Primitive Archer
Main Discussion Area => Bows => Topic started by: beast on February 05, 2021, 03:48:02 pm
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I have been thinking a lot recently and want to share my thoughts, and what you know about the following topic.
Right so this is hard to phrase but il try.
lets for all of these examples assume the bows won't stack and all have pretty linear force draw graphs. also lets say they draw 50 pounds because that's a nice round number.
1. Say we have a normal d style bow, no reflex, no deflex, and no recurve. and we compare this to a reflexed bow, say reflexed 10 inches. you might say that is a lot of reflex but my question is why.
The reflexed bow will be doing a hella lot more work, as we know because of early draw weight. but because its drawing 50 pounds like the d style bow, the limbs will have to be thinner, again because its reflexed.
Now, we say the reflexed bow is under more stress, but arent both bows drawn out under the same 50 pounds? Doesn't that mean they are both under the same amount of tension forces and the same amount of compression forces at the belly?
SO if I'm correct, then both bows are now stressed the same. the only difference is that the reflexed bow is under more stress at brace. but i don't think that changes the performance much, does it? besides maybe making the bow take some extra set?
Let me know what you think.
oh, and I'm new to this forum so..
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I think it’s incorrect to say that the bow is stressed more as I understand stress as being how close it is to failing. If both designed for their respective designs and characteristics both bows could be stressed equally. The reflexed bow will be storing more energy through its draw as at each increment of draw length it will be a higher draw weight so even if the FD curves are both linear, the reflexed bow will have a shallower slope compared to the straight bow. If designed well the reflexed bow may not take more set. The trade off to get higher energy storage is needing more mass or a wider limb to achieve this. If my understanding is correct, there may be other factors of increased vibration or hysteresis as well. This is why massively reflexed bows don’t always outperform more mildly reflexed ones. At least that’s my understanding.
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but that is what i am saying ryan. why should we need wider limbs on reflexed bows if their draw weight is the same, because that would mean the tension forces on the bow are the same. comparing a d bow of 50 pounds draw weight and 50 pounds reflexed bow, the reflexed will be thinner limbs, and thinner limbs will be able to bend further and stack less, thus being able to be reflexed without problems. so my point is reflexing a limb doesn't mean we will make the bow stack, or at least if my logic is correct.
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Think of it as comparing amount of total tip movement between the two bows. So you your d bow and another bow that, instead of being reflexed, just pulls another 5” of draw. The longer draw bow will need to be thinner to accommodate the tighter bend and wider to have enough wood to store the same amount of energy. The longer draw bow will be heavier overall because it has to be disproportionately wider than thinner for enough wood to do the same amount of work. Reflexed limbs experience this in the same way except that extra movement is experienced when the bow is braced as opposed to at the end of the draw.
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Stress depends also (and more) on the degree of bending, regardless the poundage. I mean both bending degree and poundage influence stress individually, but bending degree more than poundage (like a 10% increase in bending degree increases stress of wood much more than a 10% increase in poundage). For the same 50 lb, at a higher bending degree the actual tension and compression forces on back and belly will be higher (even if poundage is the same).
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Sometimes technical discussions get difficult if specific terms are used that don't mean the same to all.
Stress is understood in scientific and engineering as pressure. PSI, pounds per square inch in the US, is a common unit.
The reflexed bow will be doing a hella lot more work, as we know because of early draw weight. but because its drawing 50 pounds like the d style bow, the limbs will have to be thinner, again because its reflexed.
Now, we say the reflexed bow is under more stress, but arent both bows drawn out under the same 50 pounds? Doesn't that mean they are both under the same amount of tension forces and the same amount of compression forces at the belly?
For the same bend: the tension and compression stress is reduced when the limb is made thinner.
If you had chosen to make the recurve limb narrower to keep it at 50#, the stress would be higher because the original thickness will have had to bend further.
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the reflexed bow is more likely to blow up,, :)
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thanks bradsmith.
dilivou, u said the stressses on a material are more affected by the degree of bend rather than the poundage. but like, how would that make sense. If I'm pulling any bow back 30 inches , 50 pounds of force, the bow is also dealing with 50 pounds of force.
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ALso, im not increasing the poundage here. like ryan said, well think about reflexes like we would a normal bow with extra draw length.
Right then:
o and Ryan, you keep talking about the width of the bows. "and wider to have enough wood to store the same amount of energy."
brother we already have enough wood to store the energy, that's why where thinning it out so it would reduce the draw weight back down to 50.
a normal 50 lb bow at 30 inches, drawn to 35 inches, would reach 60 or 70 lbs or something like that in poundage.
so we would have to thin it back out.
where does the width come into play here
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Sometimes technical discussions get difficult if specific terms are used that don't mean the same to all.
Stress is understood in scientific and engineering as pressure. PSI, pounds per square inch in the US, is a common unit.
The reflexed bow will be doing a hella lot more work, as we know because of early draw weight. but because its drawing 50 pounds like the d style bow, the limbs will have to be thinner, again because its reflexed.
Now, we say the reflexed bow is under more stress, but arent both bows drawn out under the same 50 pounds? Doesn't that mean they are both under the same amount of tension forces and the same amount of compression forces at the belly?
For the same bend: the tension and compression stress is reduced when the limb is made thinner.
If you had chosen to make the recurve limb narrower to keep it at 50#, the stress would be higher because the original thickness will have had to bend further.
willie mate, im not reducing the width of the bow, just the thickness. so then, the stress will be the same, because now the reduced thickness will have to bend the same distance??
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i dunno what just happened up there, I just tried to quote willie and ended up quoting myself.
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That description of stress sounds confusing to me. Really we have to think of wood as stretching on the back and compressing on the belly. Wood can only stretch to a certain degree before breaking. If the wood is thicker the back and belly will be forced to stretch/compress further than if thin. Optimally built bows are as thick as possible without taking much set as that cross section is less massive than a bow that is made wider and thinner.
Beast, it sounds like your question can be distilled down to, if we are pulling bows to the same draw weight, why do they have to have different masses? The reason is energy storage. 50# draw weight between two different bows can have totally different amounts of energy storage. Simply speaking we need more wood as a place to store that energy. This is probably the end of my understanding before doing more reading on these concepts. ;D
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thanks for replying ryan.
but lol
no that isn't the question I'm trying to get at.
look.
say we have a short bow and a longbow.
the shortbow will be thinner to reach the same draw weight because of how short it is, but wider so we can have in increase in draw weight without making the wood undergo more compression and mention forces. but in this situation im not trying to mess with width or length, because changing to many factors gets confusing.
now as I understand it all woods have modulus of rupture wich is how much tension a back can take before, rupturing.
now, we have a D bow that is 65 inches. it pulls 50 pounds. at 30 inches.
Now, without changing the width, we make the wood thinner. the draw weight at 30 inches decreases. right? this decreases the stress on the bow.
Ok well now I want a 50 pound bow, not some thin stick, so I would steam in some reflex, such that the draw weight bumps back up to 50 pounds at 30 inches. NOw, is this final bow going through more or less or the same amount of stress as the bow we started with
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Now, without changing the width, we make the wood thinner. the draw weight at 30 inches decreases. right? this decreases the stress on the bow. correct, because you decreased thickness
Ok well now I want a 50 pound bow, not some thin stick, so I would steam in some reflex, such that the draw weight bumps back up to 50 pounds at 30 inches. NOw, is this final bow going through more or less or the same amount of stress as the bow we started with
it could be more and it could be less. it could also be the same if your thinning was done in a proper amount related to how much additional reflex you added
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Here's how my intro to engineering class explained these terms: Stress = force per area = pressure, as mentioned above. Strain refers to the change in length of a material as it is loaded. Stiffness refers to how much force causes a unit change in length (the resistance to bending) Strength = stress at breaking point.
I hope this helps.
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I would guess that the reflexed bow is under more stress because less wood is being asked to do more work. If this worked without over stressing the bow then any bow could be made to draw any length till the string came off. But what we see is that they end up taking more and more set or breaking.
This is an easy experiment. Try it and let us know how it turns out. You’ll have to make sure the bow isn’t overbuilt though before reflexing.
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You can't think of weight as the deciding factor in how stressed a bow is. Take your same thought but make it a 10" deflexed bow pulling 50lbs even though it is pulling the same weight it isnt stressed at all and won't shoot an arrow at hardly any speed. I think draw weight has more to do with arrow weight, heavier arrow heavier draw. Total tip movement has to do with stress but bow design and draw weight factor into that too
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yes allyn let me reply to what u said. u said a deflexed bow will be going under less stress right? but in fact because the deflexed bow will have to be nice and thick to reach say 50 pounds compared to a d bow or reflexed bow which will be way thinner. and so allyn, if u pull a deflexed bow back, it will stack quicker. so the tips will do what we asked them to, so in a 30 inch draw, the 10inch deflex tips will now move 20 inches. but well find the bow will reach higher draw weight weigh faster past those 30 inches because the limbs are extra thick.
right now say we make a bow that isn't tillered and those limbs are a whole inch thick. now that bow wont be able to draw more than like 5 inches before exploding because the stress on the wood will be incredible, the draw weight of it will be very high.
right now I will test what I am thinking, right, now my intuition says obviously relfexed will break on me, but I dunno.
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well those mongols use sinew and horn to deal with their crazy recurves, well obviously they use those materials for a reason. but . eh. i . aw man this stuff is confusing me
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another thing, ya take a nice shaving of wood off the ground when ur working. you can bend that into a whole circle and it wont break because it is very thin
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I have not read everything so if im repeating sorry,,
if your bow is reflexed 10 inches and you draw it to 28 inches,
it is bending alot further than a straight bow that is drawn to 28 inches,
so it is bending more ,, and stressed more,, bows are wierd, what seems to work in theory,, sometimes does not in reality,,
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Distill your question :)
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Beast, your use of terminology seems like it might not be consistent with most people’s understanding. Stacking comes into play when the bow becomes less efficient at storing energy later in the draw due to string angle. Bows can stack heavily without being over stressed. The wood stretching concept is useful here. You can have a very thick and narrow bow that pulls 50#@2” versus a thinner and wider bow that draws 50#@28” with the same strain/stress because the wood is being asked to stretch the same amount. The first difference is that the wood in the thinner bow is closer to the neutral plane and can bend to a tighter radius with the same amount of stretch as the thick bow. Second, the thick bow is storing significantly less energy as it is pulling the same draw weight over a shorter distance. The narrow short drawing bow will also have less mass due to the properties of a beam being stiffer by a cube of the increased thickness.
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right, well goodmorning.
ryan, i think ur wrong there. you said the thick bow and the thin bow drawing 50 pounds at 28 inches are under the same stress. no. the back of a wider bow has more area, as its wider, therefore those 50 pounds are spread out over a wider area.
the width makes a huge difference when it comes to stress.
right so ok, I thought stacking meant something else, I thought it happened when the material starts to fail, and the weight rises faster because of that. so forget everything I said about stacking.
ok, well I was only trying to bounce ideas off of you all, because all the recurves i made took so much set. maybe I need to stop working with whitewoods and start using wood that actually has decent compression strength.
right well what I learned is either I'm a knob or theory doesn't work with bows, like bradsmith said. but it should though. let me find new wood to try a severe reflex on it, rather than the hickory I'm using.
ill let yall know when i finish some experiments. If i don't get blown up by an exploding bow.
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White woods are fine. Sounds like you are long on theory and terminology and short on experience. There is no need to try to attain too severe a reflex with wood.
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right, well goodmorning.
ryan, i think ur wrong there. you said the thick bow and the thin bow drawing 50 pounds at 28 inches are under the same stress. no. the back of a wider bow has more area, as its wider, therefore those 50 pounds are spread out over a wider area.
the width makes a huge difference when it comes to stress..
Yes under the same stress because they are being asked to lengthen the same amount. Width has to do with more stored energy. You are mixing up stress (how close it is to breaking) with stored energy. A beam of any given thickness can only bend to a specific radius before breaking regardless of width. Say it is 1/2” thick and can bend to a radius of 28” with 50# of force. If you double the width it’ll take 100# of force to pull to 28 with the same amount of stress/strain. I think you haven’t grasped that with differing thicknesses and widths that we are asking the wood fibers to stretch (or compress) as much as possible with each given bow without taking set.
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yes allyn let me reply to what u said. u said a deflexed bow will be going under less stress right? but in fact because the deflexed bow will have to be nice and thick to reach say 50 pounds compared to a d bow or reflexed bow which will be way thinner. and so allyn, if u pull a deflexed bow back, it will stack quicker. so the tips will do what we asked them to, so in a 30 inch draw, the 10inch deflex tips will now move 20 inches. but well find the bow will reach higher draw weight weigh faster past those 30 inches because the limbs are extra thick.
right now say we make a bow that isn't tillered and those limbs are a whole inch thick. now that bow wont be able to draw more than like 5 inches before exploding because the stress on the wood will be incredible, the draw weight of it will be very high.
right now I will test what I am thinking, right, now my intuition says obviously relfexed will break on me, but I dunno.
You are still using just draw weight as the only measure of stress. Thicker or thinner wood doesn't mean more or less stress. Thinner wood can bend further without taking as much set but that doesn't change the amount of stress. Wider limbs distribute stress and so do longer limbs. If the stress on a bow was just based on draw weight no one would have to change bow design for weight and wood species. If you blow up two ballons to 50psi but one balloon was smaller than the other before the air, they both would have the same amount of pressure inside but the smaller ballons would have thinner walls and be closer to popping(more stressed)
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I’ll add that a specific amount of wood is needed to do the work being asked of it. If you make a bow thinner (less mass) and expect it to do more work, it’s going to take set or break.
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More bend means more strain. And more strain leads to more stress (higher force per area in tension and in compression). For two 50 lb bows the tension force on back and compression force on belly are not the same if strain is different. The 50 lb bow that is more strained will deal with much higher tension and compression forces compared to the 50 lb bow that is less strained. The poundage of the bow (as a math function) is not a direct, linear result of tension and compression forces that appear when the bow is drawn.
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White woods are fine. Sounds like you are long on theory and terminology and short on experience. There is no need to try to attain too severe a reflex with wood.
but there is a reason isn't there. you want faster speeds, you need high early draw weight.
i think I'm being misunderstood here ngl.
forget reflex now:
think about this as Bow A and Bow B
Bow A is a normal bow with given thickness
Lets say Bow A pulls 50 pounds at 28 inches. alright now we reduce that thickness. now the bow A will pull like 35 pounds at 28 inches. lets call this new bow: bow B.
Ok. now lets think of bow A as being pulled from 6 inches of brace height to 28 inches.
And bow B, lets think of it working from 10 inches of brace height to 32 full draw rather than reflex because its pretty similar.
Sweet. now This bow B, pulled 32 inches back, should be under the same amount of stress as bow A pulled back 28 inches. please correct me if I am wrong.
we are not changing width or length here, only thickness.
again correct me if I'm wrong, but what determines a woods breaking point is its modulus of rupture.
hold up pause i just got a brain wave.
so is the reason why reflexes are under more stress because our back of a reflexed bow that is doing work is much thinner and therefore we have less volume of wood under the same amount of stress. because tension isn't spread across an area, its across the back of a bow which isn't just the back, its from the back to the middle of the thickness of the bow?? and now that volume is smaller because its thinner?
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I think you're starting to get it. The thinner bow would certainly be under more stress because there is less wood overall to do the work. While most of the strain is on the back and belly surface, the inside part of the bow limb also contributes just to an exponentially lesser degree as you approach the neutral plane.
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Ok everyone i have it all figured out.
the reason why the reflexed bows with the amae width as a normal bow is under more stress is because though the work dis dealing with the same amount of force, it is being distributed through a thinner limb, so less material is doing proportionally more work.
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thanks ryan and everyone else who responded.
this helped a lot.
but lol it wont stop me wanting to make reflexed limbs.
so now if I want a reflexed bow to be under the same stress, though the same draw weight it would have to be wider and or longer
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i said work up there, i meant wood. the spell check is annoying
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Ok everyone i have it all figured out.
the reason why the reflexed bows with the amae width as a normal bow is under more stress is because though the work dis dealing with the same amount of force, it is being distributed through a thinner limb, so less material is doing proportionally more work.
the stress on any bent limb varies from the surface to the center of the limb. it is highest at the front and back and least towards the center.
any design that pushes the materiel to it's working limit, will be limited by the stress at the back or belly (usually the belly for most woods)
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right. so i know what happens when a bow's back gets stressed too much, the fibers rip apart.
but what happens when a wood fails in compression?
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The cells or fibers get crushed. Pushed past the point where they can return to their original shape. Plastic deformation.
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I think that happens a bit on the back to, most materials will not return to their original shape if you stretch them to far, even if they are not broken. Total compression failure is what happens with frets and chrysals.
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Wood is not considered a ductile material.
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It's a matter of degree.
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Several years ag I built an osage bow deflex reflex design maybe 3 1/2" reflex. No matter what I did even on the best bows I would get a little bit if set. So I thought just for the hell of it I will go super wide like 2 1/2" to see if I could get close to zero set. The bow came out good but what was interesting was that the mass came out slightly lighter than a typical bow of 1 3/8 width even though it was almost double the width. The reason the mass came out so low was because it didn't adhere to the beam theory we often apply to bows. Twice as ide would normally mean about 25% thinner in thickness. Instead it was almost 50% thinner. Only thing that could explain this is that we are doing more damage to the wood than we think we are or even pick up on when we measure set.
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Steve, I don’t think I ever asked but did that wider bow shoot faster than the ones taking set?
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I believe it did briefly.
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Pat, the one you are thinking of here was a super recurve. It shot fast for a few shots but broke down quickly. The one i am talking about here I don't think I ever properly tested and ended up giving it away but I know it was a good shooter it took almost no set at all.
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If we can make bows wider with less mass and set is there any reason we shouldn’t be making wider bows from a performance standpoint?
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well yes ryan wider bows dont look too good. not good at all. in fact they are ugly
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well yes ryan wider bows dont look too good. not good at all. in fact they are ugly
Well then you've limited yourself in the goals you want to achieve.
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but im having another dilemna.
what ar the effects of reflex on the whole limb, vs reflex on just the outer 2/3ds of the limb, vs just reflexing the tips.
and then, what about the recurces because those do more than reflexes by changing string angle.
another thing. I know reflex deflex bows are very fast, faster than a D longbow. But, some r/d tips are not in front of the handle. just level with them. so the reflex does some serious work if they add some good speed despite the bow having deflex and the tips still not being in past the handle.
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well yes ryan wider bows dont look too good. not good at all. in fact they are ugly
Beauty is in the eye of the beholder.
I once built a 50”ntn 50#@25” poplar bow with 60% working limb and 40% stiff outer tips that was 3.5” in the dead center. Many people called it ugly and it couldn’t shoot an arrow but it was beautiful to me because it proved my point. ;D
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well yes ryan wider bows dont look too good. not good at all. in fact they are ugly
Agreed in most cases. It is also much harder to get a wide stave.
Those R/D bows shoot faster because they are effectively shorter at the start of the draw, which means they have a fatter f/d curve.
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what do you mean by shorter. i thought recurves did that. the recurves that change the string angle because u start out with a small bow but when the recurves are curled out the bow elongates kinda. how would r/d start out shorter
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ryan did you just say u made a 50 inch poplar bow..
that pulls 50 pounds
and to top it off only 60 percent of the limb was working?
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oh but i just read that you said it was 3.5 inches wide.
wow that's wide.
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A r/d bow is a deflexed bow with sweeping recurves, right? As the string lifts off the limb the effective length of the limb increases.
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If we can make bows wider with less mass and set is there any reason we shouldn’t be making wider bows from a performance standpoint?
from a performance standpoint, maybe no reason. guess not many have the patience for tillering wide flat short working limbs and finessing the transition to the skinny tips
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I think most of the time wider will be heavier anyway. I have run into a few since that first one that came out light but I don't believe that is always typical.
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tradcraftsman . i dunno if me and u are thinking about the same thing. I'm not talking about deflex bows with heavy reflex to the point it becomes recurve. I'm talking about very simple deflex maybe 2 3 inches and reflex to bring the tips level with the handle.
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I think most of the time wider will be heavier anyway. I have run into a few since that first one that came out light but I don't believe that is always typical.
Maybe the challenge is to get it thin enough before bending to avoid any wood damage at all as any damaged wood could be excess mass.
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I think most of the time wider will be heavier anyway. I have run into a few since that first one that came out light but I don't believe that is always typical.
Maybe the challenge is to get it thin enough before bending to avoid any wood damage at all as any damaged wood could be excess mass.
Like trying to tiller it halfway before you actually put it on a string
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I think most of the time wider will be heavier anyway. I have run into a few since that first one that came out light but I don't believe that is always typical.
Maybe the challenge is to get it thin enough before bending to avoid any wood damage at all as any damaged wood could be excess mass.
always a good consideration, maybe especially more so with wide and thin limbs
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When y’all get to the bottom of this I will guess the bow with as close to perfect diminishing have the most reflex without set will be the fastest wood bow in the world. That’s my story and I’m sticking to it till somebody proves me wrong. Arvin