Primitive Archer
Main Discussion Area => Bows => Topic started by: Kegan on April 14, 2008, 09:39:32 pm
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I asked a question about performance hindrances earlier, and got some help. From what I gathered, an under-stressed bow that doesn't weigh alot is a good thing :). So...
Would putting in an inch of deflex (say on a hickory backed red oak bow) on a rather narrow flatbow (pulling 80# at 27", 1 1/2" wide in the middle two feet, tapering to 3/8" nocks, 70" long) be acceptable? The first good bow I made was a purposefully deflexed hickory bow, and it hit hard and fast- despite three inches of deflex and HUGE tips, and have gotten similiar testaments from others.
Thoughts? All but some of my most recent bows show about two or more inches of string follow anyway, so I figured, "what the heck"?
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One good benefit of deflex is it allows a high brace height also if you have a long draw it might help keep the wood from being over stressed. on the other hand it also shortens the power stroke to have a high brace and with only deflex and no reflex I would think the string would be looser at brace height.
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dont wnat to change the subject, sorry kegan, 1 quick answer.what exactly is string follow?
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Kegan, one of the main advantages to a D/R bow is to lesson the stress while braced. Also it is easier to brace whenyou only have to move the limbs a few inches. If a bow is braced all day as often is the case while hunting. A bow that is under less stress while braced is more likely to have the same poundage at the end of the day as it started with in the morning.
Reaching the desired poundage is abtained with stiffer limbs, either by a traped design as with "reflexed and bamboo backed" or by heavier stiffer limbs.
Brace higth dosen't have to be higher then normal, but can be if desired. I do feel you need some reflex near the tips to aid in string angle.
The dimensions sound about right. I would do the deflex right in the handle if possible and a slight refled tip. Go for it and at the very least your will learn more pros and cons. Keenan
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hunter,it is when you unstring you bow and it is still bent like a strung bow instead
of flat or backset/reflex.An inch or 2 of string follow ant bad but something I strive not
to have if possable. :)
Pappy
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Deflex is no good unless you have reflex in another part of the limb to make up for the loss of early weight.
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Ok food for thought here. If a bow has say 20 lbs of "early weight" (Braced) and is 60 lbs at 28" and another bow has 30 lbs of "early weight" (braced) and is the same 60 lbs at 28" And both are hypothetically the same mass and power stoke and so on, just to elliminate the other factors. Which bow will transfer the most energy to the arrow??? ;)
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The 2nd. :)
Pappy
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The one with the 30lb of early weight will transfer more energy because there is more energy stored during the draw.
Deflex doesn't help performance, but if you have enough performance to do whatever task you want to accomplish then it really doesn't matter. If your bow is pulling 80lb then there ought to be performance to spare, even with a less than hot-rod design. The one thing I would observe, is if your design took 3" of set when you tillered and shot it in, then your wood was fatigued a little more than it should''ve been and it might be better to use a slightly beffier design (width, or length, or reduced poundage) to get better results.
The huge tips probably do negatively affect performance too, again you are compensating with the high poundage though. And its tough to compare to other bows because every piece of wood and tiller is unique.
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I do agree with Ryan about reflex needing to accompany deflex for optimal performance. Deflex near the handle means that mass of wood moves over a shorter distance, which reduces the work required to return it to brace. Letting the outer limbs travel farther is OK because they are skinnier/thinner, meaning less mass moving the longer distance. How much work is required to return your bow's limbs to brace position, is what determines the partitioning of stored energy between bow and arrow. You total up the work required for each, then add these values and the percentage of each of the total, is your bow's efficiency. For example, if it takes 30 work units to return your limbs to brace, and 60 work units to shoot the arrow, then your bow efficiency is 66% (60/90).
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Performance, for me, isn't the be all and end all. I'd much rather have a sweeter shooting bow, with less hand shock, and quieter, than a harsh, noisey bow that is 5 fps faster. Especially for hunting! A bit (2" or less) of string follow is a GOOD thing.
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Ok just to challenge the thoughts some more. Support your decision.
You both said the second. that starts with 30 lbs. If you take 30 from 60 you have 30lbs being transfered to the arrow. What is still stored in the bow at brace is not spent energy but is still energy stored in the limbs. Now take the 20 lb braced from the 60 and you have 40 lbs of transfer to the arrow. and only 20 lbs. left stored in the limbs.
I know this is really tweeking some heads but is good food for thought. Ofcoarse the senario comes into play of what if you shoot a bow with zero lbs. of braced and we all know what that would do. But I'm challenging to do some thinking outside the bow here. If you had a bow of an incredible 40 lbs of early draw wieght but only went to 60 lbs. at 28" basically 1lb. per inch how would you expect it to perform. ;D
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The energy that is applied to the arrow is equal to the area under the f/d (force/distance) curve, not the difference between early draw weight and final draw weight.
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Tim Baker once stated that if we think of things to the extream example then sometimes we can see what it is really doing. Thus my challenging questions. I do have my own thoughts or conclusions on this but am asking others to expound on the why's and theories so that everyone can learn and maybe even challenge thier own way of thinking. ;) Keenan
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For the benefit of others would you care to explain that Jack. ;D
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To answer the earlier question regarding whether a bow with a 20# brace vs. one with a 30# brace would impart more energy to an arrow, here are my thoughts. Folks are encouraged to pick this apart.
I would guess that both the 20# at brace and the 30# at brace bow would impart the same energy into the arrow.
If we assume that two bows that pull 60# at full draw store the same amount of potential energy, they have to release the same amount of energy, since energy cannot be created or destroyed (i.e. First Law of Thermodynamics). There are three places for this potential energy to go: (1) Limbs, (2) arrow, (3) dissipated as heat due to friction. When you draw the bow, most of the energy will be imparted into the arrow early in the release (i.e. the limbs move fastest soon after release, and are moving more slowly as they return to brace height). Since the limbs are moving fastest earlier, they are turning the maximum amount of potential energy into kenetic energy, and this is where the arrow gets most of its momentum.
Since we're assuming that the limbs in both bows have identical mass and if we assume an identical brace height, the amount of potential energy absorbed by the limbs should be the same. Even though the limbs of the bow with a 20# brace height will be moving more slowly as the string hits home, the arrow already has most of its resulting momentum from the earlier stage of release, so this slower ending stroke doesn't matter. Unless there are meaningful differences in friction, we are forced to conclude that the arrow's resulting kinetic energy must also be the same, or so similar that we probably wouldn't be able to see a difference with your typical backyard chronograph.
Others' thoughts?
-Eric Garza
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Thats the kind of disecting discusion that I was hoping this would spark.
Kegan; I hope this is not view as a hijacking of your thread I am hoping this will really pull in some answeres and thoughts and theories. ;) Keenan
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Hmmmm....What, explain what energy is? ;D
If you're familiar with physics, this stuff is a piece of cake....but if you're using your intuition it's hard to understand.
The easiest way to think about how a bow works is to think about the draw in small increments. If a bow is 30# (at brace) and you draw it back 1" then the energy is 30 (30x1). If you then draw it back another inch the energy might be 33, etc. If you add all the pieces together you'll see that a bow with a higher brace weight stores more energy.
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Ok Jack so if that were the case, A bow with zero at brace would be (0x1) and (0x2) and so on ??? ;D ;D Sorry but It's my nature to think to the extream of theories. ;D
"If you add all the pieces together you'll see that a bow with a higher brace weight stores more energy" I agree (stored energy) but is it transfered energy?
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You're right, a bow with 0# at brace will have near zero energy when drawn 1" (0x1)....but at 2" you would start to feel some resistance, right? Lets say it's 1# @ 2"....so the energy would be 2 (not zero). Lets say it's 2# @ 3", so the energy is 6 at that point, etc, etc.
Stored energy becomes transferred energy when you release the string. Not all of the stored energy goes to the arrow because the energy has to move the bowstring, bow limbs, and even the air around the arrow, string, and bow. There are lots of things that affect how the energy is transferred to the arrow....but again, the physics is a bit complicated. Basically, MOST of the stored energy is transferred to the arrow (about 70%).
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When you think of energy in "pieces" you will also begin to understand that longer draw lengths also store more energy (and vice versa). ;D
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Keenan, Patrick is correct. The stored energy is expressed as the sum of each minute movement of the string.
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Great explination Jack.
Now;
"There are lots of things that affect how the energy is transferred to the arrow....but again, the physics is a bit complicated. Basically, MOST of the stored energy is transferred to the arrow (about 70%)."
What if any difference is there if I shot a bow with a slack string (unbraced) (what amount of absorbed energy) vrs. a braced bow at roughly 70% absorbed energy. ;D
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Guys I totally agree with the thoughts and theories I'm just challenging our minds to examine things from a radical prospective. ;)
I wonder if I strung a bow and shot at 30 lbs. (normal brace) then took the same bow and shot it with a slack string (unbraced) but again pulled to 30 lbs. what would the difference in cast, speed and transfered energy? ;D
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Hmmmm....a bow with a slack string is not a bow....it's a fishing rod. That's the difference. >:D
Actually, I'm not sure I understand the question.
I wish I could devote more time to this discussion but I've got to go. I'll be back later tonight. ;D Keep asking those questions....
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Ok, so after reading Jack's response I feel like backing my claim up a little better (i.e. doing a little self-dissection).
I'm not a physicist, but I do study energetics of natural ecosystems and human societies, so I'm familiar enough with energy to explain it a little more thoroughly than I did before. What we're interested in is potential energy, because the potential energy stored in the limbs of a bow will end up as the kinetic energy embodied in the arrow. There are several types of potential energy, including gravitational, electrical, chemical, and elastic. We're obviously interested in elastic potential energy. Elastic potential energy is calculated:
E = (λ χ2) /2 l
Where:
λ is the elastic material's modulus of elasticity,
χ is the elastic material's extension beyond resting, and
l is the elastic material's natural length or length when at a resting state
The bow's natural length represents the horizontal length of the bow from the point on the string where the arrow is nocked to the front of the bow's shooting shelf. This is, effectively, the bow's brace height. The extension beyond resting is the distance from the string at the arrow's nock when the bow is drawn to where that point sits in space when the bow is braced but not drawn. These points equate to the braced bow's setup envisioned as if it were a spring, with the braced bow being the resting state.
Note here that the only variables that influence the potential energy at full draw are these two distances. The wood's modulus of elasticity is constant (or at lease we're assuming it to be). So this means that the potential energy generated by drawing the bow will be the same in both bows, and again since most of the arrow's energy will be given to it very soon after release the fact that the bow's tension at brace is different doesn't matter. It doesn't factor into the equation.
Alternatively, one might argue that the bow's resting state is actually as an unbraced bow. In this case there would be a difference in potential energy stored if the deflexed bow's tips were not reflexed enough to being them to the same plane as a bow without deflex. I'm not convinced that this is the right way to look at the problem, though, as the websites I looked at to put this together all showed a braced bow when describing its resting state. Just for argument's sake...
Overall, though, I'll stick to my guns. There's no reason why a deflexed bow with lower tension when braced won't transfer just as much kinetic energy to an arrow as a bow that isn't deflexed, given the same limb, string and arrow mass, brace heights and draw lengths.
All the best,
-Eric Garza
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Very good food for thought Eric. ;) hopefully others will engage ;D
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Can't resist....There is one main problem with Eric's theory:
Bows with different limb cross sections produce different amounts of energy (when shot) even if things like limb mass, draw length, final draw weight, etc. are identical between the bows. This is the basis for the science of bow design.
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Eric, the flaw in your math is based on the fact that the equation doesn't account for the bow being braced. A deflexed bow will generate less poundage at 1" of draw, than a reflexed bow. The arrow gets pushed the whole time its on the string, not just at the instant you let go. Thats why Jack is right about adding the area under a F/D curve to calculate stored energy. It all adds up to the arrow speed at the moment it comes off the string.
Now a deflexed bow might have a similar efficiency to a reflexed bow, if its limbs are about the same geometry and mass. But it won't store as much energy, so the arrow won't travel as fast as the one coming off the reflexed bow.
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The way I see it is that the limb cross section is only a design feature dictated by the design of th ebow. A bow with a lot of bending limb can use a lot deeper limb cross section than a bow with a short bending area. Bows should always be built at a high level of strain without going past their elastic limits. Wide flat bendy handle bow of long lengths woud not make much sense unless they were made from very light woods. Just as narrow thick cross sections would not make good sense used in a stiff handled, stiff tipped, r/d design. The limb cross section should always be dictated by the design and tiller of the bow to give the working part of the limb at least an 80% of maximum strain value. Steve
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"...since most of the arrow's energy will be given to it very soon after release "
Not true. Something like half the energy imparted to an arrow, occurs in the last 2/3 of the time its on the string. Look at a F/D curve, it is usually a straight line. But it doesn't start at 0lb, it starts at whatever poundage your bow pulls initially.
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Keean,
Calculus mathematics is required to provide a precise answer to your question. However you can get a good visual approximation by looking at the area under the F/D curve for each bow. I'm certain you would see that the bow that starts the draw with a slack string will have less stored energy than the one with a taut string for the same draw length.
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Eric, a bow is acelerated from the moment it is released to the moment it hits brace height. The total stored energy in the bow will always reflect how fast the bow will travel. A well made self bow will usually be around 70% efficient. So if a deflexed bow and a reflexed bow were both 70% efficient and the reflexed bow stored 20% more energy it would shoot considerable faster. Typicaly the difference in bows of the same draw weight will range from about 140 fps up to about 180 fps, this is a huge difference both in stored energy and efficiency. Steve
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most of the arrow's energy will be given to it very soon after release
Unless an arrow is somehow accelerating on its own power I don't see how that is possible.
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Ok now that I got the pot all stired up I better get to work, ;D ,Opps Always remember that this is for a learning senario and hypothetical thoughts. ;) Good info Guys. Just for kicks I just shot a bow (Slack string) Want to see the welt ;D ;D ;D
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Jack, bows with limbs of different cross sections do not produce different energy. Energy is not produced by a bow. The energy is produced by the archer when he draws (applies force to) the bow and gives the limbs potential energy. What limb cross-section changes is the efficiency by which the limb's potential energy is transferred to the arrow as kinetic energy.
And Tom (assuming this is your real name), I think I understand your post after reading it a few times and doing something else for awhile. First you assert that there is a flaw in the equation I used because it doesn't account for the bow being braced. I disagree. I didn't make up this equation, I got it off a physics website that listed different potential energy equations. And I think comparing two braced bows is also perfectly valid. They're both likes springs at rest, even when braced. The equation I gave was developed for springs, and bows behave like springs in the way they generate elastic potential energy. Bows have resting lengths, even when braced.
In my second post I said specifically that you could adapt the equation for an unstrung bow, but to compare bows with the same resting lengths you'd need to have both with tips in the same place (in front of or behind the handle) when the bows are unstrung. So to compare two similar bows so that the only thing that differs is the fact that one is deflexed, you'd have to reflex the tips of the deflexed bow so that the tips are in the same place as those of a non-deflexed bow. I thought I made this clear, but maybe not. If you applied this equation to two unstrung bows, one deflexed (without reflexed tips) and one that is reflexed, of course they'll generate different amounts of potential energy when strung and drawn, because their resting lengths will be different. You aren't holding everything save the presence of a deflex in the handle constant.
Tom, you say, "half of the energy imparted to an arrow occurs in the last 2/3rds of the time it's on the string". This is true enough, but look at an F/D curve. Assuming you use D (draw length) on the horizontal axis and the 'curve' is actually linear starting at 6 inches on the horizontal axis and heading to higher force and draw weights at a diagonal, the incremental area under each inch of the triangle is small at lower draw lengths and increases at higher draw lengths. This means the bow is easier to draw early on (the archer needs to apply less force), and also stores less energy. If you were to release an arrow from an underdrawn bow, it would be given a very small amount of energy relative to if it had been fully drawn. Most of us should agree on this.
On the other hand, when you fully draw the bow, that first inch of travel that the string moves through after release imparts a huge amount of energy into the arrow, and the incremental amount of energy given to the arrow for each further inch of movement diminishes as the string inches back towards brace height. If, for the sake of easy math, we assume that the F/D curve is linear with a positive slope, then 3/4 of the total energy stored in the limbs has already been released into the arrow by the time it passes the halfway point back to brace height. So, Tom and Gordan, as I said, "most of the arrow's energy will be given to it very soon after release." This doesn't break any physical laws, and the arrow doesn't have to accelerate by itself. If you look at an F/D curve and read it from right to left to interpret how energy flows from limbs to arrow through the release process, this shouldn't be a controversial statement.
So while the arrow is being given energy from the time you first release the string to the point at which the arrow leaves the string, the incremental energy has fallen off so much as the string approaches brace height that I don't think it does much to the momentum and velocity of the arrow. The early stages of release for the most part define the ending velocity, while that last half just fine-tunes it.
And Badger, see my above clarification about resting lengths for two bows you want to compare.
Take care folks,
-Eric
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I'm Lennie, not Tom. See my tiny signature line?
I've looked at a few F/D curves in my day. At least two. hehe
Some points to clarify the situation.
First, on a 28" draw and a 6" brace you have a 22" power stroke. So the "D" in the F/d curve starts at 6" and if you plot every inch you have 22 points. Generally each one rises around 3lb/inch. Bows don't start at 0 force, that first inch of draw generates maybe 15lb on a snappy 60lb bow. And a bow with 3" of deflex, might start out at 10lb. So it isn't a negligible amount of energy being stored in that first inch of draw. And the difference is fairly significant between the two examples.
You did say "most", so if "most" is "over half" then so be it. You defnitely don't find 90% of the stored energy in the last 6" of draw. I think you're underestimating the contribution of the first few inches of draw. That is what separates good bows from mediocre ones, and yes it isn't a huge difference (nobody is saying a reflexed bow shoots twice as fast) but it is most certainly not negligible. More importantly, its worth pursuing since it isn't that difficult to avoid getting 3" of set with practice.
Something else, if you set your bow wood 3" or more in tillering, there's a good chance your wood is fatigued to the point that it is not as springy anymore. This means its more elastic, less rigid and you've reduced the modulus of elasticity as well as having reduced the early weight. This wouldn't apply to a bow made from a deflexed stave of course.
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Eric, the last 4" of the power stroke right before the bow hits home can affect the speed of the arrow by more than 10fps. And when you stop to consider the resulting fdc from a bow with high early draw weight the resulting difference can easily be more than 20 fps. Steve
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My apologies Eric - I misunderstood by what you mean by the terms "most" and "very soon". Thank you for clarifying. That said, I agree with Steve and Lennie that the potential energy stored in those first couple inches of draw are worth optimizing to the extent the bow is not overstressed or is overly difficult to build or handle.
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What Steve said. If you weren't getting a lot of benefit from the last few inches of power stroke it wouldn't make sense to lower the brace height to get 2 more inches of bottom end power stroke. Justin
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My hickory bow (80#, 3" of string follow) had side cast, which shows small visual cracks in the finish on the edge, and was made from wet hickory (12%). Similiar bows of red and white oak (self) stand prefectly straight though (same dimensions, 75# though)- so it's not the design. Even with the lack of storage on behalf of my hickory bow, the bow is still a favorite of mine- so I'd be happy with anyhting that shoots as well. However, I don't completely like the straighter bows, as I am so much mroe used to the hickory longbow. Assuming that the glued in delfex (I was planning on a gentle curve over the limbs, not primarily in the grip) will take some of the strain off, than it should stand at only about 1 1/2" of string follow at most. Which is something I'd be very content with. I'm not talking about a slack string, just not as taut. Personal preference sorta thing I guess.
But I'm glad I opened the door to more "bow formula" ;)
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Hey this is good healthy debate guys and everyone grows in knowledge from it. ;D In thowing out all the hypothetical senarios I wasn't trying to say or state any absolutes or golden rules, just give others food for thought and it looks like some great thoughts and valid statements came out. ;) And no one got shot ;D ;D ;D
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I finished up a red oak bow today that has about 1 1/2" string follow. I think it is about as fast as any self bow I have ever made. It has what I call hard follow. The bow still has high early draw weight even though it took some set. Some bows can take 1" set and be soft others might take 2" and still stay hard. Really can't tell without feeling and shooting them just by the amount of set or deflex as long as it is within reasonable norms. Steve
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Might be that the difference is in how much the bow sets during shooting? Sometimes a just-unstrung profile is pronouncedly different from when it relaxs back. Whats the density like? Quartersawn or plain?
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Lennie, 1 /4 sawn. Interesting thing is that I checked it after a couple hours and it only settled back about 1/2". Steve