Trapping

[mmattockx]

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.

That is as expected. The fibreglass is ~3x stiffer and 5+x stronger than the wood is, so it shifts the neutral axis well towards the back and overloads the belly. The only way that could work at all would be by heavily trapping the back and having just a narrow strip of thin fibreglass lam carrying the tension loads on the back. The realities of how narrow the glass needs to be would likely cause problems with shear failures between the glass and the wood or stability issues.

I have seen a wood child's bow that was made with a rectangular section limb that was backed with a narrow strip of fairly thin hickory (I think it was). The hickory strip was just glued on the back and the rest of the limb left rectangular. It looked odd, but supposedly worked fine. Something like that might work for fibreglass backing on a wood bow, but I would have to run some numbers before being certain of the concept.

Back on topic, for OP. I would always trap the back as the compression side will fail through buckling well before the tension side fails. Raising the stresses on the tension side of a glass lam bow is the safer option. As buemaker notes, trapping a narrow limb or tip is tricky because it is easy to take a bit more off one side and introduce twisting issues.

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...

+1 (see, I can agree on things  ). The strain experienced by the materials is how the draw energy is stored. No strain = no energy.


Mark

[Flem]

So as far as the original question goes, I think we all can agree on a few things.
The neutral plane does exist, even though we can’t really locate it. And on either side of the neutral plane the potential for opposing forces exists, compressive and tensile. When we build bows, we are balancing those forces, do they require an equal amount of mass to achieve?
For me thats the Bowyers paradox, where is the neutral plane?
We count on the materials we use to deform and then return to their original resting state as fast as possible. So it’s becomes a question of how you want those materials to return and there are two choices; pushing or pulling. Have to have both and they do have to balance out. Also need to keep in mind that with a laminated bow, each individual component  has its own neutral plane in addition to the bow’s. Which leads me to my preference. I prefer a thinner, denser, more compressive material on the  belly and consequently a thicker more elastic back. The goal being to move the neutral plane are far towards the belly as possible.
So if it was me, I would trap the belly side, which I believe would scrub off more weight with less material removal, but only for the purpose of trying to make a specified draw weight.

[Pat B]

A few years ago, when selfbows really began to take off a few commercial bowyers, some were selfbow builders anyway, tried making all wood bows with FG backings. Many of these bows failed and the bowyers gave up the idea. FG is just too strong of a backing for a wood bow. There are many other backing materials that will protect a wood bow and some add performance.
 Shedd, I'm not trying to argue about something I know little about but trying to get my uneducated thoughts about it out. What I don't understand is, is FG stretching, as in stretching and returning to it's natural state, elastic, or is it just under more tension as the bow bends and relaxed as the bow recovers? How much energy does it take to stretch FG? Is a 35# bow enough to do so, 45#, 55#, ? That's what I don't understand.

[Shredd]

I don't have the answers to all your questions but yes, fiberglass is stretched and compressed in the limbs...  This may not be the best example but take about 30 or 40 sheets of copy paper...  Hold one end tight with your fingers and the other side loose and then bend the papers in an arc...  Notice what happens...  On the loose end the papers are not in a square bundle any more... Lets make pretend that that is a bow limb and the layers are shearing... Some stick out more than others...  Now go back to a flat bundle and hold both ends tightly...  Notice how there is some resistance when you begin to bend it and the papers are wanting to separate and shear??  It's because one side is under tension and the other under compression...  If the glass did not stretch and compress your limb would shear like in the first example... I hope that helps...

[Crooked Stic]

Yeah thats pretty simple if it did not stretch it would break and not rebound. 

[monterey]

Gotta think on that one

[mmattockx]

So as far as the original question goes, I think we all can agree on a few things.
The neutral plane does exist, even though we can’t really locate it. And on either side of the neutral plane the potential for opposing forces exists, compressive and tensile. When we build bows, we are balancing those forces, do they require an equal amount of mass to achieve?

The neutral plane (also called the neutral axis) of a cross section in bending is a well established and defined thing in mechanics of materials theory as used in engineering. We can most certainly locate it with utmost precision for any given cross section shape. There is not just the potential for opposing forces, a beam in bending absolutely has opposing forces on opposite sides of the NA. The neutral axis is called that because it is the point where there the material sees no compression or tension during bending. You don't have to try and balance the forces, they balance themselves automatically or your limbs wouldn't bend in a controllable, predictable fashion.

Also need to keep in mind that with a laminated bow, each individual component  has its own neutral plane in addition to the bow’s.

Once the lams are glued together the NA of the individual components no longer matters because the glued up assembly functions as one solid piece. It is not that hard to calculate the bending stiffness and strength of a composite section as long as you know the properties and dimensions of each component in the stack.

So if it was me, I would trap the belly side, which I believe would scrub off more weight with less material removal, but only for the purpose of trying to make a specified draw weight.

If the lams are the same thickness on both sides (which seems to be most common on FG bows) then it doesn't matter which side you trap, it will take the same amount of weight off. That is because the materials are the same stiffness in tension or compression in the elastic range. If the lams are not the same thickness then trapping whichever side has the thicker lam would probably take more weight off. I would have to do some number crunching to see if the neutral axis shift that you get with the trapping increases the bending stiffness more than removing the thicker lam material reduces it. It is not a foregone conclusion that it will always go in the same direction in the case of different lam thicknesses.

FG is just too strong of a backing for a wood bow.

This is correct. I have looked at it and even very thin FG lams just overwhelm the wood because of the much higher stiffness and strength. It might be possible to back a bow with a very light FG cloth in the same manner as using fabric, linen or rawhide but I haven't looked at it. I suspect the FG would still cause trouble because of its properties, even if cut on a 45 degree bias.

What I don't understand is, is FG stretching, as in stretching and returning to it's natural state, elastic, or is it just under more tension as the bow bends and relaxed as the bow recovers? How much energy does it take to stretch FG? Is a 35# bow enough to do so, 45#, 55#, ? That's what I don't understand.

If the limb bends then all of the limb materials stretch (technically the stretch is called strain), except for the neutral axis. There is no other way for it to work. The stiffness of the FG is why FG limbs are so much thinner than wood limbs for the same draw weight, they develop the draw force required with much lower strain levels than wood needs.


Mark

[monterey]

This is correct. I have looked at it and even very thin FG lams just overwhelm the wood because of the much higher stiffness and strength. It might be possible to back a bow with a very light FG cloth in the same manner as using fabric, linen or rawhide but I haven't looked at it. I suspect the FG would still cause trouble because of its properties, even if cut on a 45 degree bias. 

I've used fiberglass cloth of the auto repair type to back board bows and it didn't do any damage.  It's just not in the strength class as bow building glass.   It didn't overpower the belly at all.   However,  it's no better than rawhide or paper bag.

[mmattockx]

I've used fiberglass cloth of the auto repair type to back board bows and it didn't do any damage.  It's just not in the strength class as bow building glass.   It didn't overpower the belly at all.   However,  it's no better than rawhide or paper bag.

That is good to know. What did you use for the glue?


Mark

[Pat B]

Thanks Mark. Your explanations help me understand somewhat.
When I think of stretch I think lengthening and when I think of strain I think of pulling without stretch(lengthening). Doesn't stretch relieve strain?

[mmattockx]

Thanks Mark. Your explanations help me understand somewhat.
When I think of stretch I think lengthening and when I think of strain I think of pulling without stretch(lengthening). Doesn't stretch relieve strain?

You are welcome. I'm trying to simplify it so non-engineers can get a handle on it. The whole subject isn't particularly intuitive but it is very helpful to have an understanding of it for building bows.

Stretch is strain. Anytime the material is deformed from its natural shape it is seeing strain. You may be confusing stress and strain. Stress is the internal forces in the material, strain is the deflection the material experiences.


Mark

[monterey]

I've used fiberglass cloth of the auto repair type to back board bows and it didn't do any damage.  It's just not in the strength class as bow building glass.   It didn't overpower the belly at all.   However,  it's no better than rawhide or paper bag.

That is good to know. What did you use for the glue?


Mark

I used a twenty minute epoxy that is a 50/50 and sold in hobby stores.  I would have used EA40  but it seemed like overkill.  That bow has made it's way through the family for a few years.   Don't know where it is now.

[Flem]

Mark, I think you might be overlooking the point I am trying to make. Which is, a bow is not automatically balanced when we take it out of the oven. You have to tiller it. And you can arrange the materials you have made it from to take advantage of their individual properties. For instance an ELB of Ipe backed with Bamboo. I am going to have to rethink my bow making if you tell us that it will have the same effect on draw weight, no matter if you removed the Ipe or the Bamboo. Or if it was removed from the sides or edges.

[Shredd]

Woven glass can be used and may work but it is not the best choice... The glass gets damaged when stressed with too much tension...  That's why it is not normally used in bow making...

[Pat B]

We aren't mixing FG bows and wood bows are we? There is a big difference due to the materials used ever though there are similarities in function.

[williwaw]

Hi.
New guy here.... so I might mention that my experience has been mainly with wood bows.  I have come here to learn a bit more about fiberglass. and working with laminates.

Crooked Stic,
One thing to keep in mind if your longbow limb is pyramid shaped.
Assuming an inch wide limb and quarter inch limb thickness and a 45 degree slope to the trapped side, you would be making the back half as wide as the belly. As you progress to the tip, you will run out of back when the limb narrows to a half inch.

Not saying you would actually consider such a radical trapping, but just pointing out the ratio of back width to belly width will change as you move down the triangular shaped limb.

[mmattockx]

Mark, I think you might be overlooking the point I am trying to make. Which is, a bow is not automatically balanced when we take it out of the oven. You have to tiller it. And you can arrange the materials you have made it from to take advantage of their individual properties. For instance an ELB of Ipe backed with Bamboo. I am going to have to rethink my bow making if you tell us that it will have the same effect on draw weight, no matter if you removed the Ipe or the Bamboo. Or if it was removed from the sides or edges.

I thought we were talking about fibreglass lam bows, not wood bows. You are absolutely correct that the properties of bamboo and ipe differ and removing one over the other will have a different effect. I don't understand the 'removed from sides or edges" part?

Woven glass can be used and may work but it is not the best choice... The glass gets damaged when stressed with too much tension...  That's why it is not normally used in bow making...

We are talking about using it as a backing material, not as the primary structural component as in FG lam bows.

We aren't mixing FG bows and wood bows are we? There is a big difference due to the materials used ever though there are similarities in function.

It appears we are.

Not saying you would actually consider such a radical trapping, but just pointing out the ratio of back width to belly width will change as you move down the triangular shaped limb.

This is correct. Trapping should be done as a percentage of the thickness and limb width, not just a fixed angle, for the reason you note. It is more work to do it this way but you maintain the same effect on the cross section properties for the full length of the limb.


Mark

[mmattockx]

In a private conversation with Shredd he mentioned I made it sound like all materials are equally stiff in tension and compression. I wanted to clear that up because that is certainly not the case. If the following is unclear, please say so and I will try to clarify whatever it is.

When I said materials are equally stiff in tension and compression in the elastic range I meant that each material is the same stiffness each way, not that it was equal to the stiffness of another material. Only that the stiffness is not different in one direction compared to the opposite direction for that particular material. Fibreglass is not the same stiffness as carbon, bamboo is not the same as ipe, maple is not the same as oak, etc. But any particular piece of wood or a FG lam will be give the same stiffness measurement in either compression or tension as long as you remain in the elastic range of stress for that material. The point of this is mostly that lams are not inherently stiffer in one direction than the other.

Another thing that occurs to me is a source of confusion is stiffness versus strength.

Stiffness is the measure of how much deflection a force will give. If material A has a stiffness of 1 unit and material B has a stiffness of 2 units, material B will deflect half as much as A for any given load. Another way of putting it is that it will take twice as much force to deflect B the same amount as A.

Strength is a measure of how much stress a material can withstand before it breaks. Strength limits are an issue for wood bows and the core material in FG bows, but not normally with the FG lams themselves as they are usually never loaded anywhere close to their actual stress limits.

When bowyers talk about how strong a limb is they really are talking about how stiff that limb is. A limb that is twice as stiff will require twice the draw force to achieve the same draw length. This is not related to the actual strength of the limb in terms of how much force is needed to break the limb.


Did that help or is it still clear as mud?


Mark

[Flem]

"I thought we were talking about fibreglass lam bows, not wood bows."

Same rules apply. Tension and compression. If I am laminating a bow, I am going to put the materials with more compressive strength near the belly and the high tension materials near the back. And if I happen to have some glass with a higher percentage of epoxy to glass fiber, I will be putting that on the belly side for the same reason. Since wood and bow composites will return to a neutral position faster from being put in tension than being compressed, I will be removing material from the belly side. Which in theory should shift the neutral plane and cause the back to work harder. If somebody can show me how compressed wood and composites have more elastic rebound than stretched materials, I'll eat my words.

[mmattockx]

Same rules apply. Tension and compression.

Yes, this is correct. But I was only referring to bows with the same material for the back and belly lams as in FG bows.

Since wood and bow composites will return to a neutral position faster from being put in tension than being compressed

Why do you believe this to be the case?

I will be removing material from the belly side. Which in theory should shift the neutral plane and cause the back to work harder.

If you remove material from the belly you shift the neutral axis towards the back, which loads the belly more and the back less.


Mark