Primitive Archer
Main Discussion Area => Bows => Topic started by: willie on April 20, 2017, 02:49:59 pm
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In a nearby thread that features a bow with static recurve tips, a poster asserted
"A sharp static tends to have much higher brace tension"
Is this because the lever at the end of the limb can be considered to be "torquing" the tip, rather than simply applying a bending force?
If so, would not the outer portions of the limb, just inboard from the levers, need to be stiffer than otherwise?
Does the angle of the the levers relative to the rest of the limb matter much?
Can most agree with that singular assertion, or is more required to gain high early draw weight?
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I'm going to admit that I don't quite understand how recurves, reflex, and such improve early draw weight. I would love a scientific-ish explanation as well if anyone can chime in. I was under the impression that once you brace it it essentially zero's out all the work put in to get it to that point, but this must be a wrong assumption.
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You guys are the science guys, we just make bows. ;)
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The balls in your court, Pat. Take a stab at it.
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I'm not sure I agree that a sharp static has higher brace tension than a rounded static.., but here's how I explain what a contact recurve does. Think of two pencils, a regular one and a "golf" pencil (short). Both are the same thickness, but the long one is easier to bend because the ends act like levers.If you hold the regular pencil with both hands near the middle, it is hard to bend just like a golf pencil.
When a recurve is at brace, it "acts" like a bow that is shorter- it acts like a bow that is only as long as the distance between the points of contact. This short bow is harder to draw because it is short.Hard to draw= high early draw weight. At full draw, the string lifts off the recurves, and now the bow "acts" like a bow that is longer, giving you more leverage to bend the bow. So a recurve acts like a short, stiff bow early in the draw cycle, and a long bow later in the draw cycle.
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Also, I think, because it's a "shorter" bow it has to be bent more to get the same brace height so you are effectively further into the force curve.
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The balls in your court, Pat. Take a stab at it.
I AM making a bow. ;)
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::) (-P
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Also, I think, because it's a "shorter" bow it has to be bent more to get the same brace height so you are effectively further into the force curve.
Yes, if you make the working limbs shorter by taking some of the length to add reflex it will make them bend more. But, hypothetically, imagine you have two bows with the same length working limb. One is straight and the other has some static recurves added like siyahs onto the end. the working limb is going to bend the same distance for both to get the same brace height right? Wouldn't that equate to the same early draw weight, unless there's some kind of lever thing going on like aaron said?
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Yes but we build bows to a certain final draw weight. The recurve bow gets "longer" as the string lifts off. This lowers the final draw weight. So if we have your hypothetical bows the straight one will keep climbing to a higher draw weight while the recurves weight will level off. If you retiller the straight one to the same final draw weight you loose the early draw weight.
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Also here's a thought experiment with bracing. Say you have a bow who's limbs are bent on a long string the same distance they usually are for 28" draw. Now let's say the same bow is braced really high, then the bow is pulled only till the limbs bend that same distance. If the fist scenario was pulling 50 lbs for that limb movement what will the second pull for that same limb movement? I think I might try this experiment on my own and see what I find.
What I'm trying to figure out is if bracing reduces the stored energy at all of if it just zero's out the brace height at 0" draw only.
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Yes, but we build bows to a certain final draw weight. The recurve bow gets "longer" as the string lifts off. This lowers the final draw weight. So if we have your hypothetical bows the straight one will keep climbing to a higher draw weight while the recurves weight will level off. If you re-tiller the straight one to the same final draw weight you loose the early draw weight.
So really recurves don't increase the early draw weight. They decrease the final draw weight and thus only give the illusion of increasing the early draw weight.
Thus going back to the earlier post about the bow that reached final draw but had no string let off. Since it had no string let off it didn't have any lever advantage to decrease the final draw weight. Therefore, just because it is a recurve, it doesn't necessarily mean it had better early draw weight since it didn't have string let off. That is unless there is something we are missing still.
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Yeah, I guess ;D ;D
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Only a compound decreases final draw weight. ;)
I think what happens is that people struggle to make sharp statics so they convince themselves that there is no benefit. :-D
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I think of it like pretentioning a spring.
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The bow wants to bend in a arc but the recurved resist ,placing more tension on the limbs,thus raising early draw weight.smooth draw as it is referred,means there is hardly any noticeable weight gain from brace to full draw from my experience.
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It's all about string angle........
DBar
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It's all about string angle........
DBar
Not really. ;)
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yeah, I mean, you can have 90 degree short hooks that are vertical at full draw but still have plenty of stack..
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Ok maybe Im thinking about it wrong but I always thought that the early tension had nothing to do with recurve angle but how far foward of the back the tips are regardless of design ,strait,recurve etc. I think Tim Baker did some writting on the subject !
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Ok maybe Im thinking about it wrong but I always thought that the early tension had nothing to do with recurve angle but how far foward of the back the tips are regardless of design ,strait,recurve etc. I think Tim Baker did some writting on the subject !
I think I was wrong when I thought bracing eliminates all pre-brace stored energy. I think bushboy is right in that it's like a pre-stressed spring. Even though its braced and that's its new starting point, the limbs still want to return to their unbraced profile.
Threrfore I would say early tension is based on how far the working limb has to travel to brace. When you brace a recurve you don't get the tips moving to brace height, just the end of the working limb. The tips may still be hanging back a bit at brace, especially if it's a sharp static. However, with a reflex you do have to make the working limb travel further to brace therefore you get eary tension greater than a straight limb. If you had reflex and recurve you would have both an increase in early draw weight and a decrease in final draw weight due to the lever effect Aaron and DC said. Thus you get a nice fat force curve.
Sorry PatM i'm going to have to disagree with you in that recurves do decrease final draw weight. If you increase the lever length (moment arm) you need less force to produce the same torque. Just like a longer wrench is easier to loosen those lugnuts on a tire. You get more torque for less force. So if we have less force near the end of the draw due to longer moment arms, we thus need less force for the same bend. In other words we have less draw weight at the end than if there wasn't recurved tips.
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My comment was tongue in cheek. The rate of increase may diminish but the actual draw weight does not decrease or let off.
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My comment was tongue in cheek. The rate of increase may diminish but the actual draw weight does not decrease or let off.
Touche. That is true. It never decreases in relation to itself. You will never lower the draw weight from a previous weight at an earlier draw (unless you have a compound ;D). You're right. However, it does decrease in relation to non-recurve bows, and that's what we were talking about.
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I think you need to think about the spring and the geometry separately. Almost all springs build at a predictable rate as long as they are pulled at the same angle. Lets say a limb gains 1# per inch if pushing down on the tip. That will start before it was braced at any amount of reflex you have. Now once you hook a string to it the geometry changes so now you are no longer pushing or pulling straight down on the limb tip. If you have a static recurve with 4" of string contact on the limb you are not pulling down on the limb tip, your lever just got 4" shorter and will require more effort to move.
If you take an unbraced bow and put a long string on it and pull it down to say 28", say your string was hanging loose at 6" so your movement of the string was about the same but no preload on the limbs. The weight at 28" will read very close to what it reades even if the bow were braced and preloaded because of the geometry.
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I don't know about all the math but if the static does not work the extra mass does not have that much effect in performance. The mass is just along for the ride. That's why I have not built them anymore. Arvin
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Also not true. The outer parts of a bow are pretty dead unless a bow is whip tillered.
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I don't know about all the math but if the static does not work the extra mass does not have that much effect in performance. The mass is just along for the ride. That's why I have not built them anymore. Arvin
I beg to differ. A well built static will outperform a straight limb bow by a wide margin
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Has anyone found that when a static recurved tip is used, the limb has to be heavier or stiffer?
BTW, here is a link to last years discussion.
http://www.primitivearcher.com/smf/index.php/topic,56255.0.html
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Has anyone found that when a static recurved tip is used, the limb has to be heavier or stiffer?
BTW, here is a link to last years discussion.
http://www.primitivearcher.com/smf/index.php/topic,56255.0.html
What do you mean by heavier or stiffer? Those are functions of draw weight choice.
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What do you mean by heavier or stiffer?
thicker or wider than an otherwise comparable length and draw weight bows, with out the hooks.
If there are advantages to static recurves that increase performance, then there should be extra wood required to store the extra energy. Of course, extra performance can also come from stressing a bow harder or is that what is really happening when special claims are made for bows with static recurves?
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The recurves need not be heavy just locked in with a heavy heat treating.also the 1 to 8 ratio plays a huge roll,i think people just tend to over compensate and make overly bulky recurves.
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What do you mean by heavier or stiffer?
thicker or wider than an otherwise comparable length and draw weight bows, with out the hooks.
If there are advantages to static recurves that increase performance, then there should be extra wood required to store the extra energy. Of course, extra performance can also come from stressing a bow harder or is that what is really happening when special claims are made for bows with static recurves?
Realistically the bow is not going to be thicker. Wider is generally recommended but I would definitely say that a static stresses the wood to a more optimal degree. Plus a static effectively shortens a bow without cutting it shorter. Shorter limbs move faster.
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Some times I study theory tell Im blue in the face & think I understand whats going on but I never learn as much as I do building the bow that I'm studying about , this bow was the fastest bow I ever personaly built 65 ntn 8 1/2 in. strait levers but there nether wide or thick I think widest thickness at the lever fade is around 5/8 & I tappered to the tips both in thickness & width got the levers as light in mass as possible with out bending I agree that they dont need to be really thick or really wide , Im working on another one right now that Im going to try narrower levers but slightly thicker , I think to get performance out of these type bows is lightening mass as much as possible in the levers I dont totally understand why it works totally , but it does recurve or strait limb but light mass is key in my mind I could not of fliped the tips on this paticular bow with out taking set as light of mass as it was in the working limbs as the limbs where just starting to take set & lose reflex but Im not convinced that a recurve vs a strait limbed bow of equail reflex is faster , but I will just have to build another bow and find out , this bow eventually broke about 8 in. Out of the handle fades !
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Thickness
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shooting said desings through a chrono,, or for distance takes some of the guess work out,,
some of Marcs bows, certainly show great cast with the recurves,,
and I am sure there are others that have test results,,
straight tip bows are underated I think, and can shoot quite well when designed properly,,
when the whole bow is refelxed and the tips are in front of the handle ,, equal to a recurve the results can be surprizing,, (--)
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I don't know about all the math but if the static does not work the extra mass does not have that much effect in performance. The mass is just along for the ride. That's why I have not built them anymore. Arvin
I beg to differ. A well built static will outperform a straight limb bow by a wide margin
Marc I have been wrong before . I did not see it working for me. It was a good shooter but not that much different . I do know that getting the mass in the right place makes a Hugh difference. Maybe I had the mass in the wrong place. :) :). Arvin
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how much is a wide margin,, could you share some stats, ,thanks b
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Im not convinced that a recurve vs a straight-limbed bow of equal reflex is faster, but I will just have to build another bow and find out
I think it depends on a lot on the length and bow design/material. That's what makes these comparisons so hard because there3 are so many variables. Like I was saying in the other post, longer limbed bows don't benefit so much from reflex because the reflex helps by improving string angle (and maybe some other factor that has no good explanation yet). Longbows already have good string angle so it might be more beneficial to reflex like you said instead of recurving. However, I believe shorter bows closer to 50", rather than the usual mid 60" bows, will have much better advantages from a recurve. Regardless of length though I would say it's probably safe to say that either reflex or recurve would be better than a simple straight-limbed bow, as long as the are comparable in material and designed/tillered correctly.
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Why is a low string angle a good thing?
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Why is a low string angle a good thing?
I wasn't saying shorter bows are better or not, I was just saying a recurve on a longer bow is going to improve the string angle less than a recurve on a shorter bow. (Let's say for example's sake a recurve improves by 5% on a 68" bow, but 12% on a 55" bow. pulled numbers out of my butt, so don't hold to those values as anything real.). Sting angle is important because it affects the force-draw curve. Bad string angle leads to stacking and you have less stored energy. Good string angle leads to "smooth draw" with no spike in weight gains. This ensures a fatter force curve and thus more stored energy. The speed of a bow can be simply boiled down to its stored energy and how efficient it is at converting that energy to arrow speed.
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My question wasn't really about recurves and such. It was just a question in general. You came close to an answer but I'm greedy. You said it causes stack or sweet draw. Kind of meaningless. If I assume that stack means hard and sweet means easy(I've gotten that impression) does that mean there is a greater mechanical advantage with a low string angle? I'm not trying to be a jerk, I've been after this for a while. If there is a greater MA at low angles,and I think there is, how do you figure it. ;D ;D ;D ;D ;D
Lots of big smiles to show I'm not a jerk. If it's a big complicated math formula just tell me that.
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My question wasn't really about recurves and such. It was just a question in general. You came close to an answer but I'm greedy. You said it causes stack or sweet draw. Kind of meaningless. If I assume that stack means hard and sweet means easy(I've gotten that impression) does that mean there is a greater mechanical advantage with a low string angle? I'm not trying to be a jerk, I've been after this for a while. If there is a greater MA at low angles,and I think there is, how do you figure it. ;D ;D ;D ;D ;D
Lots of big smiles to show I'm not a jerk. If it's a big complicated math formula just tell me that.
It comes down to how the angles change and torque. a bow would act like a simple spring if the force was applied at the same angle all the time throughout the bend but it doesn't, therefore, the torque on the limbs is changing and likewise, its restoration force depends on it. I haven't seen anyone calculate it, it's probably pretty complicated especially when you add stuff like recurves. I don't understand it fully either. If someone can offer a better explanation that would be amazing. I just know a lower angle causes the poundage to increase more gradual, and a greater angle causes the poundage to increase rapidly (hence the stacking).
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Thanks. Can you explain torque in this application. I understand torque as in wrenches and drag racing. Where is the twisting taking place?
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I prefer longer bows. I feel they are smoother have no stack and less finger pinch. Thus a smoother release. As far as levers . They go with shorter bows I would think. Wide limbs to mid llimb. Just not my deal. Arvin
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Once the string angle hits 90 deg the bow begins to stack, like hitting a wall. It won't pull much farther and the draw weight increases rapidly then you hit a stop. By bending the tips into reflex a longer draw is possible because it can be drawn farther before the string angle hits 90 deg.
This also adds more stress to the bow so the tiller has to be spot on.
Once the string hits 90 deg the force stops bending the limbs and the force is pulling along the limb. I hope this makes sense. I know what I'm trying to say it's just hard to explain.
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Once the string hits 90 deg the force stops bending the limbs and the force is pulling along the limb
Yes, that makes sense, without a recurved tip, the string is trying to bend the bow and straighten it at the same time, so to speak.
It is at this point, where I imagine that the lever would be "torquing" the end of the limb, or using the string force to make the limb bend more.
Is there an optimal angle for the static tip have? So that the effect kicks in as the string angle approaches 90 deg?
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Kooi's papers address all of those questions.
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Without knowing or reading koois' findings I would say the angle has to be at least 30 degrees to benefit from any lift off of string or lengthening of the limbs.Bends below that degree are cosmetic so to speak.For looks.Lower the degree the sooner the lift off.I don't think 90 degrees is optimal either as the string will not lift off at full draw unless it's an overdraw.Right at around 50 to 70 degrees on a self long bow seems to me to be the optimum degree for a 28" draw.Many variables and designs.Short bendy handled bows benefit from short retro bent 90 degree bent tips for string angle and close to or even overdraw.
Key I see is getting the most out of the working limb length that you can.Pushing it to it's limit if a person is into that.While retaining reflex and the woods integrity.No set.That's when it'll perform the very best.To do this quality wood is required.Tillering skills being the same.
Straight stiff levers on longer bows benefit performance by being lighter while still giving the smooth draw.Making a wider shorter section of wood work increasing it's performance.Every wood is different as to width of working limb.As you can see I'm a believer and follower of Tim bakers' findings.I've proven it to myself with my own chronograph.
For target shooting and seeing the zip zang speed of a well designed shorter recurve is a lot of fun.For hunting give me a good old straight limbed slightly bent tipped self long bow.I don't need to concentrate as hard shooting those for accuracy.That's just me.For the moment anyway.I know things can change in time though too.Guess you'd call it practice....lol.
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The force of the string is applying a torque on any type of bow. If we assume that a recurved tip is truly static, then it is changing the length of the bow, the angle at which a sting applies it's force ( which is actully tilting the angle of the top end more up and the bottom more down, this is probably countered to some degree by the portion of the string lying along the limb pushing somewhat "down" on the limb), and how much the bow must be stretched to reach brace height. I suspect the most important these for determining brace tension would be the last of the three.
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Marc said a recurve will outshoot a straight tip by a wide :NN margin,, does anyone know what that is,, about,, :NN
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It's about fps and yards of flight. ;)
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It's about fps and yards of flight. ;)
Not really. ;) ;)
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More accurate is the only other outshoot variation I know. Is it that?
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Good question Brad. I suppose all good things come with a trade-off, and hope Marc can weigh in on both sides with the pluses and minuses.
For hunting give me a good old straight limbed slightly bent tipped self long bow.I don't need to concentrate as hard shooting those for accuracy.
Ed, in what way do you find shooting a lesser recurved bow easier? Or are the bows you are comparing just usually different lengths.
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ok I think sometimes a bow that is shorter and faster,, may be a bit more sensative to hold and release,,
a longer straight bow can be a bit more forgiving,, I am not sure if that is true in all cases,, for example I have seen back in the day,, the guys shooting wood bows for high scores,,,, were shooting long bows,, and if the speed of a recurve woudl have given them higher score they would have used them,, they shoot well enough to tell the difference,, not sure if that is any proof,, but I do take it into consideration,,
I have hunted with success with short recurves,, but that was close range,,, if you want me to shoot past 50 yards I will probably just shoot a longbow,, (SH) but dont have any documented proof,, it may just be a personal preference ,, or a myth that is past down from Gurus of the past :)
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willie....That's just me.I'm sure there are fellas shooting shorter recurves at the target range with accuracy but that's not at a deer in a trees stand.It's a different ball game shooting out of position at an animal on a tiny platform.A good old longer self bow suits me there,and if you check into it I'm not alone.All my friends here shoot long bows for hunting be that they may be FG yet but still long bows.We are all successful most times here with a rig like that.
When a loaded up recurve is drawn out of position for me it's harder to stabilize because of all of the early tension in the draw.The longer bow slowly gets up there but by then I'm letting the arrow fly thus being more accurate.
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thanks Ed, (and all others who have taken time to offer constructive comments about their experience)
I thought that you might have been referring to a length preference, and also have a decided liking for bows 65" +
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I generally like bows for ease of draw/accuracy/and hunting all in one of bows that are 60" to even 68" long.To narrow it down more 62" to 65" is a good happy medium.I'm not alone here either and probably in the majority with those preferences.I did'nt come by those preferences by following the leader.I shot many arrows to come to that conclusion.Like I said that's just me.I'd have to put in a lot more practice and concentration to shoot shorter recurve bows more consistently accurate.By accurate I mean at least 3 arrows within a 4" circle or less.That's good enough for me.
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I would agree that a longer bow is more stable, therefore more accurate. The hypothesis for this I heard was that the longer limbs act kind of like counter balances. Since they have longer moment arms they have greater inertia thus requiring a greater force to tilt. Therefore smaller unstabilizing tilting forces are less likely to throw off the shot.
So it depends on what factor you're talking about. Will longbows on average outshoot short recurve bows? maybe. It depends if you're talking about accuracy. If so I would say yes. If you're talking about speed or flight shooting then you can't say for sure. I would say well-designed recurves shorter than 70" will on average probably outshoot, speed-wise, long bows longer than 70".
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You also have to keep in mind maneuverability. This isn't as important of a factor as it used to be, but the reason the Mongols used such short Asiatic bows was because they can maneuver them better on horseback without long limbs getting in the way. This might play into hunting depending on your style. While most probably would prefer the stability of a long bow others may prefer the maneuverability of a short one.
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The Japanese shot longer bows from a horse.I forget the name of those bows.Starts with a Y.
I look at shorter versus longer like barrels on a gun.The slightest twich on a pistol and your way off.Not so with a longer barrel.All depends what way you look at it.It's the early tension mostly too talking about accuracy.Probably a little of both.
For manuverability ease walking through the woods brush nocks seem the most useful to me in a lot of ways.
As you said I think comparing 2 well built bows of 2 different designs recurve versus straight recurve will shoot the fastest.
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I'm not sure I agree that a sharp static has higher brace tension than a rounded static.., but here's how I explain what a contact recurve does. Think of two pencils, a regular one and a "golf" pencil (short). Both are the same thickness, but the long one is easier to bend because the ends act like levers.If you hold the regular pencil with both hands near the middle, it is hard to bend just like a golf pencil.
When a recurve is at brace, it "acts" like a bow that is shorter- it acts like a bow that is only as long as the distance between the points of contact. This short bow is harder to draw because it is short.Hard to draw= high early draw weight. At full draw, the string lifts off the recurves, and now the bow "acts" like a bow that is longer, giving you more leverage to bend the bow. So a recurve acts like a short, stiff bow early in the draw cycle, and a long bow later in the draw cycle.
^----^ correct answer!
For science types refer to the following:
D = F * E ( L ^ 2 ) / ( W * ( H ^ 2 ) ) where
D is the displacement of a beam fixed at one end with a rectangular cross section of dimensions Length, Width, Height, modulus of Elasticity, and a Force applied to the loose end.
The above approximates a limb where the fixed end of the beam is the inflexible part of the fade. Where the limb is a more complex shape use calculus to integrate the solid.
Edit: fixed the equation
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Yes, but we build bows to a certain final draw weight. The recurve bow gets "longer" as the string lifts off. This lowers the final draw weight. So if we have your hypothetical bows the straight one will keep climbing to a higher draw weight while the recurves weight will level off. If you re-tiller the straight one to the same final draw weight you loose the early draw weight.
So really recurves don't increase the early draw weight. They decrease the final draw weight and thus only give the illusion of increasing the early draw weight.
Thus going back to the earlier post about the bow that reached final draw but had no string let off. Since it had no string let off it didn't have any lever advantage to decrease the final draw weight. Therefore, just because it is a recurve, it doesn't necessarily mean it had better early draw weight since it didn't have string let off. That is unless there is something we are missing still.
If the bow is long enough that the string never lifts off the tips you have shorter long bow with really heavy tips. The string should lift off the recurves area. The earlier it does the higher the slope of the force curve early on. Once the string is off the limb, the force draw curve is linear. Not taking weight into consideration, the perfect static recurve would have a gentle curve so the effective limb lengthened continually until just before full draw. With a wood bow this could be done with a deflexed handle. A working recurve of similar concept would have the string lift off a little sooner. With the static option, one could laminate something very light into the recurved portion to save weight and have enough width for the string to rest on.