Trapping

[Bowjunkie]

Wood is stronger in its resistance to tension than compression, generally. When a wooden bow fails, the vast majority of the time, the belly crushes, frets, chrysals, whatever you want to call it.

But glass? I have to wonder if it's not the opposite, because any of the glass bows I've seen fail, usually due to fatigue in an area, fail on the back/tension side. Never saw it happen on the belly. The ones I saw fail were recurves by the way.

I've trapped glass d/r longbows both ways and didn't notice a performance difference. I prefer the look of them with a narrower back, but considering what I noted above, I'm not sure it's best... or even matters in a longbow. I've never seen the glass fail due to fatigue in a longbow like have those recurves.

[Pat B]

If you build a bow with steel lams instead of FG it would still bend. Is the back lam stretching and the belly lam compressing? 
 The wood would collapse or be crushed between the unforgiving back and belly lams.
 I've only built one glass bow and never got a string on it before it broke so I'm not talking from experience, just common sense but that could be wrong too.   

[Flem]

I think we need to keep things in context. We are talking about Bow's and the materials we make them from. Strength in this context is the materials ability to resist bending and return to its resting state without deforming(taking a set). If you take a strip of any of the materials we use to make bows and bend it into an arc, it is going to take more force to bend the part of the material on the compression side of the neutral plane than the tension side.

[mmattockx]

Then why do you reinforce the back of the bow with sinew??  A don't have to reinforce the belly if you choose not to...

Mostly for safety. When a tension failure occurs that usually means a blown up bow, which is hazardous to the archer and everyone around. A compression failure just forms set (or chrysals in extreme cases) and seldom causes a blow up. Because tension opens defects up while compression closes them the back is much more sensitive to defects such as grain run out or knots, which is often why a wood bow is backed. Wood bows seldom fail in tension without a defect to start the failure.

In the case of sinew people often use it to gain performance as well, but that is a separate thing from using it as a backing to ensure the integrity of the wood back.

If you take a strip of any of the materials we use to make bows and bend it into an arc, it is going to take more force to bend the part of the material on the compression side of the neutral plane than the tension side.

As long as you are in the elastic range the material is equally stiff in tension or compression.


Mark

[Shredd]

Well...  You all are making some good points that I could agree and disagree on...  I think I will sit out and watch this one and possibly add some points of view along the way if this thread continues..

   Pat B...   To answer your question, Take a piece of thin wire 16" long and put some marks two inches from the ends and now tape the wire at the marks flat to the back of the bow...  If the marks do not move away from the tape when the bow is flexed your theory is right... If the wire moves that means the glass is stretching...   
   In theory I guess you if you could accurately  measure  how much the marks have moved on the tension side and compression side you could make a decent guess where the neutral plane is...

[Flem]

"As long as you are in the elastic range the material is equally stiff in tension or compression."




Not exactly true in this case. Look at a strip of composite we use. It's comprised of tightly stacked/packed epoxy molecules, surrounding glass fibers. To stretch the material you will have to overcome the internal friction of the intra-material bonds, plus the composite bonds. You will also be moving the material into unoccupied space. If you compress that same composite, you will have to overcome the same friction issues plus the additional friction of compaction.
Another way to look at it, take a 8' 2x4 standing plumb and put 100lbs on the end. Then support the ends and put that 100lbs in the center. Check the deflection for each

[Pat B]

I don't think sinew fits in this conversation. Sinew does stretch but it recovers back to it's initial position and is used as an elastic performance enhancer. It does hold down the highly stressed back of short wooden bows where it is most often used. And, in the highly stressed Asiatic horn bows the horn belly takes the massive compression stresses of these highly, over stressed bow style. In these cases the wood does little, like I imagine with a FG lam bow.
IMO sinew is rarely used for safety sake although it is used for that sometimes. There are better, less labor intensive materials used for safety like rawhide, linen and silk cloth and in some cases wood but wood also adds performance.
 If you grab a FG lam at each end and pull it I would imaging there would be very little stretch, if any. Even as a backing for a bow it doesn't stretch but it's resistance to stretch giver the performance as it pushes against the glass belly.
 back to the original topic...trapping, I still say trapping has very little effect on a FG bow's performance except by reducing the physical weight of the limbs. Trapping to the back or the belly is more of an aesthetic feature than a performance feature in a fg bow.

[Buemaker]

If trapping to the back on a narrow limbed glass recurve, the angle and amount have better be exactly the same on both sides. Other wise you can build twist in the limbs. That is my experience anyway.

[Shredd]

 If you grab a FG lam at each end and pull it I would imaging there would be very little stretch, if any. Even as a backing for a bow it doesn't stretch but it's resistance to stretch giver the performance as it pushes against the glass belly.

   Obviously my last statement went over your head and you did not try the wire experiment...  The glass stretches...  From brace height to my 29" draw with a 64" bow in a span of 9 1/2" the glass had stretched approximately 1/32"...   Like I said before if the glass does not stretch or compress the wood would shear...

   As far as tension vs compression in an all wood bow I think I changed my mind about which is stronger and I am on the fence about it...  In most cases it seems that tension most always trumps compression...  But then you add in defects and such and I am not sure where to go with that...

[Pat B]

At 5'8", going over my head is no great feat. It really doesn't interest me. I am a wood bow builder, mostly because of the simplicity of it. My above discussion comes from my uneducated thoughts on the matter.
 Again, back to the topic at hand... I say that trapping a glass bow is for aesthetics only and the only performance value is in the reduction of physical weight.

[Mad Max]

At 5'8", going over my head is no great feat. It really doesn't interest me. I am a wood bow builder, mostly because of the simplicity of it. My above discussion comes from my uneducated thoughts on the matter.
 Again, back to the topic at hand... I say that trapping a glass bow is for aesthetics only and the only performance value is in the reduction of physical weight.

Exactly

[Buemaker]

I may be completely bonkers here, but I will try to explain in English what I am wondering about. I was thinking of Shredd’s experiment with a thin wire and tape.
Just to see what happened I took a thin wood lam and made the distance between the points to 24 inches in order to get a clearer reading. Instead of a thin wire I used a piece of Mercury 100 % Dynema, over such a short lenght it would have no or very minor stretch, anyway good enough for this test.
I stretched the string tight along the lams surface and taped each end of the string to the lam. Now marked with sharp pencil string and lam 24 inches apart. Put one end in a vice so the string could not slip and bent the lam severely. Of course the mark on the string moved away from the mark on the lam and when straigtening the lam again the string was of course slack.
To my thinking that is because the distance between the points gets longer  when the lam is bent and not because the material is stretching. Shortest line between two points is a straight line. I usually have difficulty thinking out things like that, so like I said in the beginning I may be complely bonkers.
If glassfiber can stretch or not I do not know.

[Bowjunkie]

I spent 4 hours processing sinew this morning. One more session like that and I'll have enough.

So what is the consensus now? Should I put it on the back and trap the belly or vice versa?  

The reasons I'm putting sinew on this bow is because it's SUPER snakey, shorter than I'd like it to be, and I had to violate the grain at the bottom flare... shouldn't bend there much if any, but still. Gonna put my best pair of copperhead skins on the limbs and snapping turtle skin on the grip.

[Shredd]

Bue...   If your statement is true then the same thing should happen on the compression side...  The points should move further apart...  But they don't...  You will find that they move closer together on the compression side...  You won't be able to use wire though you will need something very thin and stiff...

[Bvas]

I may be completely bonkers here, but I will try to explain in English what I am wondering about. I was thinking of Shredd’s experiment with a thin wire and tape.
Just to see what happened I took a thin wood lam and made the distance between the points to 24 inches in order to get a clearer reading. Instead of a thin wire I used a piece of Mercury 100 % Dynema, over such a short lenght it would have no or very minor stretch, anyway good enough for this test.
I stretched the string tight along the lams surface and taped each end of the string to the lam. Now marked with sharp pencil string and lam 24 inches apart. Put one end in a vice so the string could not slip and bent the lam severely. Of course the mark on the string moved away from the mark on the lam and when straigtening the lam again the string was of course slack.
To my thinking that is because the distance between the points gets longer  when the lam is bent and not because the material is stretching. Shortest line between two points is a straight line. I usually have difficulty thinking out things like that, so like I said in the beginning I may be complely bonkers.
If glassfiber can stretch or not I do not know.

So if you glue that Dynema to your lam and then bend it, the Dynema would be forced to stretch.

Or cut thru the lam to stay straight. 

[mmattockx]

Not exactly true in this case. Look at a strip of composite we use. It's comprised of tightly stacked/packed epoxy molecules, surrounding glass fibers. To stretch the material you will have to overcome the internal friction of the intra-material bonds, plus the composite bonds. You will also be moving the material into unoccupied space. If you compress that same composite, you will have to overcome the same friction issues plus the additional friction of compaction.

That is not how it works. If you load the fibreglass lam axially in tension or compression it will deflect exactly the same amount for the same load as long as you don't exceed the elastic limits. This is basic mechanics of materials stuff.

Another way to look at it, take a 8' 2x4 standing plumb and put 100lbs on the end. Then support the ends and put that 100lbs in the center. Check the deflection for each

What is your point here? Those are two completely different loading conditions, with very different stresses. The fact that one deflects more than the other has to do with the loading, not the material.


Mark

[monterey]

Wires? String?  I think for this to be an accurate test, the material would have to be attached to the center of the edge of the glass lam.

[Bvas]

Maybe I’m UNDER thinking this.......wouldn’t a simple test be to glue a single glass lam to a single wood lam. Then test it with the glass under tension and then under compression to see which way it deflects less. 

[monterey]

There's a fellow on another site who has been trying through multiple efforts to build board bows backed with Gordon's FG so far they have all failed on the belly.   

[Shredd]

You guys have no idea how much I learn and how my mind gets opened up to new ideas by some of the things you post...   Always a thirst and hunger for wanting to learn the inner workings of a bow...

    Thank You...

  I hope the same thing is happening on your end also...

 Monty... Good point on wire being on the center...

  Bvas...  The theory sounds good but I am not sure how you could accurately test that...

   Patb...   The only reason I  mentioned it was because you mentioned again that glass "doesn't stretch" in your post...

I just want to clarify something before one leaves the forum with the wrong info in their head...  Glass does stretch on the back of the limb and compress on the belly...  This is basic info that should be known before you start even making a bow...