Primitive Archer
Main Discussion Area => Bows => Topic started by: bow101 on March 18, 2014, 07:26:59 pm
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What is the benefit of having 3 lams versus 2..?
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one benefit is that you can use a lighter wood as a core. another is that it makes the whole stack more flexible when putting it in a form. Also you need smaller amounts of good wood.
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one benefit is that you can use a lighter wood as a core. another is that it makes the whole stack more flexible when putting it in a form. Also you need smaller amounts of good wood.
Good points Jesse........
Why would you want a lighter wood as a core. ..? I think I read somewhere that using 3 lams makes for a better performing bow (cast)
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Compression and tension are focused on the outsides. The middle is less critical so the lighter the better. It really doesn't matter that much though :)
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Not true. It does matter. The mid lam is experiencing mostly compressive forces and a poor core lam will chrysal. There was a thread here about this not long ago. Do a search and check it out. I think it was toomanyknots who posted it.
I always use compression strong core woods like ipe, osage, cherry, bloodwood, yellowheart and purpleheart, and I've used all these woods in mostly tri-lam ELBs. I have made a couple of tri-lam flatbows as well.
I've heard plenty that a tri-lam has improved performance, but I've never heard a reason why. I have my theories, but they're just opinions.
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According to the mass principle, the 'lighter core wood' is not the exact truth. The core wood IS doing something, so 'the lighter the better' is not the best approach. The core is at least experiencing a lot of shearing forces, so the wood must be able to cope with those. Since you'll be gluing wood to both sides of this core, it is also preferable to have good gluing qualities. A diffuse porous wood is therefore preferred, because it is uniform and resists shear better than most ring porous woods. In my opinion, a typical core wood is medium dense, uniform, diffuse porous and easy to glue. Walnut, cherry and especially maple are very suitable core woods. But the truth is, almost any wood will work as a core (maybe not balsa...). Myself, I prefer a third lam as core for its ability to allow me to use a third wood species. A nicely contrasting core is simply stunning. Also, this core is not at the surface of the bow, so you can actually make use of some of your laminates that did not qualify for backings or bellies. A hickory lam with a little too much grain run off, or an ipé lam with a small pin knot... these would normally not be suited for a backing and belly respectively, but are perfectly fine for a core. I've got a piece of ipé with some small drying cracks, which I don't trust as a belly lam. So I'll use it as a core lam instead.
The third core lam also allows me to use thinner belly lams. If you have a 3/8" ipé lam, for instance as a left over from milling belly slats, it is not thick enough to use as a belly alone when combined with bamboo/hickory. So I add a core of maybe 1/8" and I have enough thickness to make a great 50# bow. Using multiple lams also means that each lam will be thinner. The thinner the lams, the easier they are to bend. So the easier it is to press them into a form or shape. A severe reflex/deflex or recurve is easily obtained when using three lams, while it is difficult when using only two lams.
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The thinner the lams, the easier they are to bend. So the easier it is to press them into a form or shape. A severe reflex/deflex or recurve is easily obtained when using three lams, while it is difficult when using only two lams.
Thanks for all the points you guys made. "easier it is to press them into a form or shape" I'm aware of that and have learned what to do next time on a 2 lam bow to make it bend easier for the tips, rather than slicing the glued up lams I going to sand down one lam back about 7-8 inches then glue on a peice after and that will also stiffen up the last 4 inches of the tips. Its tough to get a clean cut without a band saw...... :P
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and especially maple are very suitable core woods.
+1000, maple is definitely my favorite core wood!
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The light core is the main reaso FOAM cores are used in many glass bows. Just sayin ;) By saying it doesn't matter that much I was referring to the fact that a light core does not make performance that much better :)
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Core material in a FG bow is not even on the same planet as an all wood bow. They have nothing in common, other than they're between something.
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Core material in a FG bow is not even on the same planet as an all wood bow. They have nothing in common, other than they're between something.
this is not the best place to discuss this I'm sure :) lets just say that having made many of both natural and unnatural bows I disagree. ;D the same physics do apply. no big deal. I didnt want to ruffle your feathers. Just giving my 2 cents ;)
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and especially maple are very suitable core woods.
+1000, maple is definitely my favorite core wood!
My favourite backing wood!! ;)
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Core material in a FG bow is not even on the same planet as an all wood bow. They have nothing in common, other than they're between something.
this is not the best place to discuss this I'm sure :) lets just say that having made many of both natural and unnatural bows I disagree. ;D the same physics do apply. no big deal. I didnt want to ruffle your feathers. Just giving my 2 cents ;)
I think this is the perfect place to discuss these things. Please enlighten us. I'm all ears.
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Core material in a FG bow is not even on the same planet as an all wood bow. They have nothing in common, other than they're between something.
this is not the best place to discuss this I'm sure :) lets just say that having made many of both natural and unnatural bows I disagree. ;D the same physics do apply. no big deal. I didnt want to ruffle your feathers. Just giving my 2 cents ;)
I think this is the perfect place to discuss these things. Please enlighten us. I'm all ears.
adb.feel free to pm me if you need clarification on something I posted.
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I guess what Im trying to say is that personally I have been able to make both natural and man made material bows on the same form and achieve similar results providing the materials can handle the design. Using a lighter core wood can increase performance but not always that noticeably. Draw weight is determined by the distance between the back and belly of a given design regardless of which core wood is used.
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Draw weight is determined by the distance between the back and belly of a given design regardless of which core wood is used.
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I'm not following you.
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Over on TradGang, I think I read that only 18% of the weight of the bow is delivered by the (wooden) core of a f!b€rgl@$$ bow. The remaining 82% comes from the FG itself. I'm not sure where the figure of 18% is coming from, since it must depend on the wood species (stiffness) of the core wood as well. But if only about 18% is coming from the wood, no wonder that you can't notice a huge difference when using various wood species in a FG bow. So if you want to compare a low density wood such as soft maple (SG 0.5) to a high density wood such as ipé (SG 1.0), there will be very little variation. Maybe (just guessing here) 14% draw weight from the soft maple, or 22% from the ipé. The difference of only 8% is nearly negligible in draw weight: perhaps the ~4 pounds draw weight is sometimes even ascribed to a different variable by the FG bowyer...
Disclaimer: figures are just for illustration purposes, and are not calculated.
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You might be right. I haven't done the math. I would have thought that a woods resistance to bending would be more of a factor as core. It turns out it doesn't have a big effect. If you use ipe as a core or bamboo as a core the draw weight at a given thickness is the same. I tried heat treating cores before gluing. Again no difference.that leads me to believe that the gaines to be had " though small" are through the cores weight rather than its resistance to bending. The real work is done by the back and belly.
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Are we talking FG bows, or wood bows here?
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Are we talking FG bows, or wood bows here?
both :)
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Well, it's my understanding that the core wood on a FG bow is nothing more than a platform for the FG, which is doing all the work.
The core on a wood bow is an entirely different thing.
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Well, it's my understanding that the core wood on a FG bow is nothing more than a platform for the FG, which is doing all the work.
The core on a wood bow is an entirely different thing.
Different materials and limitations yes but the same physical forces acting on both.
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Well, it's my understanding that the core wood on a FG bow is nothing more than a platform for the FG, which is doing all the work.
The core on a wood bow is an entirely different thing.
Different materials and limitations yes but the same physical forces acting on both.
OK... but my only point (which you seem to be missing) is: the core wood of a FG bow doesn't matter much, but the core of a wood bow matters greatly.
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I get what you're saying believe me. I just don't agree with it.. If you make 2 tri lam wood bows from the same boards at the same exact thickness only changing the core wood like say an ipe belly, hickory, back and maple core on one. Then hickory back ipe belly and ipe core on the other. The draw weight will be close to the same giving the slight performance edge to the lighter bow which has the maple core. If im wrong im wrong and im fine with that but this has been my experience :)
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It maters to both types of bows to the same degree. Example A fg bow with an all fg core does not perform as well as one with a maple core. Darn smartphone. sorry for the edits hard to see on this thing.
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It maters to both types of bows to the same degree. Example A fg bow with an all fg core does not perform as well as one with a maple core. Darn smartphone. sorry for the edits hard to see on this thing.
How do you know this? Have you done any comparative testing? The really high end recurve target bows (Olympic style) have 100% composite limbs. Don't you think, if your statement was true that all FG bows, including compounds, would have maple cores?
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They used to make all fg bows. I just happen to own one that was handed down to me. Believe me there is a good reason they dont make them anymore. If you can find an all fg longbow that people actually like I would be surprised. When it comes to compounds the issue is the ability to withstand the forces. Quite a different ball game with cams/pulleys/ cables involved.
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I get what you're saying believe me. I just don't agree with it.. If you make 2 tri lam wood bows from the same boards at the same exact thickness only changing the core wood like say an ipe belly, hickory, back and maple core on one. Then hickory back ipe belly and ipe core on the other. The draw weight will be close to the same giving the slight performance edge to the lighter bow which has the maple core. If im wrong im wrong and im fine with that but this has been my experience :)
I tiller most of my bows using the mass principle to some degree. So, 2 bows of the same design, having the same draw weight will have the same mass, regardless of what they're made from, including tri-lams.
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Nothing wrong with that. Mass is a good performance optimization tool. We agree on that :)
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I don't try to over think it. Tri-lams look nice, too. I use compression strong wood for the core from experience, and tiller using the mass principle mostly. The mass of the limb should be the same regardless of it's make up (for wood bows!).
My theories about tri-lams revolve mostly around glue joints... much like plywood is stronger than a single piece of wood. Also the neutral plane of a glue joint is a factor I believe.
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Compression and tension are focused on the outsides. The middle is less critical so the lighter the better. It really doesn't matter that much though :)
I meant this post be about wood bows, it seemed to spark controversy about all bows included FG as well. I think adb make good sense why use a lighter wood in the core. what benefit wood there be..?
My theories about tri-lams revolve mostly around glue joints... much like plywood is stronger than a single piece of wood. Also the neutral plane of a glue joint is a factor I believe.
Flexibility seems to ring a bell here to. I noticed that over an over stressed bow of a sinlge material that has taken a lot set is rather ruined where a lam bow still has some zing to it.
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Nice discussion combining therory and practical experience
to adb:
"Also the neutral plane of a glue joint is a factor I believe."
could you explain a little bit, what you mean. Because I do not see the neutral plane in the glue joint in a TriLam in general. It is possible to design a bow like that, but only for one glue joint. physics won't let you.
JG
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Surely a key point in this discussion is that the neutral plane (on a wooden laminated bow) isn't in the middle, and that's probably why the core wood is under compression.
E.G A bamboo backing will pull the neutral plane towards the back, whereas a backing of wet spaghettii (wet lasagna sheets are better ;) ) would move the neutral plane towards the belly and the core would then be working in tension.
So my guess is it's more about the combination of woods rather than the individuals. You could choose a backing to privide power or just protection, the best core wood may well be different for each.
Anyhow, what the heck do I know? I don't make laminates :o ::) :-[
Del
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I meant this post be about wood bows, it seemed to spark controversy about all bows included FG as well. I think adb make good sense why use a lighter wood in the core. what benefit wood there be..?
My personal opinion differs on this. I know from experience as well that using a light core, like a light piece of yellow heart (my yellow heart is pretty light) or maple verses a dense core like ipe does in fact make a difference in cast, at least with the english longbows I make. I like maple because it is not overly dense, but it works as a core. Same with yellow heart. Purple heart works, and looks great, but is dense. Ipe and osage, the same. I do notice it is important to taper the core at the tips if it is weaker in compression than the belly, you don't wanna have more core than belly at the tips if you use say a maple core and an ipe belly, as it will cause it to take more set at the tips. You want the belly wood you selected specifically because it is good at compression, actually being the one taking most of the compression. But other than that, I have never noticed any difference in set or any other issue.
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If we get technical and start over analyzing everything it is all very dependent on the Modulus of Elasticity (MOE) and the thickness of each lam. Like Del points out, a bamboo backing can pull your neutral axis (NA) towards the back of the bow. BUT, this requires that the bamboo be "stronger" than the belly, belly/core materials. For quick and clean examples lets take a bow limb and make each lam an equal thickness and of rectangular cross section. and we will use the following MOE information for giggles: Bamboo 20GPa, Hard Maple 12.6 GPa, Ipe 22 GPa (and for fun Glass at 80GPa which is a rough approximate)
Bamboo backed Maple: Significant shift of NA to the back of the bow, glue line is under compressive force. This puts some compression force on the belly side of the bamboo
Bamboo backed Ipe: Slight shift of NA to the belly of the bow, glue line under tensile force. Some tensile force on the back of the Ipe lam.
Now lets look at some Trilams with three lams of equal thickness and rectangular cross section.
Bamboo backed Ipe, Maple Core: Slight shift of NA to the belly, majority of the core is in tension. one glue line is in tension, the other is in compression.
Bamboo Backed Ipe, Ipe core: Large shift of the NA to the belly, probabably right next to the ipe-ipe glue line in the core. Core is mostly under tension. One glue line firmly in tension, second glue line near neutral.
Bamboo backed Maple, Ipe Core: Large shift of the NA to the back of the bow, probably near the glue line but in the ipe core. Core is mostly under compression.
Glass backed Glass, Maple core: NA at center of the limb, however due to the discrepancy in MOE the maple carries little force, that said there are a large number of shear forces due to the distortion that the maple must undergo in movement.
That is a horrifically simplified version of the relationships and assumes that all of the material properties are equal throughout the thickness of the material (bamboo in particular). Rounding the belly of the bow will shift the NA to the back, using different thicknesses in relation to one another changes it as well.
Adam
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I don't know much about bows or physics but I always thought of them like a steel I-beam in that the core is only used to separate the back and belly while also being able to stand up to the shear forces.
However I'm beginning to doubt that after having seen a trilam on this site with a fretted bloodwood core. If that fretting was caused by the shear force or the compression I don't know...
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If we get technical and start over analyzing everything it is all very dependent on the Modulus of Elasticity (MOE) and the thickness of each lam. Like Del points out, a bamboo backing can pull your neutral axis (NA) towards the back of the bow. BUT, this requires that the bamboo be "stronger" than the belly, belly/core materials. For quick and clean examples lets take a bow limb and make each lam an equal thickness and of rectangular cross section. and we will use the following MOE information for giggles: Bamboo 20GPa, Hard Maple 12.6 GPa, Ipe 22 GPa (and for fun Glass at 80GPa which is a rough approximate)
Bamboo backed Maple: Significant shift of NA to the back of the bow, glue line is under compressive force. This puts some compression force on the belly side of the bamboo
Bamboo backed Ipe: Slight shift of NA to the belly of the bow, glue line under tensile force. Some tensile force on the back of the Ipe lam.
Now lets look at some Trilams with three lams of equal thickness and rectangular cross section.
Bamboo backed Ipe, Maple Core: Slight shift of NA to the belly, majority of the core is in tension. one glue line is in tension, the other is in compression.
Bamboo Backed Ipe, Ipe core: Large shift of the NA to the belly, probabably right next to the ipe-ipe glue line in the core. Core is mostly under tension. One glue line firmly in tension, second glue line near neutral.
Bamboo backed Maple, Ipe Core: Large shift of the NA to the back of the bow, probably near the glue line but in the ipe core. Core is mostly under compression.
Glass backed Glass, Maple core: NA at center of the limb, however due to the discrepancy in MOE the maple carries little force, that said there are a large number of shear forces due to the distortion that the maple must undergo in movement.
That is a horrifically simplified version of the relationships and assumes that all of the material properties are equal throughout the thickness of the material (bamboo in particular). Rounding the belly of the bow will shift the NA to the back, using different thicknesses in relation to one another changes it as well.
Adam
Very thought provoking, thanks for posting! Am I the only one loving this thread...
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Very thought provoking, thanks for posting! Am I the only one loving this thread...
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You are not alone. Neutral plane topic is always fascinating, particularly when it involves glue joints themselves. Theoretical stuff aside, I'm finding everyone's hands-on experience especially helpful. .Ron