Primitive Archer
Main Discussion Area => Bows => Topic started by: willie on June 15, 2020, 12:16:38 pm
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A question raised in another thread made me think a bit about trapping.
what are the pros and cons?
are there different ways to do it?
how effective have you found it with self bows? and lam bows?
My experience is limited to high crowned natural back staves, but I do have some theories about things I have not tried.
My experience with sapling staves indicates any kind if imperfection at all along the peak of the crown can be fatal. should I try to decrown next?
Edit title to include decrowning
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Not to many cons if you don't do it to the extremes which I have done ,and broke the back of the bow. You can use a file ,and eye ball it, or scribe a line on the back of the bow ,and follow the line. It is very effective with Black locust bows. Most bow woods are stronger in tension than compression thus belly heat treat ,and trap, but it is a balancing act.
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Trapping gives the bow limb a trapezoidal cross section favoring either the back or the belly. It can work especially well with selfbows and wood backed bows by increasing or decreasing stress on the limb by favoring the back or belly. Most woods are stronger in tension than in compression and by trapping in favor of the belly(wider) you can increase it's compression strength or at least add to it's durability.
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Not to many cons if you don't do it to the extremes which I have done
Bassman, did you file off the top of a small diameter stave (decrown), or slope the sides of a natural back, maybe from a larger diameter tree?
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Do not touch the back of the bow. File from the sides,and if it is a high crowned stave it takes very little filing. With a flatter stave a little more trap. Black Locust I have trapped as much as 3/16's of an inch with a flatter back. If their is a crown 1/8. I Just broke an Elm bow by taking to much off the back. It had one weak spot about 14 inches down on the top limb. Elm is strong ,and I was reducing weight from the sides, and got carried away.
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Saplings are trapping taken to an extreme and yes de-crowning any iffy saplings is a very good idea. Just make parallel lines on the back and you have essentially 'chased the ring' leaving a stronger back than if you left it alone.
Trapping is best utilized on 'normal' split staves.
As mentioned virtually all wood is stronger in tension.
You are trying to match the woods abilities to resist compression or its ability to resist tension. Like a childrens see-saw works best when you have two children of similar weights.
Doing my test of making a bow, pulling it far enough to induce some set then cutting in half along the neutral plane will really opern your eyes on this one. Do this test with bows that are made from the same wood. Trap one and leave the other normal. Once cut in half you will notice a drastic difference in what shape the bellies are.....
Best to do the tests yourself and come to your own conclusions with this one :)
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Saplings are trapping taken to an extreme and yes de-crowning any iffy saplings is a very good idea. Just make parallel lines on the back and you have essentially 'chased the ring' leaving a stronger back than if you left it alone
Can you elaborate on what you mean by parallel lines? Do you mean if you take the crown off correctly you should see the lines of the growth rings running the length of the back of the limb?
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If I remember correctly from TBB, ring lines running parallel down the crowned back were observed in primitive bows. This should follow the grain well if there is no spiraling of the grain in the stave.
I have always been a bit curious why decrowning is not seen more often. Having a flatter but somewhat wider back than a half round crown, would spread the tension stress evenly, and avoid concentrations at the peak of the crown.
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I'm no fan of decrowning, I always found other things to match a high crowned stave. When the crown is homogeneous, I prefer a HLD (hollow limb design).
Trapping is good explained above by Pat and others. We should have in mind that the most change of woodcells is on the belly side. In other words the springiness of bow is done by the recoverable compression of the belly cells. On the back the cells are of course stressed also, but the stretch is minimal. The cross section of an ELB is a kind of trapping and only a few woods can handle that high stressed belly.
just my thoughts ...
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Pat and I are on the same page on this one! Arvin
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Pat and I are on the same page on this one! Arvin
Hey Arvin, Pat B, Simson, Bassman and all,
Pats explanation is right on, It is the most commonly understood meaning of the term "trap".
I was hoping to enlarge the discussion some and include decrowning and other methods of making the back narrower.
With the primitive bows as an example, how come decrowning is not done more? People just haven't tried it yet?, or are there some things to learn if you want to do it successfully? Please share you experience if you have used this to work small diameter staves.
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I've only decrowned one or 2 bows in many years of making wood bows. I never found a reason to do so and I initially learned a pristine back was a must for a selfbow and it's hard to break old habits. I know that decrowning is an option but it isn't something I consider when planning a build.
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the reason its not done more,decrowning, is its usually not needed,, theoriticly it would do as you say, but the real results not giving much advantage if any,, and probably takes alot of skill to do on most bows,, unless back is pristine,,no knots etc,,
I think you would be ok doing it if you gonna rawhide back or sinew back,, but not gonna see a signifigant increase in performance or durability ,,,thats why you dont see it done much,, yes we have tried it,, not much we have not tried,, or talked someone else into trying,, (-S
you should definitely try it to make your own conclusion,,
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I use very clean wide staves for the most part. But I will make the back as flat as possible yet not go thru the back growth ring. That is why I prefer 1/8 thick or more for my back ring. I am a flat bow guy.
But if you have round back staves they will make successful bows. Usually take a bit more set though.
Arvin
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I've never decrowned a stave. Chicken mostly. It just seems like asking for trouble. I have lost a couple of OS bows that popped a splinter at a pin right on the crown.They may have survived if I had decrowned them but I really don't think they would have survived the decrowning. For wood like Yew that can take ring violations then I can see it being a viable option.
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Usually take a bit more set though.
Arvin
Arvin,
that's comparing identical flat bow cross sections, one made from a small diameter tree, and the other made from a large diameter tree?
or comparing two limbs of a given width, one trapped, and one untrapped?
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DC I have been moving mass around in specifically Osage bows for some years. Trapped back and belly
Even in pyramid bows trying to eliminate set. Put more mass at fades , thicker non bending outer limbs, wide at the fades etc. anything and everything for faster smoother bows. Not sure I have found the answer yet. I have had the time of my life building these bows. Arvin
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Well, lots of right info above, but the facts are even more intense. All wood is stronger in tension and most wood is at least 3 times as strong in tension.
Any gain in narrowing the back of a bow is due to reduced mass, since the limiting factor is already the resilience/strength of the belly.
Decrowning in an effort to strengthen the back is wasted effort on all but very small sapling staves. The back is already stronger than the belly. Except in the case of violated growth rings, bows always chrysal on the belly before they break on the back.
I have a demonstration kid's bow that is 3/4 wide at the fades, tapering to the tips. It has a 1/16 thick hickory backing that is only 5/16 wide the length of the bow. It shoots fine and has no string follow.
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Not sure I have found the answer yet. I have had the time of my life building these bows. Arvin
Arvin.
that's what is all about with a hobby, getting some enjoyment. Since you work almost exclusively with Osage, and it has unique properties when compared to other woods, I suspect when you find the answer, it won't be a "one size fits all" kind of discovery.
Any gain in narrowing the back of a bow is due to reduced mass, since the limiting factor is already the resilience/strength of the belly.
So in the case of a wood lam bow, a lighter in density, (but full width) backing will accomplish the same effect? I think it could actually work out better than narrowing, as the back strain can be spread out over a wider area.
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I use decrowning for OS. It helps me to get a wider stave. I can take a stave that I would only get 40lbs from and get to over 60. This also solves problems with pesky pin knots. I then back with rawhide or silk. I just eyeball it, you can't follow a ring as I do with hazel. I would like to try vine maple at some point.
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I use decrowning for OS. It helps me to get a wider stave. I can take a stave that I would only get 40lbs from and get to over 60. This also solves problems with pesky pin knots. I then back with rawhide or silk. I just eyeball it, you can't follow a ring as I do with hazel. I would like to try vine maple at some point.
Is there a particular diameter OS that you decrown or do you just do all of it?
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following ring lines in birch is challenging as they are not very apparent to the eye, I am still experimenting with some stain on a rag between scrapings.
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If trapping balances the back and belly better is there any chance that trapping an existing bow a bit won't affect the draw weight? I know that seems like a free lunch but i thought I'd ask :)
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I leave saplings a few inches longer to counteract the crown. What longer means depends on your draw length. Jawge
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If trapping balances the back and belly better is there any chance that trapping an existing bow a bit won't affect the draw weight? I know that seems like a free lunch but i thought I'd ask :)
Could be, within limits. Might be a good experiment for somebody.
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One scrape at a time until I see what's happening I guess :)
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If trapping balances the back and belly better is there any chance that trapping an existing bow a bit won't affect the draw weight? I know that seems like a free lunch but i thought I'd ask :)
This is from my engineering experience, but I know of no way to remove material from a beam (really from the cross section) and not reduce its bending stiffness. Trapping 'a bit' won't do much in terms of reducing belly strains and it also won't make the limb much weaker, either. The two go hand in hand, so the more you help the belly wood the weaker the limb will get.
Mark
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Thanks Mark. I was thinking that since the boo is so strong that the "beam" might not notice if a bit was missing ;D. Just going for a lighter limb. I will trap the recurves. I've done that before. I guess because they are static(strong)they can stand to lose a little.
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If trapping balances the back and belly better is there any chance that trapping an existing bow a bit won't affect the draw weight? I know that seems like a free lunch but i thought I'd ask :)
Think it would really depend on how much you trap and the woods of the bow plus construction. In theory I would expect most cases to be something like slight DW drop with slight improvements to cast and arrow speed, but not much changed overall. Would definitely be an interesting experiment
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Interesting but unsuccessful. I had a bow at 203 and I had to be greedy. I made a new string, dropping the weight from 57 to 41 grains. Speed stayed at 203. Trapped the recurves/tips, speed stayed at 203. Trapped the last 8-9", just a couple of scrapes. Tested the draw weight. It had dropped 3 pounds and there was a big hinge in the bottom limb. That quick. Chronoed it anyway 199. And again 197. It's going away and it's also throwing the string. Darn. My last good piece of Yew. In hindsight though if it only took a couple of scrapes to ruin it, it probably wasn't going to last anyway.
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Don,
I would agree with Mark in that you will loose poundage.
If the design is one that could have been better if the woods were more closely balanced, then maybe you could see a FPS improvement (with a new arrow at the same GPP), at the new lower weight. I dunno how you would decide whether the improvement, if any, came from a lighter limb or working the back harder, or what proportion of each.
If "maybe you could see a FPS improvement " sounds uncertain, it is because of another factor to consider . If re-working a heavier bow on the sides, the bow will be using the same belly as before. A belly that already took the compression and compaction of a design that worked the belly harder.
tillering the "new" design from fresh could yield more improvement if "balancing", has merit. Of course that puts us back to the "build two identical bows, except..." method of development.
On idea I have considered is doing tests on fresh samples of smaller laminated strips. I would like to try backing possibilities of a more uniform nature than Boo, as it seems to be a composite all by itself.
:( composed but not posted before you posted the previous, Don.