Sinew question

[DC]

This has bugged me for a long time and I haven't been able to sort it out on my own. Why/how does sinew help a bow bend so much? I've read that sinew stretches something like six times as much as wood. If that's the case why doesn't the wood just break under the sinew? If the sinew stretches that much why is it any more effective than gluing a piece of rubber to the back?
How does putting sinew on the back protect the belly?
If someone has a link to a good explanation that would be great too.

[Stick Bender]

I just had a very similar conversation to your question it boils down to the outer 10% of the limbs depth does 90% of the work ,the sinew being able to stretch 10% more then wood  & the wood beneath is being relieved of part of the tension load , allowing for a highly stressed limb to function better then wood alone , thats my shot at the answer if that worked for you !

[PatM]

Sinew in a glue matrix likely doesn't stretch THAT much but wood can break under sinew. As far as sinew protecting the belly...probably not as much as the old speculations would have us believe.

[mikekeswick]

Yes sinew can stretch a lot further than the 1% wood can be stretched. It has much lower resistance to stretching than wood and is around 1.3 s.g.
It protects the wood underneath if there is sufficient thickness to stop the wood feeling much tension. If you don't use enough sinew then the wood will break if pulled far enough. Sinew never gets to the point where it will break on a wooden bow and even a hornbow won't fail because the sinew failed. It is incredible stuff.
Sinew doesn't 'protect' the belly. If anything a sinewed wood bow should 'short' for its drawlength and therefore the belly is going to be bent to a greater degree than if the bow was wood only. That's why it is really only worth sinewing the better more elastic woods.

[Mo_coon-catcher]

I wonder if the sinew starts to stretch with a bit less pressure than the natural wood back wood. Making the back stretch a bit more earlier in the flex of the limbs. So that the wood belly is under less strain for the same amount of flex it's under. Allowing even low compression strong woods to bend a bit more before giving in.

Kyle

[mikekeswick]

The outer most surface of the bow is doing the most work and the deeper you go into the limb the lower the strain. With enough sinew the wood won't be feeling tension.

[willie]

If that's the case why doesn't the wood just break under the sinew?

because the wood is much thinner than a comparable  bow without the sinew

[gfugal]

I just had a very similar conversation to your question it boils down to the outer 10% of the limbs depth does 90% of the work ,the sinew being able to stretch 10% more then wood  & the wood beneath is being relieved of part of the tension load , allowing for a highly stressed limb to function better then wood alone , thats my shot at the answer if that worked for you !

+1 to that.
Something else that hasn't been mentioned, sinew along with any other backing will keep the wood from splintering up on the back. Most times when a self-bow breaks its not because the back was over its tension capabilities, it was because of the geometry of the bend that allows any imperfection to lift off catalyzing disaster. This is way even weak thin backings like paper can still be so effective.

Sinew won't help the belly at all. The only way it could is if you make your bow thinner to compensate for extra poundage you didn't want at the same draw. In most cases it stresses the belly more either because you can bend the bow further than most self bows, of you use more material with the same bend relying on the sinew to keep the back from breaking from the greater stress. The belly benefits nothing from the sinew under that greater stress. This is why i started the discussion on compression woods, so i would know what would be best for those highly stressed designs.

[bradsmith2010]

if the wood was wider and thinner because the sinew on the back,, wouldnt that be reducing the strain on the belly???
I know after a point when the draw is extended ,, your gonna need horn,, but at a reasonalbe draw,, I am just wondering,, I have had wood backings crush a belly,, but I have never had sinew back crush the belly on a bow drawn to half the length of the bow,,

[BowEd]

Yes Brad.Good point.Sinew will stretch so much farther than a wood backing which is'nt as hard on the belly then.

[PatM]

But wide and thin doesn't stretch the sinew as much. Sinew backed Native bows come in the two extremes, wide and thin or narrow and deep. Which shoots farther?

[bradsmith2010]

a wide thin yew would shoot further?

[BowEd]

I know that sinew loves to torque reflex on wide thin limbs if applied thick enough and will stretch it plenty.Even with shrinking a certain percentage sideways too.I think there's too many factors to answer a question like that right off hand.Design and length wise are not known.That factors into it too.

[Stick Bender]

This is some thing I have always wondered if you have a sinew bow that's reflexed say 4 in.  & when you bring it to brace is the belly under more compretion then a standard brace at that point ?  This is a great thread.

[bjrogg]

I have never done a sinew backed bow other than my HHB mini bow for Marshall. I am thinking of doing a HHB and reading this post wondering thoughts on if HHB is a good candidate and what type of design? For myself I'd probably shoot for around 60 lbs at 25" draw. Hope this isn't getting to far off topic.
Bjrogg