Post For Tim Baker ( Sinew)

[Tim Baker]

No, it's not the blob from outer space. It's what happens when you make a 1/2" thick pouring of hide glue and wait for it to dry down to a hard sheet. Might add antibiotics to the next try.

[sleek]

Thats funny, cause isnt hide glue what they use for petri dishes?

[willie]

isn't that agar? jello for vegans?

[Tim Baker]

Just did a backward sinew shrink test: Measured the sinew stiff and dry, then let it soak in warm water for an hour and measured again. The original 12" became about 12.5 inches, in the range of  3% to 4% commonly reported. 

This is dramatically different than the roughly 15% to 20% the earlier-pictured hide glue tests showed. 

So when we sinew back a bow the glue is trying to shrink roughly 4 time more than the sinew, therefor working to pull the sinew shorter than the sinew's normal 4% would like to. On top of that the glue itself is able to stretch up to 10%.... So some way complicated mechanical interactions are at play when a sinewed bow is drawn. Understanding them will surely lead to better bows, but doing so is going to take several gallons of midnight oil. 

So, does it hurt or help to use a higher or lower % of glue? 

Quite a bit earlier in this thread someone suggested that sinew mixed with glue might have quite different properties than sinew alone. Who was that, so he can get a 50 point bonus?

[Tim Baker]

12"

[sleek]

Excessive hide glue on a bows back tends to crackle when drawn. I doubt it will be beneficial to add more hide glue than needed to bond sinew on the back.

On a belly, i can see it helping. I had an image flash in my mind of cut grooves in a belly as if grooving to lay horn in, filled with glue. That would be an interesting way to possibly keep the glue from deforming under compression strain.

[GlisGlis]

Might add antibiotics to the next try.

I read they say to add vinegar to hide glue to preserve from mold and microorganism

[Tim Baker]

sleek:

" Excessive hide glue on a bows back tends to crackle when drawn. I doubt it will be beneficial to add more hide glue than needed to bond sinew on the back"

The question was targeting the ideal % of glue in the glue/sinew mix itself, given the difference % of shrinkage for glue and sinew: Yes, when resting above the sinew surface, glue will sometimes crack when highly stressed. But high-quality glue can stretch 10%, so cracking glue on a wood bow is likely just the bow giving its opinion of the glue's quality.

Your idea of glue-filled grooves in the belly surface is worth experiment with. The interface would have the average or both materials. More tests....

I wonder if there's any way to cause hide glue to run up the pores of ring-porous wood. Hot wood would keep it liquid, capillary action should work to some extent, slight suction on the other end might bring the glue all the way through. Seems a challenge, but a glue/wood hybrid belly has possibilities.

GlisGlis: 

" I read they say to add vinegar to hide glue to preserve from mold and microorganism"

Thanks. I'll read up on that.

[PatM]

I suggested that sinew and glue together might be different synergistically.

 
 We did have a discussion on here before where the composite guys were pushing for higher amounts of glue as THE only way while others thought experimenting with lowering content and getting optimum infusion through the fibers might be better.

 That was back before feisty mikey had another tantrum and quit the board again.

[Bob Barnes]

If using hide glue improves the sinew backing, then isn't that a plus over using titebond?

[BowEd]

Absolutely Bob.The combo is as they say mother natures FG.The only 2 reasons to use titebond is for convenience and most times worries about humidity and moisture protection.Performance is stunted dramatically.

[Marc St Louis]

I also had a sinew backed Elm chrysal badly, it was a reflex deflex.  The bow shot quite well but lost cast swiftly.

One would think that sinew would protect a wood low on elasticity from chrysalling but it doesn't.  A sinew backed highly reflexed Black Cherry recurve I made many years ago chrysalled badly well before I got to full draw

It seems to me that even though sinew and glue is theoretically very stretchy is still acts like a steel strap when placed over weak wood.

 I had an Elm bow that I  sinewed and it chrysaled like pine just getting it back to a straight profile which didn't make much sense at all.  I pulled it back to 32 inches anyway and it still had remarkable cast for being a mushy mess.

I dunno about these generalizations about sinew. How much sinew compared to the quality of the elm or cherry has to be considered

I always mounded my sinew regardless of bow design so that most of it was down the middle of the limb.  This should have taken some stress off the belly.

Marc:

" A sinew backed highly reflexed Black Cherry recurve I made many years ago chrysalled badly well before I got to full draw"

If your cherry recurve had had a horn belly it would'nt have chrysaled; the idea here is to see to what extent this poor-man's version of horn will function like horn. Lots of possibilities if it even comes close. Several tests needed to know one way or the other.

Of course but that was not what I was making.  I ended up removing most of the Cherry and gluing down a strip of Osage essentially making the bow a complex composite. Still have that bow and it shoots quite well.

[BowEd]

That's what I've got to do to my cherry sinewed black cherry I liked.I'm just beginning to see a few chrysals.It started to chrysal after around 2000 shots or so.Just with only 4" of reflex.

[willie]

So when we sinew back a bow the glue is trying to shrink roughly 4 time more than the sinew, therefor working to pull the sinew shorter than the sinew's normal 4% would like to. On top of that the glue itself is able to stretch up to 10%.... So some way complicated mechanical interactions are at play when a sinewed bow is drawn. Understanding them will surely lead to better bows, but doing so is going to take several gallons of midnight oil.

So, does it hurt or help to use a higher or lower % of glue?

Tim, I have some glue on TP strips drying now? How long did it take for yours to shrink the 15-20% ?  Three different grades mixed with the recommended amount of water.  I am glad you did the reconstituted sinew test, I have some fresh sinew drying at the moment also. How long before you measured your sinew?

Do you think it is the % of glue to sinew in the matrix that matters about shrinkage?  I am going to try to wet out a TP strip with glue thinned with excess water.

If one dries a strand of sinew like a guitar string, it gets shorter and tighter, or becomes post tensioned. Is that the effect we want to duplicate on a bow back? Seems like if glue tends to shrink excessively, we might be better off to restrain the bow from reflexing as it dries, rather than encouraging it?

[willie]

I always mounded my sinew regardless of bow design so that most of it was down the middle of the limb.  This should have taken some stress off the belly.

guess I should have said how much and how thick also. the" taking the strain off the belly" idea has a lot to do with proportions, there is always the "too much of a good thing" possibility