How does sinewing affect belly safety?

[David Long]

I understand how a sinewed back protects a bows back from overextension and holds splinters down. But I am confused when it comes to the effects sinewing has on belly safety. Bakers neutral plane shifting discussion in TBB1 pretty much eludes me. Say you have a bow pulling 50 lbs at 28 ". You sinew back it and now it pulls 60 lbs. Is the belly now in more or less danger and why? Seems to me that the compressive force exerted by the stretched sinew would in part be exerted on the already compressed belly, thus putting it in more danger of collapse. Confused...
Dave

[Pat B]

It has to do with the neutral plain...by decreasing the compression stresses on the belly but don't ask me why!

[TRACY]

I don't know that I can offer anything other than my personal experience. The heavier/more sinew layers, the more I see belly cracks( longitudinal). I haven't had any issues with failure due to this. I just fill them with super glue for extra insurance and seal the bow.


Tracy

[Dustybaer]

the way i understand it, sinew eases the burden (compression) on the belly because it stretches more than wood.  i think sinew stretches about 7% and wood a little over one.  a selfbow bends by partially stretching the back and partially compressing the belly.  the more the sinew back stretches (basically bending "around" the belly,  rather than compressing it) the less compression is applied to the belly.

[JackCrafty]

If the sinew does indeed add strength to the bow, then the belly will be under greater compression.  That's why horn bows are made from horn and sinew...not wood and sinew.

However, I have yet to see any increase in strength on my bows with sinew less that 1/8" thick.  Perhaps a little strength is gained just after the sinew dries, but after exercising the bow, there is no difference.  But 1/8" of sinew will decrease the stress on the back of the bow significantly.  I don't think the belly is less stressed though.  If I remember correctly (my engineering classes) I don't think changing the neutral axis releases stress from the belly in this case.  If anything, I think it may increase the stress due to a deeper cross section?

[David Long]

I get what you are saying Marius, but it doesn't quite clear things up. Take two 50 lb bows, one made of wood X with no back, the other made of wood X with a sinewed back. The sinewed bow has thiner wood- remember it's the same weight as the one without sinew. The back and belly of this sinewed bow are safer than the unsinewed one, because again the wood is thinner for an equivalent draw weight, back and belly are not required to stretch and compress as much. But now add 10 lbs to the unsinewed bow by backing it with sinew. What happens to its belly?

I think I agree with you jackcrafty, but alas I remain unsure. I am surprised your bows don't gain poundage by sinewing though.
Dave

[Badger]

I would say that if you added 10# of draw weight from sinew then the belly would proably be under less stress than a 60# bow but still at least slightly more stress than the 50# bow was originaly. Anyone who has seem Mark St Louis flight bows can see how much stress he puts on his sinewed flight bows but the bellies never give out or crush. I have a couple of little 40 something inch sinew backed osage bows that I draw back to 28", the bellies still hold up fine. Shorter bows seem to gain more weight from sinew than longer bows. I recently put 2 layers on a 64"yew bow and can't really tell the difference, I just feel safer shooting it. Steve

[JackCrafty]

I think my sinew technique has something to do with the lack of added strength.  I apply the sinew as dry as possible and with thick glue (less water) so that it shrinks very little.  I also do not reflex the bow prior to applying sinew.  I also have never applied more than 1/8" of sinew to my bows.  The wood type could also be a factor: the sinewed bows I've made (with the exception of ipe) are made of wood that is not compression strong: juniper, ash, mulberry.  (I need to go back and test those two ipe bows....I don't have any data on those.)

[Badger]

Jack, that would make a big diff what you describe, the wetter it is the more tension once dry. I have A couple of Marks bow that are sinewed and they are incredibly light, I think the sinew can store a lot of energy per it's weight. Steve

[welch2]

Shrinking sinew moves the neutral plane further toward the back ,than just the added thickness of a backing would. Because it shrinks. This also pulls the belly toward the back ,taking some pressure off of it ,unstrung anyway.

Ralph

[David Long]

OK Ralph I hear what you are saying. Rather than say "pulling the belly towards the back" we might say the sinew stretches the belly and puts it into "pretension" thus when it is compressed as the bow is bent it is more able to hack it. I have always considered Perry reflex another version of prestressed bows like sinew backed ones.
Dave

[welch2]

Yep , that's what I would have said ,if'n I could .   

Ralph

[GregB]

I've never sinew backed a bow yet, but plan to do so soon. For those with sinew experience...wouldn't sinew backing a bow help add longivity to the life of the bow, and help the bow keep its reflexed shape over time and a lot of shooting/use? Where an unbacked bow with a lot of use is more apt to show signs of string-follow over time?

[TRACY]

Absolutely Greg. I've sinewed mostly osage of narrow and short design and I'm amazed at how well it performs and adds some poundage when fully cured. I had one sliver stave that was slightly deflexed and ended up with 2" of reflex after 3 layers of deer sinew had dried and cured. Just my experience.

Tracy