Post by: Crooked Stic on July 09, 2020, 10:04:59 AM
Post by: kennym on July 09, 2020, 11:42:56 AM
Yer gonna have to build two alike and do one each way and test... :thumbsup: LOL
I trap to back and like you , only to lose weight. I think Widow does back too...
Post by: monterey on July 09, 2020, 12:32:55 PM
It's not a bow killer but is a performance robber.
Anyway, that's my take on it.
Post by: Mad Max on July 09, 2020, 01:08:17 PM
Post by: PV on July 09, 2020, 03:10:55 PM
Post by: Crooked Stic on July 09, 2020, 08:31:32 PM
Post by: Shredd on July 09, 2020, 10:34:17 PM
I find the only way to make gains in performance through removal of stock on a limb is, One, you have too wide of limbs to begin with and excessive dead weight that should be removed... Two, If the last third or half of the limb is too stiff you can remove stock and make a significant gain in performance with a better bending limb...
If you think you are gonna make a gain by trapping or narrowing your limbs you could shoot yourself in the foot by changing the bend in your limbs for the worst...
If I have to trap I usually only trap the last half or third of the limb and I do it from the belly side being that it is easier to do with a convex surface when the bow is unstrung... I guess the back could be just as easy when the bow is strung when working with a longbow...
I like the first half of what Monty said...
I don't trap much but when I do it's on the back. My reasoning is that the belly is generally weaker in compression by the tension of the back.But I feel you don't have to worry about trapping the belly if you are only trapping the last third of the limb... There is not nearly as much stress out there as there is just off the fades or mid limb... If for some reason I had to trap the whole limb to lose weight I would probably trap the back of the bow...
There are always exceptions to the rules... One being ( it still kinda falls under the rule) Designing a bow that is expected to be trapped... Meaning with standard square limb sides and width as narrow as you can go and the last third is on the stiff side, trap the whole length of the limb to lose weight and you might find a gain of 1 or 2, maybe 3 fps I don't think you will get more than that... To me I would only do such a thing if I was to enter some kind of contest where speed was involved such as flight shooting or the WTT contest...
Here is something to think about... If Trapping were such a sure thing in gaining performance then everyone would be doing it as a standard in building bows...
Post by: Shredd on July 09, 2020, 10:50:10 PM
There is also something to be said for trapping the whole back of the bow to possibly make the belly a bit more stronger in compression in comparison to the tension of the back of the bow... Which could possibly move the neutral plane more towards the belly of the bow, which could aid in less stacking but still maintaining a good rate of return through the shot cycle... This is just a theory...
Post by: Crooked Stic on July 10, 2020, 09:20:19 AM
Post by: Pat B on July 10, 2020, 09:31:55 AM
Post by: Flem on July 10, 2020, 11:05:35 AM
And since most wood is stronger in compression than in tension, it would make sense to trap the compression side to reap the most weight loss.
Post by: Pat B on July 10, 2020, 12:39:59 PM
Post by: Flem on July 10, 2020, 03:11:24 PM
Other than gluing surfaces and of course aesthetics does the wood in a glass bow have that much to do with performance? I guess it could be lighter physically than an all glass bow.
Yes, but it depends on the particular style of bow made. The wood in an old style glassed longbow, with a deep/thick core is going to be doing a lot of work.
Post by: monterey on July 10, 2020, 03:30:09 PM
I think that other than giving an inelegant slab a little visual appeal, the only reason to trap is to scrub some weight off without loosing any width. Which can be important on occasion with a narrow ASL style.
And since most wood is stronger in compression than in tension, it would make sense to trap the compression side to reap the most weight loss.
I think the compression vs tension works the opposite of that. Self bow builders can probably sort that out for us.
Post by: Pat B on July 10, 2020, 06:43:58 PM
Post by: monterey on July 10, 2020, 08:52:08 PM
Post by: Pat B on July 10, 2020, 10:51:32 PM
I don't know what a typical ASL is.
Post by: Shredd on July 10, 2020, 11:07:26 PM
Pat B... I am not sure that your last statement is correct though... If the glass did not give the bow would not bend and if it did the wood would shear...
Post by: mmattockx on July 11, 2020, 12:03:48 AM
And since most wood is stronger in compression than in tension
I believe that Flem is correct in his statement that wood is stronger in compression than tension...
This is backwards. Wood is significantly stronger in tension than compression, sometimes upwards of 3x more in tension.
Mark
Post by: Shredd on July 11, 2020, 01:09:41 AM
Post by: Bowjunkie on July 11, 2020, 06:09:11 AM
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.
Post by: Pat B on July 11, 2020, 08:19:10 AM
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. :dunno:
Post by: Flem on July 11, 2020, 10:17:12 AM
Post by: mmattockx on July 11, 2020, 10:33:45 AM
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
Post by: Shredd on July 11, 2020, 11:26:47 AM
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...
Post by: Flem on July 11, 2020, 12:08:00 PM
"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
Post by: Pat B on July 11, 2020, 02:12:14 PM
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.
Post by: Buemaker on July 11, 2020, 02:20:05 PM
Post by: Shredd on July 11, 2020, 03:05:04 PM
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...
Post by: Pat B on July 11, 2020, 03:59:34 PM
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.
Post by: Mad Max on July 11, 2020, 04:08:30 PM
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
Post by: Buemaker on July 11, 2020, 04:21:22 PM
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.
Post by: Bowjunkie on July 11, 2020, 05:38:11 PM
So what is the consensus now? Should I put it on the back and trap the belly or vice versa? :dunno: :laughing:
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.
Post by: Shredd on July 11, 2020, 06:39:23 PM
Post by: Bvas on July 11, 2020, 08:00:50 PM
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.So if you glue that Dynema to your lam and then bend it, the Dynema would be forced to stretch.
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.
Or cut thru the lam to stay straight. :dunno:
Post by: mmattockx on July 11, 2020, 08:02:53 PM
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
Post by: monterey on July 11, 2020, 08:51:42 PM
Post by: Bvas on July 11, 2020, 10:07:48 PM
Post by: monterey on July 12, 2020, 10:42:35 AM
Post by: Shredd on July 12, 2020, 11:09:18 AM
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...
Post by: mmattockx on July 12, 2020, 11:11:56 AM
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 :cheesy:). The strain experienced by the materials is how the draw energy is stored. No strain = no energy.
Mark
Post by: Flem on July 12, 2020, 11:54:08 AM
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.
Post by: Pat B on July 12, 2020, 12:16:39 PM
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.
Post by: Shredd on July 12, 2020, 01:53:33 PM
Post by: Crooked Stic on July 12, 2020, 02:20:58 PM
Post by: monterey on July 12, 2020, 02:55:31 PM
Post by: mmattockx on July 12, 2020, 05:46:07 PM
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
Post by: monterey on July 12, 2020, 07:54:19 PM
Quote
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.
Post by: mmattockx on July 12, 2020, 08:07:24 PM
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
Post by: Pat B on July 12, 2020, 09:20:17 PM
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?
Post by: mmattockx on July 12, 2020, 09:53:30 PM
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
Post by: monterey on July 12, 2020, 10:41:50 PM
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.
Post by: Flem on July 13, 2020, 08:59:51 AM
Post by: Shredd on July 13, 2020, 09:25:27 AM
Post by: Pat B on July 13, 2020, 09:30:21 AM
Post by: williwaw on July 13, 2020, 10:54:05 AM
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.
Post by: mmattockx on July 13, 2020, 11:52:27 AM
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
Post by: mmattockx on July 13, 2020, 12:24:55 PM
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
Post by: Flem on July 14, 2020, 09:20:15 AM
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.
Post by: mmattockx on July 14, 2020, 11:03:33 AM
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
Post by: williwaw on July 14, 2020, 01:18:24 PM
Quote
If I am laminating a bow, I am going to put the materials with more compressive strength near the belly
this is often done with wood laminated bows, for instance making a bamboo backed osage bow.
It must be said that the the osage is "stronger" not in the sense that Mark is using the word, but more as Flem is referring to osage ability to bend further than other woods or have "elastic rebound". Different qualities both being described with the same word?
There is similarity between a laminated wood bow made with two different materiels and a glass bow with a trapped back. The back and belly are stressed differently to a useful effect.
Shredd mentioned earlier,
Quote
Here is something to think about... If Trapping were such a sure thing in gaining performance then everyone would be doing it as a standard in building bows...
No one really responded to whether trapping is done universally in any particular designs. My question is related. Are there proven designs where different thicknesses of glass are used on the back or belly, or limbs glued up with asymmetrical stacks?
Post by: Flem on July 14, 2020, 04:17:59 PM
This is an erroneous statement for sure. let me clarify my thoughts on the neutral plane. My goal is to get it as close to the belly side as possible. To that end and within reason, the preference would be for a material to be of low hysteresis and compact/dense with the goal of using as little volume of it as possible in relation to the material on the other side of the zone. With the desired result of shifting the neutral plane to the belly, forcing more material into tension.
"Since wood and bow composites will return to a neutral position faster from being put in tension than being compressed"
This is also an erroneous statement. I should have said, materials suited for tension applications will generally, in the context and scope of bowyering, return to a neutral position faster than materials suited for compression applications
Post by: Mad Max on July 14, 2020, 04:55:46 PM
I'm not really following the thread but I was thinking :)
NOT that anyone would do this.
If you backed balsa wood with a bamboo slat the neutral plane would have to be the belly of the Boo,YES?
so any other wood the neutral plane would be moving towards the belly, Yes?
So Boo backed Ipe would be closer to the belly than Boo backed Osage?
From what I know Bamboo and Ipe are the best for a laminated Tri lam, so the neutral plane would be closer to the belly. If you used Ipe for the core that would move it even more to the belly?
JUST SAYING, I'm no Brainiac but it sounds like what I said would be true.? :dunno:
Post by: Flem on July 14, 2020, 08:12:37 PM
I can't believe I got sucked into the Neutral Zone again! My character would have been killed off on the first Star Trek episode. The worst part is being sucked into the intangible. Yeah there are formulas for estimating the location of the neutral zone, but how thick is it? one cell? one molecule? one atom?
How do you locate something that itself can't feasibly be measured? And in a dynamic environment like the ones we create will bending limbs and fulcrums, does the plane shift progressively like the increase in resistance to the bending force?
Some folks wish to be revealed the meaning of everything on their death bed, me, I would be fine with just the mystery of the neutral plane
Post by: Mad Max on July 14, 2020, 08:21:26 PM
Post by: monterey on July 15, 2020, 12:12:27 AM
Quote
or limbs glued up with asymmetrical stacks?
What is meant by "asymmetrical stack"?
Post by: williwaw on July 15, 2020, 01:51:30 AM
Post by: mmattockx on July 15, 2020, 09:37:22 AM
I should have said, materials suited for tension applications will generally, in the context and scope of bowyering, return to a neutral position faster than materials suited for compression applications
This is possible, but I have never seen any testing of materials in this fashion. Have you seen/done any to confirm it?
From what I know Bamboo and Ipe are the best for a laminated Tri lam, so the neutral plane would be closer to the belly. If you used Ipe for the core that would move it even more to the belly?
Yes, that is likely correct and is the end result when you do a two lam bow using 'boo and ipe. Since most of the stresses are carried by the surfaces of the back and belly you may get a faster bow by doing a tri-lam with a lighter core wood and thinner back and belly lams because the core isn't doing much work aside from holding the back and belly surfaces in place while the limbs bend. I haven't yet made one to find out, though.
The worst part is being sucked into the intangible. Yeah there are formulas for estimating the location of the neutral zone, but how thick is it? one cell? one molecule? one atom?
There are no intangibles with the neutral axis, it is a geometric property of the cross section shape. Unless your limbs see significant shear deflections (which should not ever be the case that I know of for a bow) it doesn't shift or change in any way due to bending. The thickness is zero because it is a geometric plane. It doesn't have to be measured, it is a calculated property.
Mark
Post by: Flem on July 15, 2020, 10:02:55 AM
Post by: Crooked Stic on July 15, 2020, 10:03:45 AM
Post by: mmattockx on July 15, 2020, 10:17:09 AM
Its still intangible. The same way electricity is intangible. Ours is a tactile endeavor.
This is true. It certainly is a mathematical construction and is not something of physical substance.
Mark
Post by: monterey on July 15, 2020, 12:29:49 PM
having different glass front and back or having a core that is not the same under the front as it is under the back.
Thanks, I'm familiar with that concept but had never heard that terminology.
Post by: monterey on July 15, 2020, 12:38:34 PM
I'm not sure if the balancing act can be done with a calculator given that wood varies from piece to piece within the same species but then again with our glass bows that may be less important than the comparative properties of the glass on each side with some consideration needing to go to what lay immediately under the glass.
Post by: mmattockx on July 15, 2020, 01:53:48 PM
It's my own thought that the ideal is to balance the tension and compression forces equally regardless of where the neutral plane ends up.
Can you explain or expand on what you mean by this?
Mark
Post by: monterey on July 15, 2020, 03:00:21 PM
Post by: williwaw on July 15, 2020, 04:10:50 PM
It is common to see references made in bow building forums to the wood database or other similar resources when bowyers ask questions about the materiels we work with. The published data is certainly useful for the basics, but I often wonder if wood, when bent as far as we make it bend, can act in ways that are not well understood.I should have said, materials suited for tension applications will generally, in the context and scope of bowyering, return to a neutral position faster than materials suited for compression applicationsThis is possible, but I have never seen any testing of materials in this fashion. Have you seen/done any to confirm it?
In the bow building world. we are our own testers when we build bows and make improvements as we go.
the preference would be for a material to be of low hysteresis and compact/dense with the goal of using as little volume of it as possible in relation to the material on the other side of the zone.I have never found much information about hysteresis specifically in tension, but one only has to make a bow out of wet wood and watch the arrows drop more and more especially as we draw the bow further, to see hysteresis in action. Presumably increasing on the compression side as we can easily see the belly fail.
Any good ideas how we could test for hysteresis on the tension side?
Post by: mmattockx on July 15, 2020, 06:57:23 PM
IOW, tension is not overwhelming the belly and vice versa.
How would you know or sense if one is overwhelming the other?
The published data is certainly useful for the basics, but I often wonder if wood, when bent as far as we make it bend, can act in ways that are not well understood.
In the bow building world. we are our own testers when we build bows and make improvements as we go.
This is very true. We are doing things with wood that none of the standard reference sources ever considered. Pretty much all of the data available was collected with the intention of using wood as a structural material for buildings, bridges or aircraft structure. No one was thinking of taking a long, slender piece of wood and bending it to near 90 degrees then letting it snap back to the original shape.
Any good ideas how we could test for hysteresis on the tension side?
Not without big $$$ lab equipment. My gut feel is that hysteresis on the tension side is near zero. If you take a wood bow (either a selfbow or a lam bow) and cut the back off the belly you will find the the back typically stays straight or goes into reflex and the belly pulls well into deflex. All the damage to the wood appears to be on the compression side, which is what my gut feel says as well. Tension doesn't cause the same kind of cellular damage because a tension failure normally occurs in one single, violent event as soon as the tension limit is exceeded.
Mark
Post by: monterey on July 15, 2020, 07:21:18 PM
Quote
How would you know or sense if one is overwhelming the other
Well, that's the crux of it. Maybe a bow that delams would offer clues.
I'm going to take the path of more compression resistant material in the belly. Starting with thicker glass.
One approach to knowing what is happening is looking for deflex (string follow). My bows are primarily ASLs which makes string follow easier to observe and more likely due to the higher ratio of core lams.
What if there is no follow observed over time? Then an indicator would be the changes in draw weight over time. It calls for careful record keeping. If I can get to no string follow and no, or very little, loss of draw weight I'll feel that balance has been reached.
Blacky Schwarz always came up with a figure for hysterisis but I got no idea how he arrived at it.
Post by: Mad Max on July 15, 2020, 07:27:03 PM
Horn bows were made 1000's of years ago using wood cores. :o
Post by: mmattockx on July 15, 2020, 08:10:33 PM
Horn bows were made 1000's of years ago using wood cores. :o
I understand that, but those were still crazy bowyers. All the wood test data we use comes from people that were using wood as a structural material that sees static loading for the most part and small deflections due to that loading. Bowyers are using wood in a completely different way and there has been precious little testing done for our uses. Our situation is not helped by the fact that we are trying to walk right up to the edge of failure with each piece of wood and that the variance even among pieces of wood from the same tree are very substantial.
Mark
Post by: williwaw on July 15, 2020, 08:48:18 PM
I'm going to take the path of more compression resistant material in the belly. Starting with thicker glass.....If I can get to no string follow and no, or very little, loss of draw weight I'll feel that balance has been reached.
your choice of working the back harder seems very reasonable if consideration of the underlying core properties is called for. Maple (if similar to other hardwoods) is said to be twice as strong in tension than in compression. whether it can stretch as much as the glass above is doubtful, but it will be harder to stretch, being in tension.
when sinew is applied to the back of a bow, it is sometimes mounded up so the outermost layer can stretch even more. Perhaps you could consider applying the extra glass you propose for the belly in a similar fashion, rather than spreading it across the full width.
Post by: monterey on July 16, 2020, 11:03:46 AM
Post by: mmattockx on July 16, 2020, 11:37:27 AM
I'm going to take the path of more compression resistant material in the belly. Starting with thicker glass.
I once asked an experienced FG bowyer about using thinner glass on the back in order to raise the stresses on the back and use more of its capacity (FG bows normally don't use anywhere near the full capacity of the glass). He warned me against going too far with that as the wood core tends to fail in shear right at the glue line if you work the FG lams too hard. The problem is the FG can withstand strains that are 2-2.5x higher than the wood can.
Using thicker belly glass is OK, but you can't go too far with making the back lam thinner when building an asymmetrical limb stack.
One approach to knowing what is happening is looking for deflex (string follow). My bows are primarily ASLs which makes string follow easier to observe and more likely due to the higher ratio of core lams.
How much set do you see happen with your bows? A thicker FG belly lam might be a very good idea if the core is taking set that is measurable. It would shift the neutral axis and lower the strains on the core wood at the glue line to the belly lam.
What if there is no follow observed over time? Then an indicator would be the changes in draw weight over time. It calls for careful record keeping. If I can get to no string follow and no, or very little, loss of draw weight I'll feel that balance has been reached.
I agree, set shows up first in loss of draw weight (when tillering a wood bow, which is all my experience). That seems to be a very good way to detect it before it becomes visible.
I understand what you mean about being balanced now. You are trying for a no set bow in the end, which is a very worthwhile target.
Maple (if similar to other hardwoods) is said to be twice as strong in tension than in compression. whether it can stretch as much as the glass above is doubtful,
I use maple boards for bows and it definitely is twice as strong in tension as compression. No wood can survive the strains that FG can.
when sinew is applied to the back of a bow, it is sometimes mounded up so the outermost layer can stretch even more. Perhaps you could consider applying the extra glass you propose for the belly in a similar fashion, rather than spreading it across the full width.
Since FG is heavy this would probably not be the most efficient way to use it. I suspect you would get the same effect with a thinner lam that runs the full width of the belly and has less weight overall. Would have to crunch numbers to see how it all works out, though.
Mark
Post by: williwaw on July 16, 2020, 02:26:18 PM
when sinew is applied to the back of a bow, it is sometimes mounded up so the outermost layer can stretch even more. Perhaps you could consider applying the extra glass you propose for the belly in a similar fashion, rather than spreading it across the full width.
Since FG is heavy this would probably not be the most efficient way to use it. I suspect you would get the same effect with a thinner lam that runs the full width of the belly and has less weight overall. Would have to crunch numbers to see how it all works out, though.
Mark
well maybe not mounded up as much as is done sometimes with sinew, but perhaps a thicker glass finished out with a crown? I agree the performance might not be all that much different no matter how the glass is arranged, as it is so thin.
Perhaps I should clarify the context with which I tossed that idea into the discussion. I have drifted a bit from the OP's question about trapping for performance gain, and presume the ASL style Monterey is primarily building is designed with different goals in mind.
Quote
The problem is the FG can withstand strains that are 2-2.5x higher than the wood can.
Good Point. I am not sure if FG is too stiff to be used effectively with some designs if it cannot be strained to its full potential. Maybe something with half it's stiffness (but still stiffer than the core) would be a useful laminate. One of the benefits FG brings to the table is its uniformity for use as a back or belly, reliving the bowyer from the need to find pristine wood with suitable grain orientation for the back if building without FG. A thicker FG replacement would allow more "shaping" of the bow limb to emulate classic wood designs.
Admittedly, I am spouting off a bit with some untested theory about FG, but coming from a wood background, mixing and matching of materiel's of different qualities is more easily done.
I would be interested to hear of any results or view links someone can post, that demonstrate what happens when thicker limbs are shaped more rounded such that the outer FG lams are reduced in width.
Post by: Flem on July 16, 2020, 03:52:31 PM
Or this?
Check out what has been said in reviews about Schulz or Sunset hill longbows. Sorry I don't have any links, but info should be easy to search for.
Post by: williwaw on July 16, 2020, 04:55:35 PM
I found lots of pics, but not many that can give a true sense of how round the limb might be getting.
I suppose my question was seeking comment on what happens when the rounding of the corners is taken to the extreme., ie approaching an elb cross section.
Post by: mmattockx on July 16, 2020, 06:17:06 PM
well maybe not mounded up as much as is done sometimes with sinew, but perhaps a thicker glass finished out with a crown? I agree the performance might not be all that much different no matter how the glass is arranged, as it is so thin.
The problem with crowning is that it strains the material more in the high part of the crown and less in the rest. To use FG most effectively you want to strain all of it the same amount.
Good Point. I am not sure if FG is too stiff to be used effectively with some designs if it cannot be strained to its full potential.
It is never used to its full potential in a traditional bow. Usually it is stressed to no more than about 50% of its limits (often much less than that) because the core can't withstand much more or you run into stability problems with narrow and/or thin limbs. If you want to see what FG can do, look at current compound bows or compound crossbows. Thick, stubby limbs that generate enormous forces (and energy storage) with very small deflections. That is what a more optimal FG limb design looks and performs like.
Maybe something with half it's stiffness (but still stiffer than the core) would be a useful laminate.
Yes! A lam or backing material that was closer in stiffness to wood but still significantly stronger in terms of the maximum strain it could withstand would be a great step forward. It would be somewhat of a holy grail of traditional bow material.
I would be interested to hear of any results or view links someone can post, that demonstrate what happens when thicker limbs are shaped more rounded such that the outer FG lams are reduced in width.
A high crown acts the same as trapping in its effects. It shifts the neutral axis away from the crown, raising stresses on the crown side and lowering them on the opposite side.
Mark