Primitive Archer
Main Discussion Area => Bows => Topic started by: Selfbowman on February 20, 2017, 07:40:58 pm
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You guys may think I am to dumb to explain this to me! Is this a good force draw on a 51@27 ? I have studied the BB1 about design but a bit confused. :-\ I assume the higher above a straight line the better. What is the best we can hope for ? According to the chart where in the limb should mass be added? In thickness or width? Arvin
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This is the bow.
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Can you tell I am confused ? ;D Arvin
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All I know is humped is good and sagged it not. ;D I don't think that you can tell if a bow needs width or thickness. It's like a report card, it tells you how you did, not how to improve it.
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I'm confused by your question a bit but I'll try here.Yes higher above the straight line is better I would say.The more early draw bow weight is determined by the amount of healthy sustainable reflex it holds.....ie..Stored energy.A humped fdc.So a design is made and tillered properly to get this or materials are used that can hold more reflex and be efficient with it's poundage.I don't believe having lots of reflex is the only factor here either though.Design and tillering will get you a lot too.Efficiency....fps per pound of draw weight is all that should matter to an archer that wants to test these bows.
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Ok DC how did I do ? O:) :-[ Arvin
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B,B+ I dunno, it's got a hump, most are straight I believe. What I found interesting when I did an FD on one of my recurves is that you can see when the string lifts off. I would guess that your string lifts off at about 22". Am I right?
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Here is a braced pic .
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I don't think you can get much of an upward bow to the F?D curve unless you have reflex or recurves. How the bow delivers the energy back on release relative to losses is very important too.
The F?D curve only tells part of the story but some people practically use it as the performance of the bow.
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You need to take a measure at least once every 2" to get a decent curve reading. The early draw is very important. Ideally, it would climb very fast in the begaining and then start flattening out to as low angle as possible. We just don't see anything that dramatic on self bows though. If you get a slight hump you are doing pretty good. I expect with your profile you should get a little hump.
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Here is a braced pic .
Oops, no string lift off there ;D ;D
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looks like horizontal is draw weight and vertical is draw length... (it's sideways?) looks good
efficiency matters arguably more, especially as arrows get lighter
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My spring scale is not the most accurate around. Curious ! If you had a full scale grid and you weighed your bow in two inch increments .Marked those spots on the grid. Laid the tip of your bow on the starting point at brace. Would the back of your bow unstrung match these points? I don't have a tiller board with grid like that or I could tell you. Arvin
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So, I think you did pretty darn well. Any F/D curve above the line at all in an essentially straight limbed bow design is pretty good. And, that is a pretty bow, too.
The flipped tips and almost straight unstrung profile on your bow should give it a pretty good start: high string tension at brace, along with higher early draw weight.
Your F/D curve would be fatter if the bow was longer, had more recurve, or reflex, or took less set during tilllering, etc, but, as mentioned, that may or may not make it shoot faster or farther. Really high humped F/D bows CAN be less efficient.
So, the questions you asked about mass placement have more to do with ensuring you have enough wood to do the job (avoid set and breakage), and in the right places not to slow down the shot. Little to do directly with F/D curve. Geometry allows a badly built, poorly designed bow to show a nice F/D plot while the bow is slowly dying from a crushed belly.
So the answer is, if indeed you needed more mass, it would be wherever the bow took more set than you planned on during tiller. Probably in the form of width, but maybe length, as I didn't see it mentioned.
Likewise, now that you have a curve plotted, shoot some arrows. That F/D curve is adequate (at least) to make your bow a pretty quick little shooter, assuming a couple things. First and biggest is tip weight. If you managed set and tip weight well, you probably have a bow there you should be reasonably quite proud of.
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The bow is 69ntn. 500gr arrow.
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This is not one of the best I've built but a good one. 200yds with linen string. 500 gr arrow . The bow weighs 23 oz.
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Springbuck the bow took some set at inner midlimb and in fade area.it is 1-1/2 at fades.lost 1in off the caul. Tips between 1/8 -1/4 wide .Arvin
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You need to take a measure at least once every 2" to get a decent curve reading. The early draw is very important. Ideally, it would climb very fast in the begaining and then start flattening out to as low angle as possible. We just don't see anything that dramatic on self bows though. If you get a slight hump you are doing pretty good. I expect with your profile you should get a little hump.
Steve my spring scale starts at 15 pounds. At 10-18# , 12"22# , 14"28# , 16"-30# ,18-33# ,20"-38#
22"-42# ,24"-46# , 26-50# , 27" 51#. I think I need a better scale. Arvin
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Those graph numbers look pretty normal and good.I think really the fdc tells a person exactly how a bow will feel when being drawn is all.Different designs show different type lines.Delivering the energy efficiently after release is a matter of proper design and tillering.More gizmos to use testing for that like a chronograph or measuring the distance an arrow is shot.So all in all it goes back to design and tillering.
For accuracy I seem to like the nice easy pull of just a plain self long bow opposed to the highly reflexed bow demanding more concentration from me during the draw,and have to be up on my A game shooting.Length of bow can make a difference to me too.That's just me.
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The numbers listed above are actually a little too good. I wouldn't trust them. Force draws are such a pain in the ass to be accurate on. best off just using a chrono.
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Beadman I agree with you about bottom out shock , noise and recurving. I am also a smooth pulling good casting longbow guy. But right now I am trying my best to build for speed and cast. I can always go back to the other. Steve it might be in my 25$ scale. I used a arrow with one of my wife's hair ties on it to make sure I did not over draw each pull so the out of whack readings may be the scale. Like I said this is not a super bow but a good one. Cast don't lie much when you put three arrows within 3 yds at 200. Arvin
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Arrow speed is the best measurement you can rely on. However, it doesn't sound like you have a chronograph. Your force curve draw looks great. anything that isn't less than straight is good. Even straight is good. You're above straight.
Faster bows will usually have fatter force draw curves. This is because they store more energy. It is possible that a bow that stores more energy will shoot slower than a bow that stores less due to inefficiency of the former. But it is less likely to be the case. The same things that can decrease efficiency on a better f/d bow can plague a straight-limbed bow. Even if it is less efficient, to paraphrase Tim Baker, 60% of 100 is more than 80% of 70. The latter is more efficient but still shoots slower.
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Gfugal I have a crono but perfer the cast method . Sometimes I get better flight than the crono says I should. I get off the string faster and smoother shooting for flight verses shooting thru the crono. I always use the same arrows try for no wind under a high pressure for flight testing. So I can get
Better info. Arvin
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Gfugal I have a crono but perfer the cast method . Sometimes I get better flight than the crono says I should. I get off the string faster and smoother shooting for flight verses shooting thru the crono. I always use the same arrows try for no wind under a high pressure for flight testing. So I can get
Better info. Arvin
The flight of an arrow is proportional to its velocity. There are other factors such as the shape and weight of your arrow, lift, wind direction and the such that may be aiding in your flight. I feel a chrono would be easier, more reliable, and more consistent but that's my own preference. If you prefer distance all power to you.
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I am hearing you gfgual and am not saying you are wrong. I am going thru my on learning curve and with a pretty hard head. If I live long enough and keep persuing my goal maybe. I have learned more in the last 3 years than the 10 years prior. It shows in my bows also. I am at the point where I am trying to figure out why things are working better and not just accepting my ability to feel deminishing mass as I build the bow. I feel testing is going to get me the rest of the way to my goal. I don't think I will ever build the perfect bow. I do believe I have room for improvement. Thanks to all that has been helpful . I am fat and need to walk 200 yards as often as possible! ;D Arvin
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So this is actually a continuation of the compression woods topic that I felt was more pertinent here. Based off your drawing I plotted your force curve so I could do some calculations off it. But I also wanted to check to see if I got it right. The first number is the draw length, and last number is draw weight. Does that look right? It’s what I was able to make out from your drawing. I changed the starting point though. You had it starting at 0 inches but really it should start at your brace height (6 in.) since the 6 inches before brace can’t perform any of the work. By-the-way by doing this it makes your shows how impressive your force curve really is. From my excel program I calculated your potential energy as 59 ft•lbs (really it’s work in not potential energy, but it's fine for our purposes). The kinetic energy of your arrow (work out) is around 32 ft•lbs. Therefore, your bows efficiency (work-out/work-in) is around 54%. This was surprising to me, but is reasonable. All the bows I've tested have come out around 50% efficient as well, even store bought fiberglass bows. It may seem low, but I just think that's just how bows work. Basically, the reason your bow is so fast is because it has a great force curve, allowing it store more energy. I don't think it was because of super efficiency. My worst bow with more than 3 inches of set, and horrible stack, was about 48% efficient. Yours is clearly much better but even if it was 48% efficient than it would still shoot an arrow 164 fps than the current 170 fps at 54% efficiency. What did you use for string? I wonder how much of a difference a light nonstretching string would make over a heavier stretchier one.
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Gregg that fdc is real close according to my spring bow scale.my question is ? Why does the fdc go down and back up. It took a bit of set at mid limb. Does that have anything to do with the fdc? Arvin
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Gregg that fdc is real close according to my spring bow scale.my question is ? Why does the fdc go down and back up. It took a bit of set at mid limb. Does that have anything to do with the fdc? Arvin
I'm honestly not sure why it went down. Ir could have been a glich in the scale (especially since there's so few measurements), or maybe something to do with the string angle at that specific spot, i'm not sure. Set might decrease the early draw weight, overal draw weight and efficiency, but i don't see it doing that. Any body else have an explanation?
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Any body else have an explanation?
you would probably be ok to smooth that curve. loose a lb or so @ 13", add a lb or so @19" etc....
just measuring "one the way up" vs "on the way down" makes a difference.
Steve my spring scale starts at 15 pounds............................... I think I need a better scale. Arvin
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Greg on that one I think it was shot with 6 strands of linen. If not it was 8 strands d97.
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Efficiency will usually be in the 65% to 75% range for wood bows. The stored energy on that bow is unreasonably high. Geometry is the best predictor of FDC. Very unusual for a bow to exceed 100% SEPDF ( Stored energy per draw force) 95% would be considered good and usually produce a bow that shoots in the 170's. A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.
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just measuring "one the way up" vs "on the way down" makes a difference.
Yeah, this is the difference between work-in and potential energy i was mentioning earlier. Measuring weight on the way up is work-in. Measuring on the way down is the closest approaximation for determining potential energy. To me i just elimanate the extra step, unless someone can offer an explanation that bows will have drastic variabilites in in their efficiency differences from work-in and potential energy. I'm figuring the decrease in effecieny for work-in calculations will be similar for all bows.
Efficiency will usually be in the 65% to 75% range for wood bows. The stored energy on that bow is unreasonably high. Geometry is the best predictor of FDC. Very unusual for a bow to exceed 100% SEPDF ( Stored energy per draw force) 95% would be considered good and usually produce a bow that shoots in the 170's. A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.
Is that 65% to 75% efficiency obtained from potential-energy/kinetic-energy or work-in/work-out. If it was from potential it would be a higher efficiency than what i've been getting. I would love to see some of your force draw curves and analyze them. I'm trying to figure out what to expect from bows. Maybe you could direct me to some links.
Yeah i'm getting about the same for SEPDF. I think i figured a straight line to be around 90%, so anything ocer 90% is a great, and if you can get above 100% its excelent.
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Efficiency is a fraction of the work in which would be work out. Same as stored energy.
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Yes. What i'm saying is that there are two different force curves. One if you measured the poundage on the way up, and another if you measure the poundage on the way down. The force curve measured on the way down will show less stored energy due to energy lost in the friction in the limbs during draw, heat lost in the muscles, and etc. I define the force curve measured on the way up "work-in" and the one measured on the way down "potential-energy" or "stored-energy". So the efficiency for the one on the way down will be higher and the efficiency for the one on they up less. What i'm asking is which curve you measured your efficiency from. The one on the way up or on the way down?
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The force draw curve should be measured only on the way up for a reading of SEPDF, the losses you mentioned will be reflected in the efficiency of the bow. Honestly, If I get a bow that reads more than about 1# difference it is going in the burn pile anyway.
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Steve can I have all the burn pile bows. I might learn more from those than I already know. :laugh: Arvin
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Wow! Those are high efficiencies then! I haven't seen a force draw curve with that arrow speed yet. If thats the case how do you get it so high. Could you or somebody else provide a link of one i could look at, maybe i'm calculating things differently.
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those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.
there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.
the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed.
perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?
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those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.
there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.
the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed
perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?
So there lies the reason for quick release on selfbows? Just asking. Arvin
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those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.
there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.
the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed.
perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?
Changes in the springs may be a factor, but there are real differences. With any energy transfer your going ro loose some energy to heat. The stored potential energy is always going to be less than the work-in you put to get it there. But as Badger mentioned, that difference should be prertty small. I didn't realize how small. It makes sense not to ever use the way down method cause some difference could be the scale not the bow, plus you can't know how much more will be lost do to slower limb return with a light arrow.
As for arrow weight, lighter arrows definately go much faster than hevier ones, but that doesn't mean they are more efficient. Quite a bit of the energy is lost in the limbs and only so much is transfered to the arrow. Lighter arrows in fact decrease this transfer, and hevier arrows increase it. If you think about it, if you have too light of an arrow you will be essentially dry firering the bow and get more recoil. Dry fireing is an example of no energy transfer. A hevier arrow may go slower but the bow is dampered by it and you have much less recoil. The bow limbs may go slower than before, but that doesn't mean they imparted less energy, if anything they gave more because if an arrow is too light it starts moving faster than the limbs can recover. Here's an article that talkes about it. http://archeryreport.com/2011/02/bow-efficiency-care/
They are using modern compound bows, so expect some differences. For example their efficiencies are much higher than what we are going to get, but thats due to their freak bows and the fact that they have better scales so they can use the potential energy instead of work-in. However, the principles should b the same.
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Again i wanted to point out that efficency does not equal arrow speed. If we want faster arrows then lighter is better (up to a certain point like you said) but for more efficiency hevier is better.
The use of having a more efficient but slower arrow is up for debate though. You could argue that it would be better for hunting cause it has more energy behind it, but like i said its still debated.
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A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.
Sorry to hijack ...but, Badger...
I hate to trouble you, but do you have a picture or link or name of a thread for that bow? I'd be very interested in seeing what you did, because my fastest bows to date have consistently been deflex recurves with string bridges, and what most consider too much curve for a wooden bow.
Also, I recently broke a 60" R/D BITH bow before I got too far into it, but it had a big rounded deflexed middle, maybe 40% of the bow's length. Unstrung from handle to deepest deflex was 4" and the tips came back to rest almost 2" behind the handle. Strung, it looked kinda like the Karpowicz "low stack" design, but much bigger middle. The string sat really parallel to the outer limbs and almost touched it for 25% each end. That thing had INSANE tension on the string when braced.
So, I want to learn more about this.
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plus you can't know how much more will be lost do to slower limb return with a light arrow.
hmm...
but if you shoot increasingly lighter arrows, you can see with a chrono, when speed gains top out?
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plus you can't know how much more will be lost do to slower limb return with a light arrow.
hmm...
but if you shoot increasingly lighter arrows, you can see with a chrono, when speed gains top out?
No doubt that's right. Sorry what i was refering to was when your measuring a force curve on the way down, your going down so slow between meaurements that your going to get no information on the recovery speed or whether or not that energy will be available due to too slow recovery. This is because the rocevery speed can really only be identified in the way you described.
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Springbuck no worry about hijack here. I am guilty way to often myself. Arvin
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plus you can't know how much more will be lost do to slower limb return with a light arrow.
hmm...
but if you shoot increasingly lighter arrows, you can see with a chrono, when speed gains top out?
No doubt that's right. Sorry what i was refering to was when your measuring a force curve on the way down, your going down so slow between meaurements that your going to get no information on the recovery speed or whether or not that energy will be available due to too slow recovery. This is because the rocevery speed can really only be identified in the way you described.
There is no point where speed gains top out that I have found. The lighter you go the faster the arrow goes. Dry fire speed means exactly what it says dry fire, any load at all will slow the limbs. I can shoot arrows as light as 120 grains and get 300 fps. Lighter arrow would even go faster.
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Virtual mass is a pain in the ass to accurately but it clearly explains why lighter arrows are less efficient than heavier arrows. Virtual mass can also give you a way to measure histerisis in a bow. It works like this, you measure your arrow speed and weigh the arrow. Next you very accurately weigh the stored energy in the bow (FDC).
The next step you can use one of the kinetic energy calculators they have on line for. Here we see how much KE the arrow had example 500 grain arrow traevling at 170 fps has 32.054 Ft# of KE, next you use the stored energy figure and plug that into your calculator. You add weight to your arrow figure on the calculator until the KE matches the stored energy your bow has at the same 170 fps. Example bow has 48 ft# SE, a 750 grain arrow carrying 48# of KE will travel at 170 fps. I subtract 500 grains from 750 grains and I have a virtual mass of the bow that is 250 grains. On a wood bow a portion of that 250 grains is histerisis that is sensitive to bow speed so it Lessens the value of this number but if you make about a 5% allowance for histerisis you would have a virtual mass of about 238 grains. This will allow you to predict arrow speeds of different weights.
Suppose you wanted to know how fast a 300 grain arrow should travel, just add the 238 to the 300 grains and see how fast a 538 grain arrow would have to be traveling to give you 48# KE. It should be about 202 fps. Now look at the efficiency of the 500 grain arrow and the 300 grain arrow. You have 66% and 56% efficiency.
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Pappy warned me about you guys. ;Di am paying attention as best I can. I will bring a pile of bows to the salt flats and y'all can laugh and tell me what I need to do better. Keep up the debate though the light might come on you never know. Arvin
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Arvin, none of that really helps you make a better bow directly. It does help when you are trying to understand your losses and get them under control. It all boils down to less set better shooter. I was never satisfied with the explanations of hysteresis in bows. I wanted to know more about it so after so many years finally came up with some tests that could identify it and show more how it actually affected performance. I would never go through that testing again because it is way too much trouble, but very satisfying to do once.
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Thanks Badger that's a helpful calculation. I know I'll be using it for sure.
I just made a Google Sheet from what you described. Here's a link to it for anybody who wants to use this without calculating it themselves.
https://docs.google.com/spreadsheets/d/1ipLC7uDFJPRbkyJXQzJn9WsjlnU6j17GX8luwm7CJDQ/edit?usp=sharing
Or here's an Excell sheet you can download and edit yourself.
https://drive.google.com/file/d/0B0oOIHJuFoYBazVjanc3RkVKX1E/view?usp=sharing
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There is no point where speed gains top out that I have found. The lighter you go the faster the arrow goes. Dry fire speed means exactly what it says dry fire, any load at all will slow the limbs. I can shoot arrows as light as 120 grains and get 300 fps. Lighter arrow would even go faster.
guess I stand corrected by Badgers experience on this, as I cannot find the graph that led to the contrary conclusion I posted. Perhaps I was recalling a slightly different aspect of arrow weight/ limb performance.
Maybe something about no significant gains in speed (due to arrow weight reduction), past some point, if the limbs were not designed well???