Cores breaking problem

mikekeswick · Post by **mikekeswick** » Fri Sep 19, 2014 6:05 am

I recently got out my first Turkish bow which had always had problems - kasan eyes too thick, twist and dodgy sinew layer.....and decided I could fix all the problems in one go by removing the sinew, adjusting the core and then resinewing it. However once I removed the sinew I saw that the core had been failing in multiple places along the 'back' and had cracks going across it. After inspecting it fully I found about 15 of these cracks so my plans to fix it went out of the window!
Thinking back I have had 3 failure like this where the core has failed.
Especially frustrating was my last Turkish bow, this one didn't need a single correction once braced, no twist at all. However once I got it to around 25 inches I heard the dreaded 'tick' and the core cracking turned out to be the problem once again. This core was made from prunus avium.
I'm presuming that the cause has to be the sinew layer simply being too thin? However I'd like to have this confirmed before spending all that time making another. I am fairly certain that the sinew was 2mm thick along the center of limbs as per Adams recommendations in his book.

Bj?rn Sofeit · Post by **Bj?rn Sofeit** » Fri Sep 19, 2014 12:54 pm

Say, how much sinew did you use for the bows each? Like, how many grams?

Bede · Post by **Bede** » Sat Sep 20, 2014 1:02 am

I am wondering whether this is a suitable core wood for a Turkish bow over 60 lbs. I remember it being used in longer bows which were not quite as stressed. I know there can be many other reasons both mechanical and structural, but I think Adam once told me about this in an email.

mikekeswick · Post by **mikekeswick** » Mon Sep 22, 2014 5:55 am

I've always tried to get close to the 100 grammes recommended by Adam in his book. However this first bow was a long time ago and I really can't remember exactly how much I used? I didn't get the back anywhere near flat enough though.
The core used on my last Turkish bow was maple however I don't think I used the full 100 g on this bow - more like 70 g.
Has anyone else come across this problem of the cores cracking? Is it just the sinew thickness that is at fault? Eg the top of the crown of the core is too far above the neutral plane?

Kviljo · Post by **Kviljo** » Mon Sep 22, 2014 7:29 am

I guess the same could happen if the sinew is not stretched out enough on the bow? I doubt that is your problem though. Perhaps you could also pull the tips together more inbetween the layers, to put more of the stress on the sinew.

mikekeswick · Post by **mikekeswick** » Mon Sep 22, 2014 3:04 pm

Interesting thought. Maybe this could be the problem.
I have just been exercising my latest bow and...i've heard the dreaded 'tick' again,a whiter (than it should be) area has appeared on the sinew and it's acting weak there. I suspect the same problem.
The core was made with bulletwood for the handle, maple sals and black locust outer limbs. It's not a Turkish design more like a Magyar bow without reflexed outer limbs.

Pat Menzies · Post by **Pat Menzies** » Mon Sep 22, 2014 3:10 pm

Would it be possible for you to use an outer growth ring piece as your bending portion?
Seems like if the core was totally sound like a good selfbow the problem could be mitigated even if other growth ring configurations have proven to hold up for others.

Uli · Post by **Uli** » Tue Sep 23, 2014 1:31 pm

I d go for the more sinew plan.
old bows have quite lots of sinew.

fusizoli · Post by **fusizoli** » Tue Sep 23, 2014 4:08 pm

Mike

What was the dimension on this bow's limb. Mean horn-core-sinew thickness and width.
Was that wood natural seassoned?

I think the back of the bow was overstressed.
May the core was too thick and the sinew too thin for that.

If the horn thick and the core too that indicate more stress on the back.
It needs more sinew.
As you know there is a neutral line in the core of the bow.
If this line going towards the back or the belly of the bow (from the center), thats make trouble as you mentioned before.

I dont think you've made any error with the wood as a selfbow expert.
But I wouldn't use black lochust for the limbs in a hornbow.

Pat Menzies · Post by **Pat Menzies** » Wed Sep 24, 2014 12:57 am

The Koreans use Black Locust for siyahs.. Maybe that's the part Mike means by "outer limbs"?

fusizoli · Post by **fusizoli** » Wed Sep 24, 2014 8:47 am

I know they use mulberry.
Bl is a new info for me .

mikekeswick · Post by **mikekeswick** » Wed Sep 24, 2014 9:46 am

I only have a minute now....
I will take some pictures to give you all a better idea this afternoon.

mikekeswick · Post by **mikekeswick** » Fri Sep 26, 2014 3:53 pm

I suppose it must be down to not using enough sinew although i'm sure i've had at least 2mm along the center of the back on all my bows.
The bending sections of the newest failure was hard maple (kiln dried board but only glued in reflex no steam bending) and the 'levers' for want of a better word were black locust.
Fusizoli - The limb at mid-sal was 33mm wide and 12mm thick. The horn was around 4mm, maple 6mm and the sinew 2mm thick.

mikekeswick · Post by **mikekeswick** » Fri Sep 26, 2014 3:54 pm

These pictures show the Turkish bow with the sinew stripped off and the other bow that I suspect has had a core failure. As i'm sure you can all see the pencil indicates the problem areas.

mikekeswick · Post by **mikekeswick** » Fri Sep 26, 2014 3:56 pm

These pictures show the pretty major failure of the core on my Turkish bow. This is the bow with an air dried cherry (prunus avium) core. The draw weight was around 60# @ 28 and it was 48 inch ntn.