Primitive Archer
Main Discussion Area => Bows => Topic started by: Springbuck on April 10, 2018, 11:28:44 pm
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DC's post got me on to this train of thought. I have to say, I still have a lot of questions about backings, mostly because guys I know make fine bows are doing things I don't get, or even wouldn't agree are correct. Here is what I THINK I know.
The main purpose of the back of a bow, unless sinew or cable backed, is to hold the bow together. "A bow is a bow until it's back breaks." Everything else is secondary.
Uninterrupted fibers from a single ring, either under the bark, or "chased" are best. A sawn wood backing seeks to replicate this strength, by the nature of the wood specie chosen and our selection and milling of the grain. We back staves that otherwise could not stand on their own, and to improve their profile.
Do we all agree so far?
Now, Baker originally said that a backing too thick for the belly wood could overpower the belly, and listed a bunch of ratios for hickory + other woods. He also extolled the rectangular cross section. I still see this idea touted a lot, and I don't buy it at face value. Baker later issued a correction, saying backings should be NARROWED by trapping rather than thinned, to match their bellies. He offered a bamboo-backed red oak as an example, the bamboo surface being barely over half as wide as the belly. I know I have had a good handful of very successful bows that ended up half backing and half belly. I have likewise made good tri-lams with "tough wood/light wood/strong wood" combos, the belly slat being less than half the total thickness.
We know most woods are tough to break, but stretch very little before breaking. I.E. A hickory backing may perhaps be stronger than maple, but both will break if stretched their length + 2%, give or take. Some woods are weak in tension, thus we see very few poplar-backed osage bows, right? Commonly chosen backing woods are proven to be tension strong. Hickory may be stronger than maple, but maple is stronger than average.....
So, all that said, I truly believe that backed bows FAIL for exactly the same reasons self bows fail: I guess a backing could overpower the belly, but far more often I have either had a backing fail from a FLAW (as do selfbow backs), or I had a belly fail because I expected too much of it....... I.E. I'm back to TBB basics, DESIGN IS KING!
It's a sloppy sample, because backed bows are often profiled by Perry reflex or whatever, but what I mean is the back still has to not blow, and the belly still has to take the compression, or it'll take set. WIDTH, length, profile, and thickness are still key, more so than the specie of the backing.
If I take a 2" wide elm stave, plane down the back, and back it with hickory, maple, or white oak, I have essentially done nothing as far as I can tell. Perhaps I have added a little bit of weight. Tillered in, ALL of those woods will "hold" an elm belly, and all will stretch a similarly tiny amount. All will ask the same amount of compression of the belly, and all those bows could be tillered to similar draw weights at the same width and essentially the same thickness. Plane down a 1-1/2" wide osage stave, and the story is the same, unless you tiller toward a very high draw weight.
So, I'm thinking the question is "will the backing hold the stiffness of the belly" whether that comes from thickness or a stiffer belly wood. I haven't done it, but I know an elm backing would hold a black locust belly, ASSUMING the width, thickness, and (by extension) the draw weight is appropriate, as well as a pristine BL back would have. I expect maple is slightly less tension strong than elm or hickory, but not so much it won't hold the locust. MAYBE a hickory back could be trapped more (or an elm back more crowned) than maple, but its probably a tiny difference. Maybe a 1-1/2" wide locust belly should have a lower targeted final draw weight, than a 1-7/8" BL belly slat, but that would be true of a self bow, too.
If I try to make a 100 lb maple-backed locust only 1-3/8" wide, well, I better tiller better than I ever have in my life. If I back a fir 2 x 2 with maple and try to tiller THAT to 100 lbs, even worse. BUT that has little to do with the backing. It's more about width, belly wood, and design.
Anyway, I could be wrong here and there, but I don't buy that backings overpower bellies. It's more likely that we back marginal staves or even marginal designs. I don't think thickness of the backing kills bellies, I think stiffness of the belly does. I do think trapped AND crowned backs are a fine strategy, assuming there is enough strength there to hold the bow. I also think different woods used as backings act about the same and interact with the belly about the same, except it may take more to break a hickory back than a maple back. Finally, It seems backings are not as strong as pristine wood, but, unless flawed, or badly designed, are plenty strong. Likewise, I have to assume a truly perfect maple backing would be better than a sketch hickory backing.
I'm leaving bamboo out for now.
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I don't buy that backings overpower bellies........I don't think thickness of the backing kills bellies, I think stiffness of the belly does.
isn't strain (% stretch) proportional to stiffness? ie. a backing that is twice as stiff as a belly will stretch half as much as the belly will compress? this holds true when the backing and belly are substantial in thickness. (roughly a third? of the total limb thickness). A disproportionately strong back will make a less stiff belly act thicker/ take set sooner. If the limb cannot be as thick, it cannot be as stlff.
If the thickness of backing doesn't matter, why not a backing only a couple of thousandths thick? Perhaps I have not understood the question well?
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I see your point Springbuck. Years ago I was really big on matching bellies to backings. Now I don't see much difference. I have backed ipe with maple on plenty of occasions. I have also used bamboo backings on woods like maple that ended up being 50% of the thickness. I feel the belly woods and thickness requires more attention than the backing. Very easy to go to thin on some belly woods. I know maple and several white woods will chrysal if the belly is too thin. Ipe and oasge can also cave in if too thin but they are more tolerant than the white woods.
The biggest thing I have noticed and I am not really sure why is that if a bow is tillered out without any set it will perform as well or very close to as well as a backed bow that didn't take much set. I used to get a much more pronounced difference in performance between the two. For example. I have always felt that any self bow doing better than about 174 fps was a very fast self bow, but I would expect 184 from a backed bow before I would refer to it as very fast. I am finding more and more self bows hitting that 180 mark which really closes a lot of that gap between the two. I always like to add that I am talking hand shot bows which means more to the bowyer than an actual hard number shot from a shooting machine. Each archer will get different numbers depending on his shooting style.
I still lean heavily toward bamboo backing primarily because I feel they are more reliable. I can't say for sure that are any faster than other backings though because i have repeated the same performance levels from all kinds of backings. I have never believed in trapping so I have no experience or comments to make on that subject.
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The thing that's eating at me is that we are taught to never, ever violate a back and yet quarter sawn backing, which almost certainly has grain violations, is used regularly. Does this mean that we could tiller back and belly as long as we keep the back flattish?
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DC, one of the things I like about 1/4 sawn backing is that you can tiller on the back if the belly starts getting too thin. Done it many times. Overall I have good luck with 1/4 sawn backing but not as good as I do with bamboo as far as being reliable and holding up over time.
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I am finding more and more self bows hitting that 180 mark which really closes a lot of that gap between the two.
Steve, have you had a chance to build a backed bow using your no set approach?
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I did a whole slew of them last year and they all took world records. I didn't chrono any of them. I just started using my chrono again after not using one much for a couple of years. These were english longbows. I will be doing an r/d bow this week or next, I will likely just skip the deflex and go right into reflex. I never liked the looks of a bow that deflexes right out of the handle.
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I did a whole slew of them last year and they all took world records.
just curious what some of the backing/belly species combos were?
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Preach! haha. I agree with you Springbuck. Willie makes a good point, but I think you already know that and are simply referring to backings of similar qualities like hickory and maple. like you, I disagree that "too thick of a backing will overpower the belly". It's more of a depends. If the belly is significantly less stiff, as Willie suggests, then of course it will (Backer showed this with his flax pine bow in TBB1). However, if you backed hickory, with hickory, unless the difference between the two trees was so great, I doubt that it would matter that much if the backing was 3/4 the thickness or 1/4 the thickness. Like you I've never understood backing things like osage with something like maple, unless the osage you wanted to use was marginal, in a way that it couldn't be a self-bow or else why not leave it as is? However, I do understand backing some compression strong woods that are otherwise marginal in tension, like juniper, ERC, IPE, ect.
But maybe there is something more to it, like Badger suggests. Is it true that backed bows, no matter the pairing, outperform self-bows? according to his experience, it would seem that way but why? Maybe that's just crazy sample selection in his case, or maybe some subconscious difference in tillering. More likely there is something more to it that we don't fully understand yet. I've heard that backings are good because it more evenly distributes the weaknesses in the wood. For example, if you had an internal unseen imperfection in a limb, if you back it with some other piece of wood (even of the same species) then that imperfection only accounts for a partial amount of the wood thickness in that section, whereas before it would be all of that section that has that weakness in a self-bow.
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I will be doing an r/d bow this week or next, I will likely just skip the deflex and go right into reflex. I never liked the looks of a bow that deflexes right out of the handle.
This is confusing me. Wouldn't an R/D with no D just be an R or are you going to deflex the handle like Marc?
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Backed bow always seem to be glued up with a more favorable profile. That's likely where the help is coming from.
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I will be doing an r/d bow this week or next, I will likely just skip the deflex and go right into reflex. I never liked the looks of a bow that deflexes right out of the handle.
This is confusing me. Wouldn't an R/D with no D just be an R or are you going to deflex the handle like Marc?
On a multi lam bow I deflex the handle area but on a stiff straight handle and fade area I don't like the sudden deflex. In the past I have always pulled it down mid limb about 1" and then start my reflex but I think I will just skip the deflex on the next one. I like the deflex when I use the handle to deflex.
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Gotcha, I agree. :D
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willie; "If the thickness of backing doesn't matter, why not a backing only a couple of thousandths thick?"
I wonder. Baker talked about backing some bows with hickory veneer. Thickest veneer I have seen is 1/40". MY real thinking here is that, while following longitudinal grain exactly is so vital to the process, it's essentially impossible to do. A backing that thin is just too prone to flaws. But, I've seen PearlDrums suggest someone slap on two 1/16" hickory backings. Does that equal one 1/8" backing?
I think the same about bellies, too. I once glued on a belly lam of ipe, less than 1/8" thick, to a black locust bow that I had screwed up. I ground the belly flat on the belt sander, clamped some of the set out of it on a form, and applied the ipe to the belly. That ipe basically crumbled at half draw.
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I would think that the thinner you go with backings or belly strips the more critical the lamination process becomes and may require epoxy. At 1/16 backing I think you would be ok with a good backing strip. I have reduced them down in my outer limbs on several occasions during the tillering process.
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Badger: "I have also used bamboo backings on woods like maple that ended up being 50% of the thickness."
Some people would call you a maniac. :o
I left bamboo out of the discussion, because the fibers inherently MUST be unviolated, unless I was dumb enough to flatten the nodes.
And I agree, too, that gluing up very thin lams then requires both perfect selection and perfect handiwork, esp on the back and belly.
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DC: "Does this mean that we could tiller back and belly as long as we keep the back flattish?"
This is partly why I brought it up. YES, this is exactly what decrowning is, and it's exactly what working with a board stave means, too. Again, TBB basics. "it won't break if it isn't under a breaking strain, and you can reduce strain a lot by making things wider.
The key, of course, is to saw out a backing from a straight tree, and follow the grain perfectly. I, too have done just a bit of sanding to my backing when the limb ended up thinner than expected and I was worried about belly thickness.
gfugal: "I disagree that "too thick of a backing will overpower the belly".
Yeah, I'm saying that if your backing is going to overpower your belly, maybe your limb needs to be wider and thinner. Maybe THAT could require you to use a thinner backing, but that comes at it from the wrong end.
Badger: When I first started making bows, I must admit that I was surprised when reports (and my own experience) began to come back that vert-lam bamboo flooring didn't make a good backing. Looking at it, I was sure it would have acted like quartersawn wood, which it doesn't. Likewise, I tried a few "board" bows from flooring, and very soon discovered they needed linen cloth backs to survive much draw weight at all.
I suspect, Steve that since you use bamboo, the crown takes on the function of the trap.
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Willie: "isn't strain (% stretch) proportional to stiffness? ie. a backing that is twice as stiff as a belly will stretch half as much as the belly will compress?"
I think this is the main crux of both my confusion, and my argument. That's what I understood early on, that the backing should stretch a bit to balance the belly compression, move the neutral plane, and not overwork the belly. Then TBB says, nope, we were wrong.
Somebody had a great thread on here maybe a year ago with a graphic about amounts and percentages of wood stretching, and that really seemed to show that all woods stretch about the same. That convinced me that the differences in wood species, width, and stiffness really came down to "will it break?" rather than "how far will it stretch?" Because the answer for all woods was basically "somewhere between one and two percent".
But, here you and other guys I know, good and successful bowyers I have been learning from for 20 years, do it and think it this other way, and it works.
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I think that when two craftsmen do things different ways for different reasons and they are both successful that their reasoning is probably wrong. At least one of them, anyway. ;) So many things are done because,"That's the way I was taught to do it for this reason." Question everything :)
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Then TBB says, nope, we were wrong.
having too strong a backing did not work out well in some experiments I made last winter, in fact I came to the conclusion that stiffer backing is helpful only in the amount required to keep an iffy back from breaking, but beyond that, more stiffer materiel on the back only serves to crush the belly sooner.
I guess what we need to define is what constitutes the "balance" we look for in a good design? Should we build to make the back and belly have equal strain? I have always wondered if lesser performing designs often suffer from more hysteresis because one side stretched more than the other.
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I understand that the back is under tension and belly under compression. I cannot understand why back material would increase or decrease the amount of compression the belly is under?? Isn’t bend radius and limb thickness as a whole responsible for the amount of compression?
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Morgan,
if the back cannot stretch, then the belly has to compress more
a different but extreme example of the effect is
http://www.primitivearcher.com/smf/index.php/topic,62668.msg878994.html#msg878994
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I understand that the back is under tension and belly under compression. I cannot understand why back material would increase or decrease the amount of compression the belly is under?? Isn’t bend radius and limb thickness as a whole responsible for the amount of compression?
It's called the neutral plane. In a homogenous material of similar compressive stiffness and elastic stiffness, the neutral plane is in the middle of the limb's thickness. However, once you start combining materials of different stiffness, the plane shifts to the side that has the stiffer material. If it shifts to the back the belly is under more compression if it shifts to the belly the back is under more tension. Generally, bows break in tension before compression so we tend to either place a less defect riddled backing or a stiffer backing to shift the plane towards the back. The latter reduces the tension the wood fibers under the backing experience. However, if you do very stiff material, or way too much of just a little bit stiffer material, you'll end up crushing the belly.
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It's called the neutral plane. In a homogenous material of similar compressive stiffness and elastic stiffness, the neutral plane is in the middle of the limb's thickness. However, once you start combing materials of different stiffness the plane shifts to the side that has the stiffer material. If it shifts to the back the belly is under more compression if it shifts to the belly the back is under more tension. Generally, bows break in tension before compression so we tend to either place a less defect riddled backing or a stiffer backing to shift the plane towards the back. The latter reduces the tension the wood fibers under the backing experience. However, if you do very stiff of a material, or way too much of just a little bit stiffer material, you'll end up crushing the belly.
But, but... heat treating! Why would you do heat treating if bows gave up in tension? Anyway, nice explanation for neutral plane, that's what i tell to myself also.
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If generally,wood is 8x stronger in thickness,wouldn't this put more strain on the belly with a thicker lam?
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But, but... heat treating! Why would you do heat treating if bows gave up in tension? Anyway, nice explanation for neutral plane, that's what i tell to myself also.
I'm not quite sure what properties heat treating changes. It's possible that it doesn't make the belly that much stiffer, but maybe rather improves the wood cell's integrity so they are less likely to collapse and damage. So the benefit it gives of keeping the belly sound may be worth the risk of it potentially causing the back to break, I would still not heat treat a wood that is already weak in tension or has some defects if it was going to be a self-bow for the very reason you mention.
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If generally,wood is 8x stronger in thickness,wouldn't this put more strain on the belly with a thicker lam?
So we're not talking about width vs thickness in this discussion I don't believe, so I'm not sure how this is relevant. Were only concerning ourselves with the differences in stiffness between materials.
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I'm picking up what your putting down gfugal!makes sense!
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And stiffness isn't affected by thickness right?not a engineer here.
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And stiffness isn't affected by thickness right?not a engineer here.
I'm not one either. I actually learned most of this from this site haha. Either that or my brother or father who are engineers.
You bring up a barrier in language. I'm using the word stiffness in replace of Modulus of Elasticity (MOE) also known as Young's Modulus. It's true that a thicker piece of wood is going to be stiffer when you try and bend it than a thinner piece. However, if you have two sperate pieces of the same thickness but different material that's where MOE comes into play. If you try to bend a quarter inch thick by 2 inch wide piece of steel, it's going to take a much larger force than quarter inch thick by 2 inch wide piece of plastic.
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I understand know .I was posting in regards to what I have witnessed in lams that I've tried though the years and not science type stuff!I should have taking more time to read the complete thread!my bad!
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I don't mean to be a smart a$$,all I know is if the belly gets to thin it's not good.
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I don't mean to be a smart a$$,all I know is if the belly gets to thin it's not good.
Ther might be something to that but I think.
willie; "If the thickness of backing doesn't matter, why not a backing only a couple of thousandths thick?"
I wonder. Baker talked about backing some bows with hickory veneer. Thickest veneer I have seen is 1/40". MY real thinking here is that, while following longitudinal grain exactly is so vital to the process, it's essentially impossible to do. A backing that thin is just too prone to flaws. But, I've seen PearlDrums suggest someone slap on two 1/16" hickory backings. Does that equal one 1/8" backing?
I think the same about bellies, too. I once glued on a belly lam of ipe, less than 1/8" thick, to a black locust bow that I had screwed up. I ground the belly flat on the belt sander, clamped some of the set out of it on a form, and applied the ipe to the belly. That ipe basically crumbled at half draw.
I wonder if there is like some critical thickness for the wood fibers where if you go too thin then you hinder its ability to compress?
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I wonder if there is like some critical thickness for the wood fibers where if you go too thin then you hinder its ability to compress?
I think that the often repeated adage that the top 10% of thickness does half of the work, is only part of the story. Proportionately less work is done as the depth approaches the neutral plane, but the other half of the work is still done below the top 10. If you have a thin but stiff layer taking strain, over a weaker underlayment, once the top half chrysals, things go down hill fast.
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I'm not quite sure what properties heat treating changes. It's possible that it doesn't make the belly that much stiffer, but maybe rather improves the wood cell's integrity so they are less likely to collapse and damage. So the benefit it gives of keeping the belly sound may be worth the risk of it potentially causing the back to break, I would still not heat treat a wood that is already weak in tension or has some defects if it was going to be a self-bow for the very reason you mention.
I've had Oak, Osage, HHB and Maple bows explode violently after heat-treating. It's pretty hard to say what type of failures they were but I would be willing to bet they were tension failures. The collapse and damage would be related to the elasticity of the wood and not it's strength/stiffness, I've always maintained that heat-treating does nothing for the elasticity of the wood.