Primitive Archer
Main Discussion Area => Bows => Topic started by: mmattockx on November 26, 2020, 11:23:14 am
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Say you have a 67" NTN, ~42lb@28" pyramid profile flat bow and wanted to put the nocks 1.5"-2" ahead of the handle. Stiff tips with very little flex. Would you flip the tips or reflex the whole limb in a smooth arc? Why? Assuming the wood can handle the extra strain either way (ie - no extra set) would you expect one method to outperform the other?
Thanks,
Mark
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In TBB3 Tim did a section on this in the bows of the world chapter, it basically said regardless of limb profile bows with nocks equal distance forward of handle would store the same amount of extra energy all else being equal
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this is a question I have asked before and has been talked about quite a bit. Reflexing the whole limb puts the most amount of stress on the bow. I've done it before with white woods and if they are heavy draw weight bows they can be extremely hard to string. Imo. The consensus was/is that recurves are more beneficial than reflexing the whole thing.
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Since this appears to have run its course, I will elaborate. I am working on the bow described in the first post and analyzed it as a flatbow, then with flipped tips and a reflexed limb. I wanted to know if my results matched the experience of those here and it appears they do.
In TBB3 Tim did a section on this in the bows of the world chapter, it basically said regardless of limb profile bows with nocks equal distance forward of handle would store the same amount of extra energy all else being equal
My results say they aren't identical, but are close enough that you would never know the difference if you were testing the two on a tree.
this is a question I have asked before and has been talked about quite a bit. Reflexing the whole limb puts the most amount of stress on the bow. I've done it before with white woods and if they are heavy draw weight bows they can be extremely hard to string. Imo. The consensus was/is that recurves are more beneficial than reflexing the whole thing.
Thanks for this. My analysis results agree that reflexing raises the stresses in the whole limb while recurve tips raise the stresses a touch more than reflexing at the base of the limb, but less as you move towards the tips. By mid limb the recurved limb has lower stresses than the reflexed limb. The recurves give higher string tension throughout the draw and slightly higher early draw weight but finish with a bit lower final draw weight. Stored energy is basically the same, as I mentioned above. Considering the smoother draw and generally lower stresses of the recurves I would say the consensus was correct.
Mark
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That is all well and good BUT the reflexed limb would take less set :)
Assuming that they would take the same set is wrong. The recurved limb has a shorter woking section opposed to a proper pyramid limb of the same intial length. The pyramid can also have significantly less weight at the tip/last 6 inches or so.
Fresher wood that has taken less set is going to give a greater limb return speed.
Unfortunately in bow making you can't really change one thing without it affecting another area....doesn't half make things interesting though :)
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Are you including string length in your figuring? If you have two bows like you say, at brace height the reflexed bows string will probably be full length ie tip to tip but the recurve string won't have lifted off yet. This will skew your DW at various DL's.
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That is all well and good BUT the reflexed limb would take less set :)
Assuming that they would take the same set is wrong. The recurved limb has a shorter woking section opposed to a proper pyramid limb of the same intial length. The pyramid can also have significantly less weight at the tip/last 6 inches or so.
These were just short flipped tips, not big hooks. The pyramid flatbow I started with has stiff tips. The flipped tips were shorter than the stiff section, so the working limb length was unchanged (unless there is something there I am missing). I agree the tips can be narrowed significantly on a flat pyramid design, but optimizing that wasn't what I was interested in. I wanted to know the effects of reflexing the tips forward and if one method was better than the other overall to achieve that.
The goal of the experiment was to take a flat bow design and modify it exactly as you could in real life with an existing bow in your hand, by either flipping the tips or reflexing the whole limb without any other changes. I changed nothing on the limb thickness or back profile, so the flipped tips were the same weight/width/thickness as before flipping them. As expected with this approach, draw weight increased along with limb stresses. In reality you would likely want to retiller some, to either get back to the original draw weight or fine tune the bend because of the changes made from the original bow.
Are you including string length in your figuring? If you have two bows like you say, at brace height the reflexed bows string will probably be full length ie tip to tip but the recurve string won't have lifted off yet. This will skew your DW at various DL's.
Brace height was kept constant. I would have to look at the string lengths to see how much they differed. The flipped tips didn't touch the string at brace so there was no lift off point to worry about. I just wanted to see what effect the two options had on the outcome, if the string rode the recurves a bit that would have been one of the effects to watch.
In the end the recurved design stored the same energy as the reflexed design at a slightly lower draw weight, had lower overall stresses and a smoother draw with less stack. The differences aren't huge (because I didn't move the tips a huge amount forward), but they are definitely there.
Mark
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How much lower draw weight ,and what does all this convert into speed in comparison?
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How did you measure stress
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In the end the recurved design stored the same energy as the reflexed design at a slightly lower draw weight,
how do the designs compare for stored energy if you adjust up the thickness of the reflexed model to where both bows pull the same weight at full draw? or are the differences very slight?
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How much lower draw weight ,and what does all this convert into speed in comparison?
Draw weight was about 2% lower for the flipped tips than reflexed. No idea on the speed, that is not something that can be analyzed mathematically. Or at least not something I can do with math.
How did you measure stress
The design was analyzed using Super Tiller software.
how do the designs compare for stored energy if you adjust up the thickness of the reflexed model to where both bows pull the same weight at full draw? or are the differences very slight?
The differences are very slight with the small amount I moved the tips forward. I would expect to see bigger differences with more reflexing, but at that point you would need to design the bow specifically for that side profile. What I was looking at was just the typical small changes people do to an existing bow to hot rod it a bit after it is mostly (or completely) finished.
Mark
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A pyramid taper for a recurve isn't optimal and stiff tips on a pyramid also isn't optimal. So its a bit like comparing apples and oranges - that is what I was getting at. You are not making a 'fair' comparison. Indeed it is next to impossible to do so and come up with anything meaningful.
An optimised pyramid and an optimised flip tip bow - the pyramid will be faster.
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Mark I have done this with a glass bow it was full of shock . Bad plan to do in glass but may be different with wood. Just flip the tips. Arvin
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If energy storage is largely a function of how far the nocks are ahead of the handle, what specific benefits does a deflexed handle offer? If any? This assumes that the limbs are reflexed enough to move the tips ahead of the handle.
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An optimised pyramid and an optimised flip tip bow - the pyramid will be faster.
When you say an 'optimized pyramid' what is that in your mind? Are you thinking a flat bow, one with lightly reflexed limbs, or something else? Narrowed tips or straight taper to the nock? You are also pretty much the first person here I have seen say that stiff tips aren't desirable on pretty much all bows. I respect your opinion and would like you to elaborate on these a bit if you would.
Mark I have done this with a glass bow it was full of shock . Bad plan to do in glass but may be different with wood. Just flip the tips. Arvin
You built a reflexed limb pyramid with glass? And it was nasty to shoot? Can you PM me more details and a pic or two if you have them?
If energy storage is largely a function of how far the nocks are ahead of the handle, what specific benefits does a deflexed handle offer? If any? This assumes that the limbs are reflexed enough to move the tips ahead of the handle.
I can't make a blanket statement about it, but in the bows I have modeled in this style the deflexing allows you to use a big hook or heavily reflexed limb that gives very high string tension at brace and high early draw weight without overstressing the limbs at full draw. Essentially you have a really stiff limb (or big hooks) that would fail when pulled to full draw on a straight bow, then you rotate them back until they can stand the strain of bending to the full draw position.
As an example, on the bow above I took the flipped tip version and rotated the limbs back until the tips were back to in line with the handle (so the same as the flatbow I started with). With the same brace height this design gives a slightly lower draw weight at full draw and stores a bit more energy than the straight bow. Stresses also go up slightly, but less than the extra energy stored for a gain in efficiency. All the changes are small because I am looking at small recurves and a small amount of deflex, but the gains are there as well with bigger amounts of reflex or recurves and more deflexing.
Mark
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It took me a while to figure this out, but now I realize the 'experiments' you are running are being done on a computer program, and not with actual bows. How do you know that the computer program accurately represents actual wood?
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No idea on the speed, that is not something that can be analyzed mathematically. Or at least not something I can do with math.
Mark
This struck me as an odd statement based on the whole point of the topic and your interest in the engineering side of things,
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No idea on the speed, that is not something that can be analyzed mathematically. Or at least not something I can do with math.
Mark
This struck me as an odd statement based on the whole point of the topic and your interest in the engineering side of things,
Pat,
there are methods to make speed calculations, but there are also quite a few more variables that have to be considered which make the results less precise. ie, more like an estimated range of possible speeds.
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Anything else being equal (so same overall reflex and let's say an even tiller, so the flipped tips are bending exactly as much as the rest of the limb) flipped tips bow will store a tad more energy and be a little faster. A simple matter of angles and decomposition of forces. Flipped tips bow simply gains more in the late draw compared to what it loses in the early draw. Same amount of stress on 2 "identical" bows that only differ in shape doesn't mean also same amount of stored energy.
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No idea on the speed, that is not something that can be analyzed mathematically. Or at least not something I can do with math.
Mark
This struck me as an odd statement based on the whole point of the topic and your interest in the engineering side of things,
Pat,
there are methods to make speed calculations, but there are also quite a few more variables that have to be considered which make the results less precise. ie, more like an estimated range of possible speeds.
Yes, and i thought that would be a key interest for those of that inclination.
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Anything else being equal (so same overall reflex and let's say an even tiller, so the flipped tips are bending exactly as much as the rest of the limb) flipped tips bow will store a tad more energy and be a little faster. A simple matter of angles and decomposition of forces. Flipped tips bow simply gains more in the late draw compared to what it loses in the early draw. Same amount of stress on 2 "identical" bows that only differ in shape doesn't mean also same amount of stored energy.
Not a engineer but I think this is spot on. One finally put it in terms I can understand. Thanks Arvin
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A pyramid taper for a recurve isn't optimal and stiff tips on a pyramid also isn't optimal. So its a bit like comparing apples and oranges - that is what I was getting at. You are not making a 'fair' comparison. Indeed it is next to impossible to do so and come up with anything meaningful.
An optimised pyramid and an optimised flip tip bow - the pyramid will be faster.
Arvin? This makes it sound like flipping or recurving the tips on your bows is a bad thing? It's all over my head, but interesting none the less... :OK
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I’ve just been doing it because it works . Flipping the tips and a ease of building a pyramid on straight grain wood. The perfect Diminishing mass in a pyramid with some added back in fades and ends. The ends to hold reflex and the extra mass at fades for the extra strain at full draw because of the flipped tips. I do this because I have not moved the mass else where yet. That was better. I’m looking at DC’s work though! Arvin
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It took me a while to figure this out, but now I realize the 'experiments' you are running are being done on a computer program, and not with actual bows. How do you know that the computer program accurately represents actual wood?
Because I have compared Super Tiller's output to David Dewey's design spreadsheet and to bows I have made. It isn't that hard to model structures and wood is not a totally wacky material that is unpredictable in its response to a load so it can be done with reasonable accuracy.
This struck me as an odd statement based on the whole point of the topic and your interest in the engineering side of things,
I would love to be able to model the whole shot, but it is extremely complicated due to the dynamics involved and the math gets beyond ugly. Developing that model would be a PhD or post-doc level project and I have no desire to subject myself to that sort of agony again.
Mark
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Hasn't someone else done that math and you can just plug your numbers in?
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Hasn't someone else done that math and you can just plug your numbers in?
Not that I have found. There are lots of people that have worked on numerous vibrations/harmonics/dynamics problems and there is software for some of it, but we are a small niche area and not on the radar of most academics in terms of areas of research.
Mark
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shooting through a chronograph,, would give you useful information,,it wouldnt take many bows to figure what design had better cast,, of if it was a wash,, and might vary from bow to bow,,I have a feeling it would be close and depend on the wood and the tiller execution, more than energy storage,,I think I read everything, and no chronograph numbers,,,wouldnt that help with comparisons,,
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I have a feeling it would be close and depend on the wood and the tiller execution, more than energy storage,,
This is the problem with comparing two (or more) bows. Even made from the same tree or board the wood will have differences and the fine differences in tillering are impossible to eliminate. It is a very challenging problem to deal with.
Mark
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I have a feeling it would be close and depend on the wood and the tiller execution, more than energy storage,,
This is the problem with comparing two (or more) bows. Even made from the same tree or board the wood will have differences and the fine differences in tillering are impossible to eliminate. It is a very challenging problem to deal with.
Mark
do you suppose it would be less challenging if we made FG bows?
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yes but there would be some patterns that would become apparent,, even with variations,, one can see that,, with the bows being posted here,,Badger DC Mark,, posting chronograph results that show performance, but so many variaitons on that,,even in one bow design, being over stressed or not,, does keep it interesting,,
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Both
Flipped pyramids are rockets.
HH~
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any fps with 10 gpp on the rockets,, :)
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do you suppose it would be less challenging if we made FG bows?
Yes. Or at least results would (should?) be more consistent from bow to bow, so trends would be easier to identify. Of course, the FG guys complain about the FG lams being inconsistent from batch to batch, so maybe it would be no different at all aside from dealing with slightly different types of inconsistency. (--)
Mark
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aside from dealing with slightly different types of inconsistency.
I believe the inconsistency with wood is primarily with set taking and when working with stresses near the elastic limit. below that level of stress, my experience agrees with your earlier observation
I have compared Super Tiller's output to David Dewey's design spreadsheet and to bows I have made. It isn't that hard to model structures and wood is not a totally wacky material that is unpredictable in its response to a load so it can be done with reasonable accuracy.
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I have a feeling it would be close and depend on the wood and the tiller execution, more than energy storage,,
This is the problem with comparing two (or more) bows. Even made from the same tree or board the wood will have differences and the fine differences in tillering are impossible to eliminate. It is a very challenging problem to deal with.
Mark
That's why simulations don't show the whole picture and you'd have to build bows to compare, this is more subtle than straight math equations because you have to shape the wood yourself. The numbers don't mean a thing if you can't make the bow
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I have several chrono's but im not shooting my SB's thru them. If they kill mature whitetails at 30 yds and elk at 25yds that's all I need to know really.
I will say this new flipped tipped pyramid I have 40@28" with a little longer riser and shorter limbs is CLEARLY this fastest SB i have ever made or seen shot at that weight. I shot it in Aug in IN and the four guys I shot with in finals, one shooting a 60@28" could not believe it was 40lbs. They current arrows i am shooting are over 10GPP 450ish. I wish i had shot this bow all three days there!! I am sure when this riser makes it's debut at the stake in a big shoot the stink will start drift dead downwind.
It is shorter than I normally shoot and the limbs do have more load on them but I also was able to drop brace height as well. This all adds up to more speed. Was not really trying to do this it was just what I had to work with in that piece of Hedge.
HH~
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I will say this new flipped tipped pyramid I have 40@28" with a little longer riser and shorter limbs is CLEARLY this fastest SB i have ever made or seen shot at that weight.
Have you posted pics of this bow anywhere? I assume it is not the 'Hind Tit' bow you posted a month or two back?
Mark
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No Sir, I have not. Looks like Arvin's target model but shorter, more pyramid at tip and reflexed bit tips are flipped more.
Shawn~
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That will be fast and smooth on the draw just guessing. Hedge is that from the stave I sent you? Arvin
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Yes it was Arvin. Took several heat treats to get one limb straight. Grain was not straight on one side. Was a real challenge with tips that narrow to bring to proper alignment.
Hint, with the Hedge: if your doing real heavy pyramid bow use the straightest grain wood you can put your hands on or your going add some grey whiskers.
This bow is very consistent in way it pulls weight from 6” brace to 28”. The Super Pyramid
To me it only matters that a bow shoots where i look. Point aim and point of impact are most important to me. Has to shoot consistently with respect to those two points. How fast it does this matters little as long as it does it with the arrow setup designed to kill efficiently.
HH
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My analysis results agree that reflexing raises the stresses in the whole limb while recurve tips raise the stresses a touch more than reflexing at the base of the limb, but less as you move towards the tips. By mid limb the recurved limb has lower stresses than the reflexed limb. The recurves give higher string tension throughout the draw and slightly higher early draw weight but finish with a bit lower final draw weight. Stored energy is basically the same, as I mentioned above. Considering the smoother draw and generally lower stresses of the recurves I would say the consensus was correct.
This is a fascinating question. But I have no skill to verify the statements with math or physics. Hence I resort to common sense and common language analogies. It appears that reflexed limbs are evenly stressed; while recurved tips make the straight inner limbs work harder. As inner limbs are thicker and wider, they can take the additional stresses better than outer limbs. If so, I guess with same draw weight and materials, all other things equal, the recurve might be slightly lighter than the reflexed.
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In the end the recurved design stored the same energy as the reflexed design at a slightly lower draw weight, had lower overall stresses and a smoother draw with less stack. The differences aren't huge (because I didn't move the tips a huge amount forward), but they are definitely there.
Does that mean at the same draw weight but at slight longer draw length, the recurve store more energy than the reflexed?
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No Sir, I have not.
Can you post a few pics for us to appreciate it?
Does that mean at the same draw weight but at slight longer draw length, the recurve store more energy than the reflexed?
I hadn't thought about it that way, but that would be the way it works out in the end.
Mark
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Hedgehunter,, since you have a chrono, would you consider shooting your bow, through one, and sharing the info with us, here,, thanks in advance, B
arrow weight ,, draw weight,,draw lentgh,,string type,, and anything else you might share, photo etc,, thanks,,
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I dont shoot selfbows thru chrono's. I made that promise to myself a long time ago. I will put a BH on a shaft and shoot it for distance with a 420-450grn arrow @28" draw when I get a chance.
HH~
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Can you get someone else to do it for you?
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I dont as a rule let others shoot my personal rigs or backup rigs.
HH~