Primitive Archer
Main Discussion Area => Bows => Topic started by: stuckinthemud on July 17, 2021, 11:58:32 am
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A comment by Bownarra made me stop and think. I've never built a bow with trapped back or belly. How do you do this ? I mean how far over weight and at what point in the tillering process? I'm guessing at a good overall tiller at half draw length and maybe 20lb over target draw weight?
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The reason for trapping is to compensate for the difference in the strength of the back or belly so in that situation I think trapping from the start is the way to go. Most woods are stronger in tension than in compression so most trapping with wood bows would favor the belly making it slightly wider than the back.
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Here's what I said in another thread:
"I've done a lot of reading about lab testing of woods and have "trapped" a lot of bows, ash in particular. It seems that wood stretches only about 1% of it's length before breaking without yielding first. The only clear gain from "trapping" the back is in the reduction of mass in the limbs.
That is a gain, I admit.
As for ash being stronger in tension, every wood is, though some are worse about giving no warning before a catastrophe. In fact most woods are 3 to 4 times as strong in tension as in compression."
I made a small pyramid bow with a strip of hickory for backing. The hickory was only as wide as the limb tip (3/8" wide from fade to tip). It's a snappy shooter.
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My usual strategy has been to narrow the width to 60% of the total width. I will get the bow floor tillered or as soon as the tiller is even, I will remove the wood for trapping. I will mark around 1/8” or a bit more from the belly as the “depth” of the trapped section. That leaves enough room for tillering without removing so much thickness that the belly has a knife edge where the belly and sides of the trapezoid section meet. As mentioned above, wood is stronger in tension than in compression but it’s always better to err on the side of the back being stronger.
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I made a small pyramid bow with a strip of hickory for backing. The hickory was only as wide as the limb tip (3/8" wide from fade to tip). It's a snappy shooter.
Jim,
What were the dimensions of the limbs and the strip? Have any pictures you can share of this? It certainly is an easier way to gain the same end result as trapping.
Mark
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i have never trapped a bow,, )P(
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I made a small pyramid bow with a strip of hickory for backing. The hickory was only as wide as the limb tip (3/8" wide from fade to tip). It's a snappy shoot
Jim,
What were the dimensions of the limbs and the strip? Have any pictures you can share of this? It certainly is an easier way to gain the same end result as trapping.
Mark
It was a small bow to be used in the living room. I gave it away recently, so only have one image. The bow is about 3 feet ntn, 1" wide at the fades and maybe 3/16 thick. The hickory backing is only 1/16 thick.
I made arrows that had bulbous tips so a miss wouldn't damage the sheet rock. At least that was my vain hope....
I don't recommend this approach. It was an experiment.
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When I make a bow I plan on trapping, I do it before it goes on the tillering tree and before I know how heavy the draw weight is. It's part of the initial shaping of the bow.
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Most woods are stronger in compression, but not every wood. Cherry, juniper, incense cedar and perhaps most conifers are stronger in compression. Though I would never trap the belly in these, just back them
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I trap the back when the bow comes off the form, but I check the limbs for straightness before I trap. When I think of trapping Black Locust always pops up in my mind.
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On white wood bows,ash,hickory,walnut,red oak etc I trap the back as part of the initial forming. Once tillered to brace height I heat treat. If I scrape quite a bit off I’ll heat treat again near final tillering. I rarely have to heat treat the second time.
This has worked pretty well for me.
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Most woods are stronger in compression, but not every wood. Cherry, juniper, incense cedar and perhaps most conifers are stronger in compression. Though I would never trap the belly in these, just back them
Brain hiccup, right?
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Yes Jim, I think he meant say tension.
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What has been said above ^^^^^^
It is the 2nd string bow woods that you notice the difference with most (and b.locust) :) woods that people don't use much because 'they take too much set'. Red oak pops to mind....out of the native woods to here fraxinus excelcior really likes a good trapping ;) I once made an awesome little flight bow from heavily trapped ash, it was one of the best wooden flight bows i've made.
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Trapping reproduces the effect of a high crown small diameter tree. Some species, such as Elm, make a better bow when they have a high crown. One thing to remember is that if the wood you are trapping has very fine rings then it's more likely to pop a splinter
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Does anyone have a list of species that work better when trapped or as a small diameter tree or sapling?
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I have trapped a number of Osage bows with decent results. I believe Marc nailed it. It reproduces a small diameter crowned effect. I will often not temper these, as I question if having done both (crowning and tempering)might take one a bit too far. Just my sense about it, nothing more.
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tension vs compression observation, a bit of an aside
I recently found a number of well weathered but dry birch sticks. when bent to failure, all broke in tension before taking any set.
Of course, as bowyers we often cite the general necessity for using un-weathered wood, but the thought came to mind as I broke stick after stick.....
if designing for proportionally high tensile stress, extra attention should be given to wood soundness on the back
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Jim Davis, yes. The first compression means tension. Need to proofread.
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I’ve never trapped a bow but have noticed naturally trapped bows(small diameter) have a advantage in speed with elm and even hickory. Perhaps the whole idea of de-crowning these woods that are stronger in “tension”(got it right this time) is a bad one.
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No question in my mind that you are right. Smaller diameter crowned bows are noticeably better performers. Decrowning might make the back less likely to break, but the net benefit in my mind is a loss in cast. I am speaking only about Osage. Can’t recall trapping anything else. Persimmon on the other hand can’t withstand the strain. High crowns have snapped every time on me.
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Perhaps the whole idea of de-crowning these woods that are stronger in “tension”(got it right this time) is a bad one.
De crowning until the back is as wide as the belly?
you could decrown a narrow width on the back and essentially have a trapped bow. a slight decrowning as such would still spread the max stress evenly across whatever width chosen instead of concentrating it along the peak of an uncrowned back.
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My sample size is way to small to draw any conclusions, but a bow I made from a 1-1/4" serviceberry stave held on the crowned back, but chrysalled on the belly.
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With white woods for me belly heat treating, and trapping the back has really prevented set ,and fret. Before learning how to do both properly I was making far to many bows with set, and fret, and the bows were lazy. 100+40=133 to 136 fps. Just finished a Black Walnut bow at 36 lbs. for target shooting that shoots a 350 gr target arrow at 154 fps average at my very short 25 inch draw. Nothing like DC's bows , but sure puts a a smile on my face. This one is fire hardened, and has an 8 strand D 97 string with very slightly reflexed tips. No set, and tunes good ,and when you unstring it it bounces right back to it's original profile. With Black Walnut that is quite an accomplishment for me.
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Great speed, Bassman...hoping to get there, as I don't even draw an honest 27 with a Redwing recurve. SHorter draws like more efficiency.
Build mostly hickory, always trapping. Even applying sinew, keep most of the "umph" on the thin layer(s) down middle.
Once at first low brace, running the knife edge down the sides, and then sanding.
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Stay with it StickMark you will.
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Just finished a Black Walnut bow at 36 lbs. for target shooting that shoots a 350 gr target arrow at 154 fps average at my very short 25 inch draw.
That's very decent for a 25" draw. A longer draw length is a big advantage when looking for speed. The testing I have seen people do on FG bows shows every inch of draw adds significant speed when keeping arrow weight at a constant GPP.
Mark
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Yes , my son with 28 plus inches of draw with glass bows shoots up to 25 fps faster with the same glass bow ,and arrow depending on the bow.
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I trap most all my bows these days. Some take less set . I think it’s more about density as far as the need for the trap as in white woods. I still trap my Osage. But I think it’s back to density on all woods.
Arvin