Primitive Archer
Main Discussion Area => Bows => Topic started by: mmattockx on March 30, 2020, 01:14:48 pm
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If one were to build a tri-lam (or more) bow it would provide the opportunity to pick each lam with just one job in mind. The back only handles tension, the belly only handles compression and the core really is just keeping the belly and back apart and in the desired shape.
With that in mind, what are the best wood choices for each layer? It seems to me that bamboo or hickory would be the choice for the back, but what about the belly? What woods are best in compression? And what wood is the lightest that will carry the shear forces in the core? Would black poplar be enough or not?
All thoughts, experiences, experiments are welcome.
Thanks,
Mark
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Bamboo-yew works good. I'm going to go out in a limb here and say that Yew may have the best weight to compression resistance ratio(if that makes sense :D) I know that the mass principle says that the mass of the wood doesn't matter, that you will use less of the more massive wood, but in my experience a lighter bow will be a faster bow. That may have something to do with my tillering abilities but there ya go.
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I have had some lams picked out setting here for a while , I choosed hickory for the back, maple for the core & osage for the belly I have made self bows out of all of them but never combined them, my thinking is hickory great in tension ,maple great light weight stable core & osage great in compression
There are other woods I could think of but those are the only prestine strait grained woods I have ! But would have to build one to find out :D
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How thick are you planning for the belly lam?
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Don I think you where asking me ? I haven't calculated the stack yet but I was going to pre grind the tappers before glue up is my plan keeping the belly as light as I can with enough margin for error & post tiller I'm still working on the osage/sinew shorty maybe that one next ! I was thinking reflex LB vs D/R !
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My opinion. Osage and boo is unbeatable on every front. I've made some sweet backed yew bows, but I've had just as many that didn't work out. Yew can be finicky in board form. The last one I attempted was just a few weeks back. I had the bow perfectly floor tillered to maybe 50# at the most. The second I low braced it several paper thin rings wrinkled right up, well, like paper flaking away. That was a waste of my time. I've had that happen enough that I wont use yew on the belly anymore. For me it will be osage bellies and nothing else. I'll use yew for a self bow any day, love the stuff.
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Bamboo-yew works good. I'm going to go out in a limb here and say that Yew may have the best weight to compression resistance ratio(if that makes sense :D)
That would be specific compression strength (strength/lb of wood) and it makes perfect sense.
I have had some lams picked out setting here for a while , I choosed hickory for the back, maple for the core & osage for the belly I have made self bows out of all of them but never combined them, my thinking is hickory great in tension ,maple great light weight stable core & osage great in compression
There are other woods I could think of but those are the only prestine strait grained woods I have ! But would have to build one to find out :D
Well, it sounds like I will just have to wait until you build it and see since you are working on pretty much what I was thinking of. Don't take too long to get that one going!
My opinion. Osage and boo is unbeatable on every front.
I was afraid of that, because osage might as well be unobtanium where I am located. Unless I can find someone to cut a few boards of it 30"-36" long for me to rip laminations off of and ship them here I will never get to touch the stuff.
Mark
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There are a few places on-line that will ship osage slats to you.
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Don I think you where asking me ? I haven't calculated the stack yet but I was going to pre grind the tappers before glue up is my plan keeping the belly as light as I can with enough margin for error & post tiller I'm still working on the osage/sinew shorty maybe that one next ! I was thinking reflex LB vs D/R !
Yes. What I was asking in my peculiar way was how thick to you try and make sure the belly lam is when the bow is finished. Just curious how much belly wood you need for it to hold up.
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My opinion. Osage and boo is unbeatable on every front. I've made some sweet backed yew bows, but I've had just as many that didn't work out. Yew can be finicky in board form. The last one I attempted was just a few weeks back. I had the bow perfectly floor tillered to maybe 50# at the most. The second I low braced it several paper thin rings wrinkled right up, well, like paper flaking away. That was a waste of my time. I've had that happen enough that I wont use yew on the belly anymore. For me it will be osage bellies and nothing else. I'll use yew for a self bow any day, love the stuff.
That's really strange, I've never had a belly problem with yew. With me trying high reflex boo yew I've been expecting/waiting for chrysals etc but it's been bullet proof. I suppose it could be a board thing but that doesn't really cut it with me, a board is just wood. That's a puzzlement.
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diffuse porous woods for shear. maple is often cited, but I do not know why birch would not work just as well. edge grain instead of flat grain might be a worthwhile consideration if you were considering something like spruce or pine. Lots of nice lodgepole around nowadays.
How much work do you expect to get out of the tension side?
If the superior elasticity of yew for a belly is to be utilized, then it would make sense to me to try to use a similar density backing. Of course you could always use less of (trap) a higher density backing if you wanted it to store more work, but you would also be at more risk for stress concentrations at pin knots or other inclusions.
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edge grain instead of flat grain might be a worthwhile consideration if you were considering something like spruce or pine. Lots of nice lodgepole around nowadays.
There is lots of lodgepole pine around here and I have access to a fair pile of it that a friend is clearing from some land. When you say edge grain I assume you are talking about quarter sawn with no bias?
How much work do you expect to get out of the tension side?
My goal would be to have a consistent safety margin on both back and belly, so thicknesses and/or widths would be adjusted to achieve that balance.
Mark
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There are a wide variety of belly options depending on what exactly your looking to get out of the bow. Osage is always a spectacular belly wood, it will make anything you want and seems to be consistently “good” for lack of a better work.
One of my favorite belly woods is black locust. It grows straighter than Osage which makes finding good grain pretty easy. BL is lighter then Osage, but with a good heat treatment it becomes very stiff and springy, similar to Osage. Osage is a more elastic wood, but black locust is really hard to beat for an all around good wood. I have had some black locust that was lackluster, but most often it is good. I’ve backed BL with bamboo and maple, I believe that maple is a better choice of the two but both work very well.
There are MANY belly woods that I have yet to try out. Two of them on my short list are maple backed black walnut and maple backed cherry. I believe both will make lightweight sweet shooting bows. Heat treated white woods also make for good belly woods. Hickory, ash, birch, hop hornbeam,hackberry and hard maple all come to mind.
The cores in my tri-lams are always hard maple. Maple is strong and lightweight, proving to be the perfect qualities for a good core wood.
Laminated bows are a lot of fun from the almost endless combinations that can be made. They are completely different then building selfbows, but both are great in their own way.
Cheers,
Taylor
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Osage is avAilable in Canada,albeit expensive....my elm stash is really low .
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I’ve backed BL with bamboo and maple, I believe that maple is a better choice of the two but both work very well.
Why do you prefer maple over bamboo for the back?
Osage is avAilable in Canada,albeit expensive....
I suppose that is true of most things. Where do you get yours from?
Mark
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When you say edge grain I assume you are talking about quarter sawn with no bias?
yes, and some bias is probably Ok too. If you say "quarter sawn" to the guy at the specialty hardwood store he might show you a very expensive board. avoid ring lines parallel to the gluelines, the more so if early/late ring densities vary much.
black poplar is cottonwood? I have seen some that is very soft.
I would be curious to know if many trilam builders find a compelling benefit for optimizing the middle lam over building a with just two lams?
some guys get by with minimal tillering by carefully tapering the middle lam while controlling lam tolerances with thickness sanders.
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yes, and some bias is probably Ok too. If you say "quarter sawn" to the guy at the specialty hardwood store he might show you a very expensive board. avoid ring lines parallel to the gluelines, the more so if early/late ring densities vary much.
I asked because I have a good supply of decent lodgepole pine and would be cutting the pieces myself. I would never dare to specify something like that at a lumber store because of the price tag, as you note.
I would be curious to know if many trilam builders find a compelling benefit for optimizing the middle lam over building a with just two lams?
I expect there is not a significant (or even measurable) performance benefit to using different wood for the core. There may be a cost savings and aesthetic benefit of being able to match contrasting woods and/or grain, etc. I can see a construction benefit of easier bending multiple thinner lams onto a form and a structural benefit in being able to run the belly lams up onto the handle like fibreglass bows do to avoid having the handle pop loose when scabbed onto the back of the lams.
Mark
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My most trusted, durable, and favored trilams are bamboo/osage/osage. But sometimes I like to mix it up a little.
Bamboo/yew/yew is excellent when it survives, but I've had a couple of compression failures with what appeared to be superb, clear, flat and quartersawn yew in trilams where I didn't think it should have been an issue. ...when I've had more highly stressed yew trilams survive just fine. I don't yet understand why.
Bamboo/yew/osage is a really good combo, but others work too.
I taper the core lam, and leave the belly lam parallel to allow wood for tillering, weight reduction, shaping, etc. That said, my goal is to get as close as possible to the needed stack height, with a little bit of working room, so that there won't be a need to remove much wood. Good tapering means little removal of material for tillering, and good stack height means little removal to hit weight. Less wood removal overall helps avoid backtracking and some possible hurdles. The bulk of the work with them is best done in preparation.
Of course it also depends on width, length, cross section, and other design features, but osage core and belly lams of .200 to .220 can make a 55-60# bow easy enough. That's with the core lam tapered .003 to .0035 per inch.