Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on June 28, 2017, 03:18:28 pm
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I'm starting on my first Black Locust bow. I've read that it should be trapped. How much is the norm for trapping? Make the back "x" inches narrower than the belly. Solve for "x" :D
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Do you want to trap it? Or, do you think you need to? The latter would be very false.
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DC, I'd say 1/8" total or 1/16" each side but I doubt it is anything cut in stone.
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I have never done any trapping, so not sure if it is necessary, but am open to learning something here,, (SH)
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It's just that this is the first time I've made a bow from wood that I'd heard could use trapping and I had no idea how much. I'm used to high crowned wood ans this is also the first no(well almost) crowned wood I've had. The back is flat so if BL does want trapping I think this is the piece. I'll just slope the sides a bit. :D
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I'm in for learning to I want to try trapping hickory !
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maybe on a design that is a bit aggressive or overly stressed it would become more necessary,,,, but I am just guessing,,
I have a black locust bow,, 20 years old and still holds straight profile,, it is wide and long,, overbuilt a bit,, (AT)
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I have done only one BL bow from a tree about 5 inch dia. the belly is 1/4 wider than the back. I think a low crowned one would want the most trapping.
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I did a yew bow this year and trapped it instead of thinning the sap wood. I've done several, usually just high tension bows. I just make my bows as normal, parallel/square, and when I get to about 10 lbs. over the weight I want, I taper the edges 45 degrees, narrowing the back. I've also tapered early and kept it going during the reducing/tillering. Either works depending on the wood. But even on a square bow I tend to roll all the edges so I have no sharp lines, so I get some tapering on the back and the belly. Point being, for me, to really be trapping I have to go a full 45 degrees or more because by the time I round everything and sand it down the edges make radius that blur those lines. So make the trapping obvious so that even after final sanding, it is still obvious.
The trapping can/should happen where the limb works the most, performance-wise, but you can carry the tapered edges clear to the handle and tips if you want that look.
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Reduce the back to about 75% of the width of the belly. Trapping on such a stave (if the wood is 'average') with a flat back will reduce set.
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Trapping makes sense on high stressed bows, as short limbs, narrow limbs, high stressed designs.
On wide, normal length, normal design - I wouldn't do.
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As suggested by the late and great Forrest Nagler, trapping should be between 10-15% of the bow's width. That means that the trapping actually changes with the width of the bow at that point - greatest at the fade, least at the nock. And, technically speaking, I think that every bow, no matter the wood, would receive benefit from trapping since most woods (if not all?) are stronger in tension than they are in compression.
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define benefit,,,,,I dont disagree,, just dont understand (--)
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Trapping is one of those bow building things you simply cant prove, but only believe it helps.
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Wouldn't heat trapping the belly accomplish the same thing ?
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Yup. That's why most of us temper locust, elm and hickory.
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Ok just a thought trapping can be done prior to tillering the bow leaving the belly less strained as where belly tempering is usually done at some point with tillering already under way would there be any advantage of doing both on the above mentioned woods ?
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for example is someone said,, I had a bow not trapped,,it shot 150fps,,10gpp
I trapped the bow ,, now it shoots 10 gpp 157 fps,, I would start to be more interested,,,
I have not heard Badger say much about trapping, he is my voice of reason most the time,,,,
Pearl seems to be right on,, as well,,
I am still listening for the "benefits",,,always willing to learn ,, just not easily convinced at this point,, (-P
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I think it just helps prevent chrysals. I don't see how there would be much performance gain, if any.
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for example is someone said,, I had a bow not trapped,,it shot 150fps,,10gpp
I trapped the bow ,, now it shoots 10 gpp 157 fps,, I would start to be more interested,,,
I think trapping could only decrease actual bow poundage and speed (you're taking away a consistent part of the bow) but you'll end up with a more balanced bow.
A less stressed design for those woods that are more strong in tension than in compression.
After a decent number of shots your trapped bow may actually shoot better than your stressed untrapped design
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If you where splitting hairs on a more stressed design bow one trapped the other not and both bows same draw weight the trapped one would be less mass , it's like any thing with this bow designs I'm going to have to try one with hickory to find out keep us posted on yours there DC. Interesting topic !
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it prevents chrysals but is a narrower back more prone to splintering/breaking in tension? i think trapping would move the neutral line further from the back, so it stresses the back more..?
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It doesn't prevent frets from happening. A solid design and tiller job does that.
It doesn't stress the back more, it wakes its lazy butt up and tells it to work equally with the belly. Proper tempering does the exact same thing.
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If tempering equaled trapping I'd much rather trap because its much simpler and less ambiguous.
Brad--Its not that you trap an efficient bow and it suddenly becomes more efficient. Instead, you take an otherwise inefficient design and make it more efficient. Its no less theoretical than tempering where we find the constant questions about which wood should be heat treated how much or maybe it broke because of the tempering.
We already know for sure that alot of wood types are tension strong relative to the compression side so we know that making the tension side more narrow balances the two out so that you get to remove mass from the bow in order to achieve a better balance.
If you took an otherwise efficient bow and then tempered the belly the only way that improves it is if you have now raised the draw weight so that you get to remove mass in order to bring it back to weight. Its the same principle, as far as I understand it. But my experience is that tempering is more subjective.
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I guess since I work mainly with osage,, trapping has not been needed much, if there are other woods that benefit,I just dont have experience with it,, it sounds interesting,, (SH)
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The gray area where the 2 methods overlap in benefiting bows is that number 1 in trapping you relieve the belly from compressing while tillering taking the back down to the bellys' compression level.Number 2 heat treating the belly makes it denser or harder to with stand the compression better to balance it with the back.A flip flop version of getting a balancing act of the 2 forces.Although a person has to remember heat treating does not make the wood more elastic.In fact less.Keeping a wood elastic is good.Trapping will still keep the woods elastic qualities.Both will make the limbs lighter but I would say heat treating and retillering does a better job of that in the performance arena because of the limb mass lost if the design is not too extreme or to the edge to risk tension failure.Good osage does not benefit as much as it is just plain too dense already.
That's my take on it as I rarely trap any wood,but do heat treat a variety of white woods.
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The key to effectively using both of these techniques is to first understand the properties of the particular wood you are working. You just can't generalise here.
As mentioned both trapping and heat treating are methods used to help 'even out' the forces in tension strong / compression so so woods. Trapping takes 10 mins and is easy to be precise. Heat treating is as mentioned above a bit more subjective and takes longer (not that time taken bothers me!).
The bend test mentioned in TBB is a great starting point to determine how your wood behaves when bent. As are half size/ scaled small bows.
As for not being able to test the effectiveness of trapping or heat treating.....take two slats of your chosen wood one heated or trapped and one not....bend them and look at results.
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great topic - learned lots already, hoping for more.
Cheers,
George
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define benefit,,,,,I dont disagree,, just dont understand (--)
The neutral axis of a bow limb does zero work (imagine a line drawn through the center of mass of a cross section of bow limb), so the idea is to get as many fibers as possible away from the neutral axis where they do not work, but instead sheer... This sheering of the neutral axis combined with the compression of belly fibers and stretching of the back is what contributes to string follow.
One way to reduce the neutral axis is to hollow out the sides of the bow limbs somewhat by shaping them into a heavy "I-Beam" cross section. However, given the difficulties in the construction of a bow limb of this shape, the same result can be had by giving the bow a trapezoidal cross section.
In doing this, the stronger back of the bow is reduced, evening out the forces between back and belly, and reducing overall fiber stress of the bow limb.
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Ok just a thought trapping can be done prior to tillering the bow leaving the belly less strained as where belly tempering is usually done at some point with tillering already under way would there be any advantage of doing both on the above mentioned woods ?
Trapping a bow should always be done before tillering! If not, then excessive fiber stress has already occurred, and the neutral axis has already taken excessive shearing forces. Doing both? I don't know, try it and let us know!
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Contrary to a post above, Badger has had a lot to say about trapping. He and I have also had lots of off-forum exchanges about it.
Bottom line, any improvement in performance is probably because of less mass in the limb.
The thing is, a trapped back does not stretch more before breaking than a full back. The back, if intact, will always fail AFTER the belly fails. Test results I have seen said that wood stretches only about one percent before failure.
Statements above about relative strength in tension and compression are correct. Almost all wood is at least 3 times stronger in tension than in compression. So, reducing the width of the back is still going to leave the back far stronger than the belly. BUT, narrowing the back in an area where the width tapers makes the grain run off the sides even more abruptly in that area. I have had several failures where the surface lifted where the "trapped" limb narrowed at the tip.
I think there is merit in the idea that string follow might be largely the result of shear in the neutral plane, but I have no evidence or even a way to get evidence.
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ok I would still like to see some specs on how a trapped bow performs,,
has anyone done any testing,, (AT)
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BUT, narrowing the back in an area where the width tapers makes the grain run off the sides even more abruptly in that area. I have had several failures where the surface lifted where the "trapped" limb narrowed at the tip.
I think there is merit in the idea that string follow might be largely the result of shear in the neutral plane, but I have no evidence or even a way to get evidence.
Very true, trapping a bow does make the grain run off the sides more abruptly, but rounding the corners of the bow reduces the tendency for splinters to lift somewhat, but this technique may be better suited to backed bows if grain runoff is a major concern.
As to the evidence of sheering forces in the neutral plane contributing to string follow, I wonder if we can get a bowyer willing to attempt Forrest Nagler's "Heavy I-Beam" construction a try? Any takers???
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ok I would still like to see some specs on how a trapped bow performs,,
has anyone done any testing,, (AT)
You and me both... The test would require two bows be made from the same tree, and given the exact same construction parameters, except one bow, before tilling is given a trapezoidal cross section where the back is given about a 10-15% reduction in width compares to that of the belly. The comparison would have to be made on a force-draw curve to demonstrate each bow's efficiency.
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I dont really need a compison, that would be nice,,
but just to see how a trapped bow shoots,,
can they shoot as well as normal self bow,,the same,,, better ,,, something???
If I had a trapped bow, I would shoot it with a 10ggp arrow just to see how it was doing compared to other self bows in general,,
:-D
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I dont really need a compison, that would be nice,,
but just to see how a trapped bow shoots,,
can they shoot as well as normal self bow,,the same,,, better ,,, something???
If I had a trapped bow, I would shoot it with a 10ggp arrow just to see how it was doing compared to other self bows in general,,
:-D
I don't know that you'll see a difference in cast... Trapping the back won't make it shoot any faster, it's just a method intended to prevent uneven fiber stress. If it's something you want to find out about, just build one and then tell us about it.
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I trapped all my bows for a decade or so. I could not compare them with untrapped, but all seemed to have good speed and low hand shock. The latter I attribute to the fact that they were all pyramid design as well as trapped.
If I had access to a chronograph, I'd test a bow, then trap the limbs and test again--but I don't.
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I see your point,,
I just see alot of speculation about the benfits of trapping,,
but no stats to back it up,,
I dont need to build one to find out,,osage really doesnt need it ,,,was curious if anyone claiming benefits from trapping had ever tested a bow,,
maybe they shoot the same as a regualar back self bow, but who would know,
if there is no difference in cast, that would be nice to know as well,,,, (AT)
if you can get a bow from what might not make a bow, even if the cast was not so great, that would still be a bennefit,, :)
thank you Jim we crossed post,, thanks for the input,,
I am understanding more and more,, (--)
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I think it goes in the durability file. It's like making a questionable stave a little longer or leaving a knot a little wider. Little things that you hope will make the bow hang together a bit better. I know for a fact that with my variable release I would never see any dif with a chrono.
"Variable release", sounds like a good thing, doesn't it. Good sales pitch ;D ;D
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"Variable release", sounds like a good thing, doesn't it. Good sales pitch ;D ;D
I call my "Variable Release" by another name... Usually it's "what the %$&# was that?" :)
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yes my release is a work in progress,, after 50 years still )-w(