Neutral plane and sinew

[sgt413]

Hello everyone.  My name is Rich and I have been making bows for one year. I am getting ready to sinew my first bow but have a question which I thought I knew the answer to but now am confused.

I thought I understood the effect sinew has on the neutral plane of a bow but I am confused again.  According to Tim Baker in volume one of the Bowyer's Bible sinew moves the neutral plane towards the belly of the bow. (page 106).  He compared the sinew on the back of the bow to a light child on a see-saw and the belly wood to a heavy child on the other side of the see-saw.  He said the neutral plane of the bow must shift towards the belly just as the fulcrum on the see-saw would have to move toward the heavy kid to maintain balance. He stated it was this shift of the neutral plane that thinned out the belly wood that has to undergo the forces of compression thus allowing it to bend farther without damage.  That makes sense to me. 

Then I go and mess myself up by doing a search here for the effect of sinew on the neutral plane and I am slapped in the face with just the opposite.  Everything I find on here suggests the neutral plane is pushed towards the back of the bow after sinew is applied making the belly wood thicker therefore able to withstand the forces of compression better.  Which is it?  Everything I have ever read about white wood states the thinner it is the more it will bend.  It seems counterintuitive to think the wood will bend further without damage if the neutral plane is shifted toward the back of the bow.

[RBLusthaus]

I have not read my BB for a while, but I would think the neutral plane would move towards the sinew there by giving you more belly wood (than before the backing was added) to take the compression.   In fact, that is the purpose of adding the sinew - so that the sinew will take the tension forces instead of the wood.  This only would be true if the neutral plane moves in that direction.  I am sure someone will more knowledge than I will chime in soon. 

[Bryce]

Brings the neutral plane to the back. Not the belly.

[JW_Halverson]

Seeing how the neutral plane doesn't do anything, what does it matter?  A better question to ask is whether the bow design is one that sinew would improve or not. 

For example, when I first started playing with bows I thought the ab-so-fabu-lutely bestest EVER bow would be an osage ELB with three layers of sinew.  Turns out with a bow that long and as short as my draw length is the sinew wouldn't "load" up with as little strain as it would be getting from such a short draw.  You'd be taking a moderately functional bow and adding unnecessary physical weight.

Whatcha gonna make?  Pictures, man, pictures!  I can't read, nor can most of us, post some pictures! And welcome to the Forum, Rich.  Glad to have ya!

[PatM]

Bottom line of course is that it really doesn't matter how it works as long as it does.

[sgt413]

I am the type of guy that needs to know what certain things do to one another and how they work in relation to each other.  It helps me put it all together in my mind before I go trying something I will end up throwing away.  I guess my biggest concern is what does sinewing do to limit the compression forces on a belly.  Since Hickory is weak in compression, I am hoping that sinewing the bow will not only keep the back from blowing out and give me more cast but it will also help prevent the belly from crysalling.  My goal is to try this one out and if all goes well shoot for a similar bow in the mid 50's length with some deflex/reflex.

Right now the bow is 60 " ntn, 2 inches wide from fade to 20 inches up the limb then it tapers to 3/8 inch at the nock. I have steamed two inches of reflex into the limbs.  It has a working 5" grip, 2 1/2 inch fades  and is made from Shagbark.  I am shooting for 50 lb. at 28inches

I colllected over 300 deer tendons from three butcher shops and I believe I have worked enough for three layers on this bow.  I have it bending evenly out to 12 inches with the long string.  I will stop tillering now and sinew the back.

If all goes well I will take photos when it is complete.

[Shaun]

The neutral plane will move towards the back. Not sure if it will make much difference in the belly side since the majority of the work is done by the last 1/16th inch of either the back or belly and moving the plane will not have a major effect on this. It is possible that it would put more strain on this thin compression area of the belly since the fulcrum point (neutral zone) will be moved towards the back.  The same way adding bamboo to the back puts more stress on the belly material. The best way to alleviate compression is to have a wide flat belly that has the most material in the extreme compression zone near the surface thereby spreading the load. Yew which has very good compression rating (heart wood) can be made into a narrow rounded belly where a small amount of wood takes this heavy load. 

 

[SLIMBOB]

If I understand correctly, sinew does alleviate some strain from the belly.  By moving the neutral plane closer to the back, the bend radius of the belly is larger and therefore more wood is at play to handle the compression.  In other words, because the back is stretching farther, the belly is compressing less.  This is as I understand it.

[MWirwicki]

Sgt413: 
Your desire to understand the mechanics is appreciated.  I too like to know how things tick.  Now here it is... HOWEVER,
What I've also found in this wonderful craft is that sometimes, getting too wrapped up in the physics can take some of the "fun" out of it.  There are some basic rules of thumb but afterall, we are on Primitive Archer constructing primitive weapons. 

The shortest I would go is 56 inches with a semi-stiff handle though you may need to flip the tips some.  You might have a string angle issue pulling 28 inches.  I've made shorter but they tend to follow the string a bit even with sinew. 

Some of the fun and best learning is by experimenting.  I'm glad to read that you've decided to move forward with your sinew project, regardless of the differences in opinions.  In response to your original question I would say, "just make the bow."  If it doesn't turn out exactly as you envisioned, make a few adjustments and make another one!  Eventually, you'll develop the "feel" for it.