Limbs cracking at the sinew line

[Mesophilic]

I've had a recurring problem of limbs cracking at the sinew line.  I never had this happen when living in OH or even the desert in AZ. 

So far this has happened to limbs of a yew flatbow, 2 sets of bamboo (asiatic style), two osage flatbows, and a couple of others I can't remember  as they completely came apart and went in the fire place in a moment of rage ; all of which were began here in the mountains of N. New Mexico.  Also, it seems to manifest as I get the limbs down closerto draw weight and start exercising the limbs, though I have had one or two just sitting doing nothing in the living room and I hear an audible snap.

I have a yew and two osage flatbows that were sinewed in OH and AZ, and they do not have this issue.   Might be that being sealed well they slowly climatized, they did gain an average of 8 pounds each.

So what I'm wondering is if there might be a relationship between high altitude and low humidity causing the sinew to dry too quickly?  A non-boyer local said the altitude and low humidity acts like a freeze dryer here.  I'm inclined to believe that.  This morning the local airport showed 9% humidity, and we are just shy of 8000 feet elevation...and it rained
briefly yesterday!!!   If my wife only knew the actual value of some of the fires we had this last winter...

Anyone had this problem?  Any advice from our more experienced sinewers?   Maybe completely round the sides of the limbs?  Trap them?  The money wasted on materials is painful,  but the wasted time much more so, could've taken on a second job and commissioned a couple of  real nice bows by now.

Below is a pic of the most recent.  I think this backing job came out aweful, but it's hard to keep up the motivation when going into it I have a 100% chance, thus far, that it will end up as roasted meat smelling kindling.

[Ryan Jacob]

Maybe applying thin layers over a long period of time might help? Just an idea

[Mesophilic]

That's an excellent idea.  Maybe start in the center and work my way out?

Would sinewing the sides all the way around to the belly help as well?

[JNystrom]

I've experienced this too, quite recently. The backing is still in place tho. I just soaked it in hide glue and clamped tight.
Could it be that we sinew backed the bows too early on? My stave was pretty thick when sinewed. I thought too high moisture content also, but I dont think so. All guesses, no real answers. I think Simson had some problem with one really hard piece of osage, the sinew backing kept popping off. I try to find it from his website.

[Eric Krewson]

I had it happen once but I used a piece of osage that had side checks in the stave.

[PatM]

It is from the sinew shrinking in width which results in the equivalent of a glueline failure in the  wood.  Osage and Yew are both known for this trait.

 Was your bamboo Asiatic bow laminated bamboo and then sinewed?

Also, don't throw out failed sinewed bows. Soak the sinew off and re-use it.

[Ryan Jacob]

PatM
So if lateral shrinking is the issue, maybe keeping the layers of sinew near the edges thin and keeping most of the mass near the center is the way to go, effectively crowning it? I’m also pretty sure removing all sharp corners will do some good.

[burchett.donald]

     What kind of glue are you using...Glue could be the culprit here...Hide glue has never failed me, so far...
             Don

[PEARL DRUMS]

Don I think how much glue is left in the sinew is the reason for a lot of these types of failures, especially hide glue. I use two fingers and squeegee it out until its border line "dry". Then lay it, smooth it into the rest and let it cure. I've never had a crack, significant reflex, twisting, lifting, loosening or anything. All have them have very stable from start to finish.

[Mesophilic]

I've had this happen both with homemade hide glue and Knox gelatin glue.

I've had horizontal cracks in sinew that I believe is from too much glue, so like mentioned above I squeeze out a bit of glue with my fingers as well.

Because of the nature of this problem,  I also won't attempt sinew on a stave with any visible checking.

The bamboo was not laminated, they were made by as kit bows by someone I don't think I can mention.  Both came in way under the target weight advertised on his website and by all accounts I should have sent them back.  But my goal was/is to learn how to sinew an Asiatic to eventually make a horn bow.

I do salvage sinew once, after that it gets a little too stinky IMO, and I worry about compromising it's integrity.

[BowEd]

I feel for the concern you have here as I have a number of sinewed bows myself and like them.The only thing I can say is it's possibly in the preperation before sinewing.I usually like sinewing onto fresh just sanded  but clean wood while sizing that very well maybe an hour or two apart with very thin glue.I'm talking hide glue here.Some will premoisten the wood on the back with a wet rag to bring the knap up of the wood first too before sizing.I size it and sand smooth between sizings till it is like glass and shiny.Then I sinew it with slightly thicker hide glue.
Some will use acetone or even oven cleaner especially on osage then rinsed of course before sizing and sinewing.I've never needed to yet.Knock on wood.

[DC]

Could it be the width of the bow? More width equals more shrinkage. Are your bows wider than "normal". Just a thought, I've only done one.

[Mesophilic]

I am at 1.5 inch width or less at the fades.  The yew was 1.25.  If you guys think I should try narrower I'll give it a shot.

Thanks for the sinew tips, don't often find them all together like that.

Mine is an amalgam.  I sand the surface,  clean with acetone, lightly score with a piece of bandsaw blade or hacksaw. 

If it is an oily wood, I will again clean with acetone just prior to sizing.  It comes from knife handle theory, that because you have an area thst was just made less oily then the surrounding wood, the oils will want to migrate.  Amd in the case of epoxy and handle scales,  inhibit the bond.  A second pass of the acetone removes it, and then because the wood gave up oil the first go around it will take it longer for the oil to rebound from second cleaning.

I size with the diluted hide glue and let that seep in and dry, then again with full strength.  I apply the sinew before that second sizing fully dries, when it's  slightly tacky to the touch.   I do gently squeeze out excess glue from the sinew.

The part I may deviate from alot of folks is that once applied  I wrap the limbs with strips of inner tubes, snug but not tight.  Then I use a heat gun to wamr just enough re-liquify the glue.  I wait until the bow has cooled down below the gel point of rhe glue I carefully unwrap.

I do this because I've been attempting bows with curves,  and the sinew dries so fast here.  The upper layers will pull the sinew off the bow before the inner layers have dried enough adhere.  I've even peeled it back and found the glue on the limb to still be completely wet.  So this method seems to work the best at turning the sinew in to  a more solid layer rhat tends to dry a little more evenly, at least evenly that I dont experience lift offs.

I do sand between courses, and shoot for a more crowned backing by building up the centers more than the sides.   The inner tubes help even it out and give a nice gentle slope to it when used on subsequent courses and requires less sanding to even it out.

[loefflerchuck]

ON some of my juniper sinew bows I have the shrinkage of the sinew pull the grain apart on the sides. I have one juniper bow with a heavy sinew backing that has huge fissures up and down the sides of the limb. The bow is 49" long and draws 26". It is 3 years old now and has been shot thousands of times. It finally is starting to show little signs on one part of the belly where the cells are breaking down but the cracks have never been an issue on this or any other bow.

[Mesophilic]

Do your cracks look similar to the one in the pic above?  Or more akin to a hairline crack?

Based on these experiences if I keep working this limb the crack will spread down along the sinew line and ultimately fail.  Superglue has worked to hold them together when they hairline but not these wider deeper cracks.  I can fit a .05 pencile lead down in this one. 

Does juniper have a different grain structure?  As soon as they start issuing permits I'm going to try to harvest some. As thats one of the species the Native Americans in this area used.  See if that makes a difference.