Primitive Archer
Main Discussion Area => Bows => Topic started by: Mo_coon-catcher on January 29, 2016, 09:21:29 am
-
I finally tried my hand at a yucca fiber backing. I will say that it seems to work. Though I don't have enough arrows through the bow to tell how well it will last. I will say though that for the amount of work involved with laying down the fibers I would rather go with sinew for performance gains. But the yucca is more easily available and easier to process. Buts it's holding and between the heat treating and fibers the strength did increase. I'll probably try it again, but apply it green gluing down with hide glue and see of Itll pull some reflex when it shrinks as it dries.
It's an osage mollegabet, heat treated and yucca backed, that came out 30# @ 27". Starting with 3/4" reflex, 1/2" deflex after shooting, and flat after resting. Not too bad for taking 30 minutes making brace tiller, and another 30 minutes getting it shooting after realizing it was so far under weight. I know the tillers not perfect, but I didn't want to lose any more weight and it shoots well enough as is. But please point out anything that's off about it. I split out the stave in November from a fence post in assuming was cut in September/October, and carved out the bow about three weeks ago. So I rushed it a little. Sorry about the lack of pictures. I'll put up more once I get it fully shot in and finish put on. It's still naked so don't want to show I off too much.
It came out under weight after fighting with some cracks. I tried leaving one ring of sapwood for the back but due to time restraints the wood wasn't dry yet. So lots of drying checks. I went for a rounded belly to try relieving some strain from the back but once I tried going to full brace and what would have been about a 50# pull. I got one limb bending well and the other a little strong. I scraped the strong limb and it didn't move. Did it again and the same thing. After the third time it looked to be going backwards so I checked over the bow again. And more cracks opened up on the back weakening the limb and making the outer limb bend more than I eanted. Before I realized what was making it weaken I had already unnecessarily scraped a bunch of wood off. I learned a few things from this bow. Dont go for a sapwood back on osage when the wood is still green, it will crack. Watch for checks forming on the back after a sudden shift in tiller. Don't scrape to correct when said shift happens. Yucca makes a reasonable backing with being a radily available material. And it's possible to take light draw osage from braced to shooting in 30 minutes.
Thanks for looking. More pics will come in a day or three. Feel free to criticize as you please. I like to know where I went wrong so I can correct it later and others can learn. More pics will come once she's fully dressed up, only wanted to give a tease while she's still naked.
Kyle
-
Awesome idea! Pic of the back?
-
Yea bud that sapwood is suseptible to crackng when air dried too fast.Now you know.I'm sure it shoots nice for ya though.Nice bow.
A few years ago I backed a KCT bow with 12 ply linen string.Was'nt on the forum at that time so I did'nt take pics but it gained I figured about 1# per 3 or 4 strands of string.Total of 28 strands.So it gained about 7 or 8 pounds.Applied it on with smooth on epoxy.Makes the bow almost indestructable.Yours too I imagine.To top it off I put about a 1/16" of horn on the belly too and it gained another 8 to 10 pounds.
-
what glue did you use for the yucca? If it is readily available, why not keep putting it on? If the belly side is good, the cracks might not matter too much if they were further towards the middle of the bow.
-
Looks fine to me!
If you want to up the poundage, why not add more backing? I guess you can raise it to 40#.
-
Looks good, looks fun. Did you process the yucca yourself?
Also, I know how sinew is used, and I have used plant fiber backings. There are ways to use them to boost performance, but usually if the stave was sub-standard to begin with.
-
The way to help bring poundage up on a bow can be done may ways.The thing about fiber backing is that it will stretch or take set with no return.Once stretched it does not spring back.Even using hide glue won't help that much.Adding extra poundage to your bow can be increased even more somewhat by reverse stringing your bow while glueing the fiber on.
-
Snip a few inches off the ends Kyle. I don't know the length you have, but the pic tells the story. Tiller looks nice for a wavy one.
-
The yucca was processed myself from my moms plants using a rock to pound them and a chuck of steel to scrape away the extras. I let the yucca dry for a couple days then I washed it to re moisten it before applying. Those plants are starting to take over the yard. So if anyone wants to try some themselves, let me know. I used TB2 since I managed to misplace the TB3 and I didn't have a heat source handy to use hide glue. I think I will put some more yucca on and clip the tips an inch each and see what happens. Think I should apply this next layer with the bow reflexed a little to see if that help or leave it straight to reduce some stress since I'll already be stressing it more by clipping it?
Kyle
-
Kyle Ill gladly take some of the bigger 1 inch round or larger stalks off your hands. Nothing better than yucca for friction fire spindle. IMO.
-
That shouldn't be any problem. That's about how large the stalks of these plants get and about 6' tall. I'm not sure if there are any still up. But I'll see if there is when I get home. If not they'll be back this summer.
Kyle
-
Awesome project, thanks for sharing. On the idea of using it green in hopes to gain some reflex through shrinkage, I am not sure that would work. I think the fibres would shrink alot in diameter, but not in length. This shrinking of diameter might mess with the integrity of the fibre/glue matrix. When we make baskets , we dry the material for a year, then rehydrate by soaking for a day. When the material rehydrates, it does not regain its original diameter, so when the basket dries, the weave remains tight. When you make a basket of green material, it shrinks and becomes loose. The same is true for cordage. If you do test out this method, you may want to do a small tester piece first and please post results if you do.
-
Your bow is reinforced very well with your yucca on the back.Reflexing it 4 to 5 inches won't break it when rebracing and shooting it.Even after piking it.Bet in the end after piking it and reflexing it you might be pleasantly surprised to be in the mid 40's or so in poundage.Not much to retiller either if applied on both limbs evenly.
-
I got some more yucca scraped clean yesterday after putting down some sinew on one of my trade bows. There should be enough to put down a good heavy layer. I'm going to give it a few days to dry out before I put it on. Sorry pearl drums, but there wasn't any stalks left on te plants. I'll remember ya when they grow back this summer. Once I get this now all dinished I'll post some more pictures of it.
Kyle
-
Is it just the stalks you use or are the fibers in the leaves any good? We have two yucca plants so we get two stalks a year. A good year, yucca is iffy for blooms up here.
-
It's the fibers in the leaves that I used. After scraping the leaves to get clean fibers I let them dry. Then I rehydrate so they are pliable again then lay them down like sinew fibers. This is something I'm going to have to play with more. I have a couple bows that need a backing and are a little too wonky to use a wood backing.
Kyle
-
Ah, I was confused by the fire starting thing. I had to re-read the whole post to figure that out. :-[ Thanks
-
The bow looks nice ... but,
I can't see a good reason to back a bow of that length and I don't believe the yucca fibers are enough strong for a backing.
-
I agree, Normally I wouldn't have worried about a backing on this bow. But it had some cracks on the back that kept wanting to open, even when filled with super glue. And I've been wanting to try yucca backing for a while. I want to do a good test for yucca as a backing. I've got some osage boards with some bad grain runout. Do you think it would be a good test on it as a backing if I make a bow from the osage board and back it with yucca fibers?
Kyle
-
I can't see a good reason to back a bow of that length and I don't believe the yucca fibers are enough strong for a backing.
Don't underestimate those fibers. Plant fibers such as sisal (and I reckon Yucca should be very close to that) are stronger than most woods, certainly than osage. And they can stretch quite a bit further than wood. I'm rather convinced it can make an excellent backing.
Joachim
-
Make a piece of a string same diameter, one sinew one yucca and test out.
Or take a slat back it with sinew and another with yucca same thickness.
I think I can estimate what the result is ...
But back to drying cracks on a bow's back. If the wood is seasoned the cracks will be stable, best is fill with super glue. When the stave is still wet, it doesn't help - the crack will increase. Tell what a backing should do when having the fibers layed down in the same direction like the crack?
If layed perpenticular o.k., but then it doesn't work as a backing and you only add excessiv mass.
I'm still not convinced this is a good idea.
-
Make a piece of a string same diameter, one sinew one yucca and test out.
Or take a slat back it with sinew and another with yucca same thickness.
I think I can estimate what the result is ...
Simson
I wasn't comparing sinew as a backing to sisal/yucca, but other wood. If wood is strong enough as a backing (in a self bow), then sisal/yucca (which aren't the strongest plant fibers) surely does the trick too.
And as far as rupture under a load is concerned: there's lots of actual data on this stuff, performed under controlled conditions. Sinew is indeed exceptional in that it can stretch up to 15-30% before breaking (at high MC), but past 5% it has nonelastic deformation (tension set). So past this stress this extra tension as a bow backing is irrelevant. (see http://paleoplanet69529.yuku.com/sreply/519341/sinew#.Vq-3UIXhBD8)
As a backing on a self bow (no horn belly), sinew will hardly ever be asked to stretch more than 1.5 let alone 2%. It takes less force to stretch a strand of sinew for 2% (150 MPa) than to stretch a strand of sisal for 1% (220MPa) (though it can safely take 2%). So even though you're right to state that the same diameter sinew string will break at a higher tension load (at c. 1200 MPa vs 800 Mpa in the best case for sisal), for a bow backing this crazy elongation of 15-30% is irrelevant.
So in the "take a slat" example the yucca (or at least sisal-backed) bow would be stronger. Less elastic (but elastic enough to back any self bow), but stronger.
You could also use pineapple fibers. They are very strong too (comparable to hemp and flax).
For those interested, I compiled a database on all these properties for all kinds of woods and natural fibers (animal and plant based). Available upon request (and a work in permanent progress).
-
Kyle
if you think that the technical discussion of other backing fibers is beyond the intended scope of this thread, please let us know.
Joachim-
I would be interested is seeing some of your data base work on the fibers especially. I will send a PM.
In these discussions of fiber elongation properties, there is seldom much said about how to determine how much of a particular fiber to use in a given application. If, for instance, a single sinew fiber can stretch 15% before taking set, then it stands to reason that 2 fibers would take the same load at 7.5%, and 4 fibers @ 3.8%, 8@1.9%, and 16@ .9% etc......
where does one optimize the buildup? It seems to me that the benefit of a fiber backing is that it can stretch more than the back could without it. so it need to be thick enough to take the load away from the wood back below, and lower the neutral plane to save the belly some compresssion. However there are diminishing returns to just adding more backing.....
your thoughts please....
willie
-
I'm completely fine with the technical discussion on the backing material going on. I find it interesting. For me, as long as I can make the wood bend and it doesn't go BANG. I'm happy. Even happier when it is done with minimal set and good speed.
Lyle