Author Topic: Sinew over yew - Why did it fail?  (Read 723 times)

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Offline simk

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Sinew over yew - Why did it fail?
« on: August 13, 2019, 03:04:35 am »
Bow building can be a hard school. I put big effort into this trade bow for Martin just to see it fail after 300-400 shots. I finally at least would like to understand what happened. What do you think:

A) Sinew delamed first, then the wood let go (more likely)
B)First the wood let go then the sinew delamed (unlikely)

A1) It was substandard glueing job that caused the delaming? I did it very careful - the only thing that I could imagine is, that I maybe didn't roughen the back enough before applying sinew...

A2) It's the yew-sap-sinew combination that made it fail? ...as discussed here http://www.primitivearcher.com/smf/index.php/topic,65625.msg923293.html#msg923293 Does hide glue not stick well on yew sap maybe?

C?

On the pics you can see the delamination area midlimb on th upper one - it's slightly brighter... tiller was good imho. The belly of the bow is intact.

Thank you for your thoughts!

Online Del the cat

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Re: Sinew over yew - Why did it fail?
« Reply #1 on: August 13, 2019, 03:43:11 am »
The Yew wouldn't have broken if the sinew was doing it's job, so my guess is the sinew/Yew glue line gave way.
Or maybe the sinew wasn't actually under tension and wasn't doing anything...was the bow in slight reflex when the sinew was glued on?
Mind sinewing bows isn't my thing... so I'm just guessing from an engineering stand point rather than experience (damn armchair expert  ::) ;) :o )
Del
« Last Edit: August 13, 2019, 03:48:40 am by Del the cat »
Health warning, these posts may contain traces of nut.

Offline Mafort

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Re: Sinew over yew - Why did it fail?
« Reply #2 on: August 13, 2019, 05:55:47 am »
Usually when sinew backing yew wood you take the sap wood off and recurve and reflex the entire bow. Hell you may be able to soak off the sinew take the sapwood off and glue the back back together with some epoxy and then re sinewback it with the wood under tension I.E a low reverse brace. My last bow failed like this because I didnít rough up the back up and it didnít adhere enough to protect anything. If you reverse brace put sone tape on the belly. Itíll help to keep the belly from splintering.

Offline PEARL DRUMS/PEARLY/PD/DRUMS

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Re: Sinew over yew - Why did it fail?
« Reply #3 on: August 13, 2019, 06:07:03 am »
Too much bend in that area. Even sinew cant help that. Your fades need to work right up into your grip when you have such short working limbs.
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Offline mullet

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Re: Sinew over yew - Why did it fail?
« Reply #4 on: August 13, 2019, 06:30:45 am »
I'm with Pearly. how many layers of sinew did you use. I just sent a 51" yew bow to Texas with 3 layers of sinew over partial sapwood and heartwood. This bow was pulling 30" at 45#. It was doing the same at 55# before I reduced the weight.
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Offline simk

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Re: Sinew over yew - Why did it fail?
« Reply #5 on: August 13, 2019, 06:55:18 am »
Thank you guys - very interesting reading your thoughts!

Del: That was my primary guess too...the bow was reflexed when I put the sinew on - after shooting it in it was flat again but the reflexed levers should make the sinew work.
Mafort: Yes, I also heard it's common to sinew-back yew without the sap. Still this one is not repairable - I'll probably reuse the sinew though.
Pearl: Yes, it's a lot of bend there - following your judgement it was the wood that failed first, before the delamination happend...?
Mullet: Only one layer - but a thick one 55gramms (2+ oz) of sinew.

Additional info for further speculations: The bow didnt break at full draw but only at 22/24". I did an identical bow which is holding up well for way over 1000 arrows now - this one was without sapwood and even had several ring violations on the back (under the sinew).

still curious

Edit: The bow was tillered out to 28"before sinewing => picture at about 26"
« Last Edit: August 13, 2019, 07:58:33 am by simk »

Offline bassman

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Re: Sinew over yew - Why did it fail?
« Reply #6 on: August 13, 2019, 07:54:35 am »
Looks to me like lack of sinew in the area were it let go unless you cleaned that area away to show the breaks.

Offline Marc St Louis

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Re: Sinew over yew - Why did it fail?
« Reply #7 on: August 13, 2019, 07:56:50 am »
I would say it's problems with the sinew and its adhesion to the wood that is responsible.
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Offline backtowood B2W

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Re: Sinew over yew - Why did it fail?
« Reply #8 on: August 13, 2019, 02:02:11 pm »
My draw length is 27" max.
We had a quite colder day than before.
I strung the bow. pulled half draw 3,4 times, nocked an arrow and draw. She let go about 3-4 inches in front of my face.
My first thought was delamination too, as there wasn't a real loud kabumm, but Iīm no expert with sinew....

Offline bownarra

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Re: Sinew over yew - Why did it fail?
« Reply #9 on: August 14, 2019, 12:32:22 am »
You have made a 'hornbow' type bow. Too short working limbs and almost certainly the sinew/wood bond failed first.
The back should be smooth when you size then apply sinew DO NOT roughen surfaces when using collagen glue. The best bond is achieved with smooth surfaces. What happens with roughended surfaces is you get lots of little airbubbles trapped in the ends of the cut wood fibers.
The other very important thing with yew (especially) is sizing. If you sized with 5 or 6 coats of 5% glue first. Used 25 - 30% glue for your first layer. 20% for any further layers you won't ever have this problem again.

Offline simk

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Re: Sinew over yew - Why did it fail?
« Reply #10 on: August 14, 2019, 07:58:17 am »
As - for future projects - its always important, to have at least an idea why past things failed, I wanna thank you for all your contributions. Fails only help if you can learn something.

Bassman: I promise to better cover the limbs with sinew - on the edges.
Marc: Thank you for your highly valued judgement.
b2w: Hope your are not dreaming about this nightmare-situation  ;D
bownarra: Very helpful information!!! I will take more time for sizing also for sure! I'm still tempted do try this design again,...we will see.

Cheers

« Last Edit: August 14, 2019, 08:05:29 am by simk »

Offline Parnell

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Re: Sinew over yew - Why did it fail?
« Reply #11 on: August 14, 2019, 08:27:38 am »
You have made a 'hornbow' type bow. Too short working limbs and almost certainly the sinew/wood bond failed first.
The back should be smooth when you size then apply sinew DO NOT roughen surfaces when using collagen glue. The best bond is achieved with smooth surfaces. What happens with roughended surfaces is you get lots of little airbubbles trapped in the ends of the cut wood fibers.
The other very important thing with yew (especially) is sizing. If you sized with 5 or 6 coats of 5% glue first. Used 25 - 30% glue for your first layer. 20% for any further layers you won't ever have this problem again.

Hmm, this caught my eye.  This seems to be contradictory thought to what I remember reading in the TBB books and with other posts over the years.  I suppose my question would be why the contradiction over the years?  People just falsely feel the textured face provides greater surface area or friction for...artificial adhesives, then?  Curious about this and people's experiences.
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Online Pat B

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Re: Sinew over yew - Why did it fail?
« Reply #12 on: August 14, 2019, 09:16:55 am »
It doesn't take but one thumb or finger print to cause the sinew/glue to not adhere or at least start lifting. I had that problem early on but once I started washing my sinew in Dawn dish soap and warm water and scrubbing the bow's back with Dawn and rinsing with boiling water did my sinew backings adhere well. Not only does this eliminate and grease or oils but it preps the wood surface to accept the glue sizing.
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