use for the belly?
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I look for the same quality grain on the belly as I do the back.
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Then what would be the point of applying a backing? I suppose you could Perry reflex, but I don't think that's why most folks back their bows. I think most are concerned about getting bonked.
So, how bad can the belly be before it's not even worth backing?
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of course pd has a point that we try to use the best possible, but using a backing often presupposes the bow wood is not quite up to snuff for a back but can still be used for the rest of the bow.
"bad grain" can mean a lot of things, straight grain with lots of earlywood may not be suitable for a belly, but wood swirls around small tight knots may work ok if the wood is dense and otherwise sound.
do you have a pic? or a better description of what you are trying to work with?
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Willie, this isn't for a particular project. I'd just like to know what you can safely get away with. Most of my bows are from boards, and there are simply more options available if I can slap a backing on a questionable piece of wood.
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Choose a strait-grained board and try to keep at least one growth ring the whole length of the belly. The wild spots in the compression wood are more prone to crushing. Wood is strongest when the force is applied to the end of the fiber. Think like the wood is a bundle of straws glued together with hot glue. It won't support much force except in the direction where the straws and not the glue is stressed. Try to keep the modulus of elasticity of each material balanced with the amount it has to stretch or compress so the backing doesn't explode or compress the belly. If the backing has a low modulus of elasticity, it can be thicker. If it has a high modular of elasticity it must be thinner. For example, if your backing is moso bamboo and your belly is ipe, thickness ratio should be 50/50 or less. However, if it is moso bamboo/ maple, make the bamboo as thin as possible and it might still be too thick.
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that hard to quantify without an example, but after you have lost some labor backing and tillering bows that break, you will have a better feel for what you can get away with. certainly high strain designs call for better materiel and there is a point where a core is not suitable for the shear stresses involved. reviewing some pics of "why did my bow break" autopsy threads might help, as well as some chapters in TBB series.
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I don't use a 1:16 ratio like I do with backings(though I probably should). I mainly do it by eye. Nevertheless I still try to use the straightest grained stock that I can find.
It is very important to use straight grained stock with an English Longbow. Not as much the grain on the side, but the grain across the belly is absolutely critical.
I had a nice hickory backed, tri lam stave. I had it on a tillering board with a loose string. Then at low brace. When I tried to brace it to full height, it blew. The grain across the belly was probably only 4 " from one side to the other, where it broke. Add to that a cambered belly and there really wasn't much holding it together. The backing was a champ and still held together undamaged but the belly fractured, and continued up through the core as well.
With a wider flatbow it isn't as critical, but once again use the straightest stuff you can get your hands on. If it looks too iffy it probably is.
All I know is if you use straight grained wood you won't have any issues.
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The belly of a bow can be made from a series of blocks butt joined together with no glue and still work! :o
The compression will hold them together.
It's knots and faults that will fail in compression that will cause problems.
Del
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Poor belly grain can make an otherwise straight bow twist at brace and it can compress and fracture on angular grain lines. I've seen both firsthand. Is it a sure thing? No, but the longer I do this stuff the less I like to play games with the materials I use.
Del, Id like to see that done and the tiller it offers. Not sure your going to get a sooth inside bend on a square butt joint. Something has to separate for a butt joint to bend.
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This is bamboo backed osage,from about 8 years ago,very bad grain.tiller was decent
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I only back bows that need to be backed unless I'm planning to make a backed bow. I would never back a sound bow stave with a hard backing. If I plan to make a R/D or Perry reflexed bow with a board stave I want it to be the best backing and belly I can use with the backing appropriate for the belly.
Over the years I've used bad bellies(big knots or unruly grain) and bad backings(grain violations and run offs) and have been fortunate to have success with most. After years of experimentation I've come to the conclusion that it's just not worth the effort to work with iffy materials.
If you have a good tree stave it generally wouldn't need a backing, the back and belly are appropriate for each other or they can easily be made that way by trapping one way or the other. I've only made a few unbacked board bows because I just don't trust a sawed board. Even brown grocery bag paper is sufficient to help keep a splinter from lifting so a backing doesn't have to be complicated but I try to only use the best materials I can find if I'm going to put the effort into building a bow. A backing won't necessarily make a bad belly safe.
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Poor belly grain can make an otherwise straight bow twist at brace and it can compress and fracture on angular grain lines. I've seen both firsthand. Is it a sure thing? No, but the longer I do this stuff the less I like to play games with the materials I use.
Del, Id like to see that done and the tiller it offers. Not sure your going to get a sooth inside bend on a square butt joint. Something has to separate for a butt joint to bend.
yeah, I'm with you on the twisty grain. I think I've seen youtube vids of balista type machines with blocks on the belly or some such.
I had a Yew bow that I was reviving, pulling out the set and heat treating. I overdid it and it cracked right across the belly to about half the thickness of the bow >:(. I was about to give up on it but thought I'd give it a chance. So I strung it, put it on the tiller and took it right back to full draw :o... no problem! So having survived that I thought I'd patch the belly. The bow is still going strong.
I was really just trying to say that not all "problems" are actually problems. I s'pose it's down to the individual bit of wood, faluts that run across or along the bow prob aren't so bad, it's the diagonal stuff that get you :)
Del
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The belly of a bow can be made from a series of blocks butt joined together with no glue and still work! :o
The compression will hold them together.
It's knots and faults that will fail in compression that will cause problems.
Del
Thanks Del, that's what I was looking for. I won't be butt joining anything together, but the idea that you could tells me something.
And, I'll be sure to avoid all of those beautiful knot-riddled eastern red cedar boards that all the lumber yards are peddling.
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All I know is if you use straight grained wood you won't have any issues.
That's not true.
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First things first. Del's idea of butt jointed blocks wouldn't work, so don't let that tell you anything. Secondly, if you use straight grained slats you wont have any issues as said, with the materials.
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Another thing you can do with a flatbow made from boards and structurally backed is rip two layers of good quality belly wood from the same piece, lay them up with the center layer turned over to the belly. Think of the slices as being really tall pages. You open the book and rip out a page and lay it down in that position over the other. In this way the faults of the wood mirror each other and cancel out twist in the grain. This does not fix a weak point in the grain such as a pinhole knot or figuring. But it does help avoiding a propeller bow. It is important to have some idea how thick the belly will be after final tiller. Your core should be about 2/3 the total belly if you are using this method. That is not applicable if you are using three different woods in a tri-lam.
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Secondly, if you use straight grained slats you wont have any issues as said, with the materials.
You're telling me that nobody has ever had a properly constructed straight-grained bow break on them?
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The sooner you break this bow, the sooner you can start a new one. To PD's defense, I don't see how that's a problem. :BB (--) (-_)
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Secondly, if you use straight grained slats you wont have any issues as said, with the materials.
You're telling me that nobody has ever had a properly constructed straight-grained bow break on them?
If anything it would be the bowyers poor judgement that broke the bow. Not necessarily the materials.
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I'm curious. I'm going to build a butt jointed bow.
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I've butt jointed two lams of horn mid-limb. But not wood, I have a sneaking suspicion that the ends will crush and leave an ever widening gap when unstrung and eventually be a pain.
Now a butt-joint in the handle I've done many times more often than not with a stiff handle and a power-lam
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Butt jointed bow challenge anyone?
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Butt jointed bow challenge anyone?
I believe it can be done.
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a more practical challenge would be to rip and laminate what usable wood you can get out of the erc boards. then apply a backing
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I thought in the context of this question the "straight grained stock = no issues "was pretty self explanatory to anyone with a modicum of common sense.
You won't have any issues associated with relating to the scenario that I had explained in my posts, and the posts of other guys answering your question.
Nothing can be done if the design, workmanship, glue up prep, or tillering judgements are bad. That's pretty obvious dude.
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I thought in the context of this question the "straight grained stock = no issues "was pretty self explanatory to anyone with a modicum of common sense.
I know what you meant. The statement is still a fallacy. ;)
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I'm curious. I'm going to build a butt jointed bow.
I look forward to seeing it. I'd suggest a reasonably thick Boo or Hickory back, with a nice thickish tapered belly lam that is cut into blocks before glue up... maybe 3" or 6" blocks?
Del
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I was going to go with hard maple, because it's what I've got. How thick are you thinking on the back and the belly?
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I'm all for trying new things. Just know going into it that it's an experiment, and prepare for failure. As long as you are okay with that, I too look forward to seeing if you can get it to work.
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I don't care if it fails. I just want to know what happens.
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Instead of being childish, please educate exactly why I might be incorrect? I have only been making bows for 21 years, though I do admit you never stop learning. Answers please?
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I'm going to do a maple backed maple bow, because I've already got the wood, and I don't want to go buy an other board just for this. I'll use a 1/4" thick by 1 3/8" wide backing and 72" long. I'll try to remember to post pictures throughout the process.
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I've butt jointed two lams of horn mid-limb. But not wood, I have a sneaking suspicion that the ends will crush and leave an ever widening gap when unstrung and eventually be a pain.
Now a butt-joint in the handle I've done many times more often than not with a stiff handle and a power-lam
I don't think anyone expects it to produce a bow for long term regular use or to actually be practical! ::)
I think it would just be to demonstrate the principle. :)
Del
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I've butt jointed two lams of horn mid-limb. But not wood, I have a sneaking suspicion that the ends will crush and leave an ever widening gap when unstrung and eventually be a pain.
Now a butt-joint in the handle I've done many times more often than not with a stiff handle and a power-lam
I don't think anyone expects it to produce a bow for long term regular use or to actually be practical! ::)
I think it would just be to demonstrate the principle. :)
Del
When I was a kid, gift shops at places like Yellowstone were selling these wooden snake toys. They were made from two pieces of wood with slits cut in the side, and they were bonded to a flexible core, so when you shook the tail they would slither like a snake. I wonder if this bow might turn out something like that, at least when it's unstrung. :) Probably won't be good for much, other than a conversation piece.
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I have posted this before; I have made 30 or more BBOs with belly grain like this, never had one fail.
Julia Norris shot a similar bow I made her for at least ten years and won many state and national championships with it. She probably put in excess of 100 K arrows through it before it got wimpy and lost cast. The belly wood never cracked or showed a sign of failure.
(https://i.imgur.com/azV8SBS.jpg)
I replaced her BBO with an osage static because they changed the rules for most primitive classes and don't allow BBOs anymore. She won the nationals at Cloverdale with her new bow this year. It is an ugly bow but shoots very well.
(https://i.imgur.com/cQLgSgu.jpg)
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That's encouraging. Thanks for sharing. Do you have a picture of the sides of the bow?
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I make the sides of my bows square with a slightly rounded edge. The wood in this bow has about 28 grains per inch, very tight stuff. The bamboo is thinner that the picture depicts, What you are seeing in the picture is the side and the hump of the bamboo on the back. The thin white line is the actual edge, about 1/16"
On the far right of the picture is a major grain swirl as well as a pin knot cluster.
(https://i.imgur.com/QWSoTHu.jpg)