Primitive Archer
Main Discussion Area => Bows => Topic started by: Kidder on June 02, 2022, 01:21:05 am
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Is there any correlation between design stress and performance? In other words, all else being equal, will a higher stressed design bow result in greater performance? One example of what I’m thinking is a 50# 66 inch bow versus the same 50# bow but only 60 inches overall. Assuming that is the case, will the performance degrade over time where the performance of the low stress design overcomes that of the high stress design after extended usage? Thanks for entertaining my wandering curiosity.
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Every piece of wood has its own set of properties. How elastic is it? How able to resist compression? How close are its resistances to stretching and compressing? Plus others!
Your job as a bowyer is to 'listen' to the wood and hear what it is telling you.
The more skilled/experienced the bowyer the closer he can get to optimal strain on the wood.
One reason I always suggest people should trace the outline of the back before ever bending the stave - during tillering you can placed the bow back against the original outline and see where/how the set is appearing.
Also get your head around 'tiller logic' as soon as possible....once you understand how width taper is married to thickness taper and that thickness taper determines 'tiller' you can make any bow in any design and KNOW how it should bend to optimally strain the wood.
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If you give wood enough width to handle the strain in the areas it needs it.It holds up over time.There is no certain script that fits all bows and designs.All things are not equal then.All woods are not the same either in density/shapes of backs/elasticity and the list goes on.Evaluating the wood and knowing it before design and tillering is started.Width's and thickness's are different then too.If the design is going to be more stressed you need more wood to handle it.
The cleaner the wood the more stressed design it can handle.Even then it's a fine slow line to walk and not always accomplished.
To me that's the beauty of self bows or sinewed bows for that matter.
It's one of the reasons I think why these type bows will out shoot FG bows as most all those are scripted.There are FG bow makers out there though that do not conform to the script,but they have their limits too.
Even the highly stressed bows will shoot quiet as a mouse with or without silencers [I like the looks of them] if they are tillered correctly/settled in and take years and years of shooting/hunting seasons and still shoot with the same performance.
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Well said mike, Ed. Sometimes our own thoughts and ideas or ambitions get in the way of listening to the wood.
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I think there’s likely a sweet spot for performance. Too stressed will obviously result in set and worse performance. Understressed May mean more mass or wind resistance in the limbs. I think there’s still a question of mass given Badger’s experience with wide limbed bows that we’re not necessarily heavier than narrow limbed counterparts.
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Is there any correlation between design stress and performance?
Yes. You will get more performance as stresses go up until you go too far and the wood starts taking set or otherwise failing due to the high stresses. As the others have noted, the fundamental problem with wood is its inconsistency in material properties. You never know quite what the limits are for the piece of wood you are working on and you can't tell the peak until you have gone past it.
Mark
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Firstly I would like to thank Alan Case for publishing his valuable work for other bowmakers and then recommend to read his comments /see the graphs/ here ( and several previous pages too ) :
http://www.primitivearcher.com/smf/index.php/topic,69529.345.html .
I ( as not very experienced bowmaker ) do not know other way to do asked task, than to test ( similarly to avcase's method, but preferably using adviced 4 point bending setup ) on appropriate samples of wood from prepared stave, determine the proportional limit and decide what set is still acceptable along different parts of limbs for your bow design. Then use bendmetering at the final part ( at least ) of tillering to not overcome the desired set ( in every stations along the limbs ).
To resolve the problem mentioned by Mark - "you can't tell the peak until you have gone past it" - I suppose one could make the bow intentionally thicker/stronger/ from the beginning and after overcoming the peak ( evenly ), you will remove damaged belly wood without changing the tillered shape of the bow and by such way achieving desired draw weight and performance.
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Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin
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+1 to what everyone else said. The TBB performance chapters have detailed tests on power stroke as well. The same bow of 45-48 pounds (if I recall correctly) was tested at low 20’s draw and shot 135 fps. It was retillered over and over to 45-48# out to 28” in one inch increments, and it gained 20+/-feet per second in the process. Power stroke is also important.
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Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin
Hey Arvin,
To answer your questions in order:
1) A bend test doesn't tell you how much stronger your wood is in tension or compression, it simply gives you the stiffness of the wood in bending (the modulus of elasticity) and the stress level where set starts to occur (assuming you test like Alan Case did in your other thread linked to above). You could figure the tension/compression balance out by trapping bend test samples different amounts and examining how they fail until you find the one that fails nearly simultaneously in tension and compression. You might run out of wood before you find this point, though.
2) The effects of trapping are not linear like that at all. Removing 30% of the back width will not raise the tension stresses by 30%. The change in stresses is based on how much you shift the neutral axis of the limb cross section and that is dependent on the back width along with how much of the side is removed and the final cross sectional shape.
3) I would say the perfect mass distribution is what you get with a quasi-pyramid back profile and constant limb thickness. The constant thickness keeps the strain/stress the same throughout the working limb length and the width changes to optimize the amount of wood used to the minimum required to keep the strain/stress the same everywhere.
Mark
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Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠
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as you make the bow shorter,, there will be a point of diminishing return,, where the perfromance will go down,, even the draw weight is the same,,
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Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠
Trapping will help balance the tension and compression capacity of the wood and let you get closer to the maximum performance the wood can achieve but it isn't magic.
Mark
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Ok Mark thanks for the answers.
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For the same reason we round the belly on yew bows I think...because with yew the weaker side is the back, imho.
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Is there any correlation between design stress and performance? In other words, all else being equal, will a higher stressed design bow result in greater performance? One example of what I’m thinking is a 50# 66 inch bow versus the same 50# bow but only 60 inches overall. Assuming that is the case, will the performance degrade over time where the performance of the low stress design overcomes that of the high stress design after extended usage? Thanks for entertaining my wandering curiosity.
When I've previously said things along the lines of what I say below, I have attracted criticism for my expression of this craft as being too divorced from the romance of bowmaking by feel. However, given your question I feel an answer such as the below merits this sort of info. Using methodologies from materials science and mechanical engineering allows us to answer questions such as yours, and has helped me to personally teach maybe 180 students how to make dependable, high performance wooden bows.
Yes, there is a correclation between stress and performance. If the stress is very (very) low, it will be because the stress is being distributed across a whole lot of wood. Either through very long or very wide limbs (or both). If your bow is very high stress, it will be because the bow is very short or very thick (or maybe both).
In the former case, all that wood across which the stress is distributed presents a whole lot of excess mass. More of the energy stored in bending the bow has to be expended on accelerating the heavy limbs.
In the latter case, the bow gets dangerously close to having stress that either causes high amounts of set (which diminshes performance) or breaking (which summarily ends all performance). A bow which is tillered to a high level stress and repeatedly exposed to that stress is likely to deteriorate in performance over time. I understand that it's not uncommon for wooden flightbows last a few shots and then break down (or break up).
So the aim of the game is to find that amount of working stress that walks the fine line between sufficient wood to withstand the stresses over time, and of sufficiently high stress to minimise the mass. In your example of the two 50# bows, ideally they'd both have the same amount of stress. There may be some who don't believe that, but mechanically it's true. A piece of timber with an ideal amount of working stress should be made into bows that experience that amount of stress regardless of the length or drawn shape.
While it is true that the variability of wood's mechanical properties can be quite broad, it is also the case that this phenomenon doesn't matter much so long as the stave is of sufficient size to extract a sample for bend testing first. There are probably many bowyers who would argue that this isn't 'listening to the bow/wood', but I'd argue that it is in fact listening very carefully after having asked the wood some very specific questions.
It is entirely possible to measure the mechanical properties of a piece of timber and from those calculations make a dependable, high performance bow. I've done it for years. What's more, I've outsourced the calculations to spreadsheets. I've written a reasonable amount about it here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450 (https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450) and here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=13765 (https://ozbow.net/phpBB3/viewtopic.php?f=34&t=13765). You can do the same with free software and engineer your own bows.
If you've the patience to go through that first link, you'll see its possible to make a bow with perfectly even stress and strain throughout the entire length of the bow, and that you can achieve this regardless of the particular drawn profile of the bow, be it English Warbow or short Mollegabet. If you go through the second, you'll see how I record bow properties in shorthand. Of note, by combining my bend test library with Tim Baker's, I found that while a wood like Rock Maple might vary in its mechanical proprties between samples, it is very often the case that the variation is sufficiently small not to matter a great deal.
I suspect the practice of rounding the bellies of yew bows is more a matter of convention than engineering best-practice.
AY
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Paul Comstock spent quite a bit of effort analyzing the subject of highly stressed bow designs in “The Bent Stick.” I’d venture to say you’ll find lot’s of useful info in that short highly informative paperback.
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ok here is a good example,,
take a 40 inch self bow,, and draw it 20 inches to 50#,, shoot it through a chrono with 500 grain arrow,, I would say 140 fps would be good,,
now take a 66 inch self bow 50# draw and draw it to 28 inches ,, not so strained ,, shoot the same arrow,, 170 fps is achievable,,165 being more achievable,, ;D
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Bradsmith, the change in speed and disgance of the arrow in your example is due to the power stroke, which changes how much energy is stored and then released into the arrow.
Ideally, both bows would be designed and built to have equal strain/stress.
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yes but one bow is stressed more than the other,,
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No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.
It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".
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ok
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No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.
It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".
So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post? I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...? thanks.
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Mass distribution is more important than mass alone. That gets into surface area and pounds of stress per square inch. For a few years I have been on and off working on getting a formula for it. I have one that works well, but isn't exactly where I want it to be. I'm still trying to improve it.
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I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.
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So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post? I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...? thanks.
Yep, absolutely can do. Stand by for a day or two and I'll pul something together.
I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.
Totally agree Badger. Fortunately, doing a bend test needn't take long. And the time it saves is immense too. I can make a longbow from a board that pretty much comes off the bandsaw tillered to within a couple of pounds of design.
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Aussie, 1st of all I wanted to publicly thank you for converting the mass formula to an excel spreadsheet for me many years ago.
As for a bend test, I was never able to convert a bend test to usable information that I could apply to making a bow. I think if you could lay out the method for doing this, if it is not too complicated it would be a huge step in making higher performing bows. I came out with several tests that were useful in comparing wood but I was never able to directly apply it to a design. Hopefully you can send us in the right direction here.
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the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
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the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
Brad, I agree somewhat. but if someone can tell me that the shorter bow needs to be (maybe) 3" wide and 3/8" thick to be strained the same as a longer bow that is "1 1/4" wide and 3/8" thick... it would be interesting and a good place to make 2 bows just to see how they compare in reality. I guess I'm a visual learner....
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me too thats a good point,, I would be thinking is the mass of the 3 inch bow gonna effect the performance,,
I would just have to make it,, shoot it through a chrono and see,, I dont know the answer,
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Aussie, 1st of all I wanted to publicly thank you for converting the mass formula to an excel spreadsheet for me many years ago.
As for a bend test, I was never able to convert a bend test to usable information that I could apply to making a bow. I think if you could lay out the method for doing this, if it is not too complicated it would be a huge step in making higher performing bows. I came out with several tests that were useful in comparing wood but I was never able to directly apply it to a design. Hopefully you can send us in the right direction here.
Most welcome Badger, a pleasure and an honour to help out.
Laying out the method can be easy to understand, but it isn't necessarily something I can cover thoroughly in a short space. Fortunately though a few years ago I did lay out the method, from plank of wood to engineered bow here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450 it might take a while to read through but I tried hard to make it accessible.
the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
BradSmith, if by this you mean that it's easy to show that a short bow will be inherently more stressed than a longer one, then no that isn't a fair demonstration of the hypothesis. All you will demonstrate is that a more stressed bow takes more set than a less stressed bow. The reason is without common metrics and methodology, there's no way to control for the amount of experienced stress in the bending bow. By this I don't mean that the stress is uncontrollable, but that you can't be sure that the stress in the two bows is the same.
An aluminium rod and a dowel feel cold and tepid, respectively, when you pick them up in your workshop. But the reality is that if you measured them scientifically, they would actually be the same temperature. Observational studies are not always reliable reflections of physical phenomena.
The amount of plastic deformation (permanent set) a piece of wood (a bow) takes is dependent on the maximum amount of stress the wood is subject to during bending. More working stress = more set. Less working stress = less set. This is completely independent of draw length, draw weight or bow length.
We've all made short bows that took little set and long bows that took more. It has less to do with the length than it does with the stress as a proportion of the elastic limit of that piece of wood. The shorter bow with less set experienced a smaller proportion of its potential stress, and the longer one with more set was caused to endure a larger proportion of its potential stress.
On the weekend I'll bring together a few different theoretical bows with dimensions and if people wish they can make their own according to the dimensions. I'll do a short Mollegabet, a medium pyramid bow, and a longer D bow.
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Aussie thank you so much for the details and explanation,, I just ment if you build the two bows,, alot of your answers will be revealed,,, :)
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Oh, I see. Orrighty then.
Okay, so I pulled some stats and calculated some dimensions. These dimensions are for a sample of timber tested by Tim Baker many years ago. The design stress should result in a set of about 1.5". For what it's worth, the modulus of elasticity of this piece of timber was 16,415 MPa and the working strain is 0.085%.
64" pyramid bow (8" rigid centre not included - "Dist" = "dist from fade")
50# @ 28"
Dist_____Thck___Wdth
0.00_____0.47____1.58
4.67_____0.47____1.40
9.34_____0.47____1.18
14.02____0.47____0.92
18.69____0.47____0.63
23.36____0.47____0.55
28.03____0.47____0.47
71" bendy handle longbow
60# @ 28"
Dist_____Thck_____Wdth
0.00_____0.80_____0.87
3.94_____0.80_____0.86
7.87_____0.77_____0.84
11.81____0.73_____0.82
15.75____0.69_____0.80
19.69____0.62_____0.79
23.62____0.54_____0.77
27.56____0.48_____0.69
31.50____0.41_____0.54
35.43____0.36_____0.39
63" Modern Mollegabet
50# @ 28"
Dist_____Thck_____Wdth
0.00_____0.89____1.02
1.57_____0.78____1.00
3.15_____0.67____1.04
6.30_____0.56____1.05
9.45_____0.54____1.05
12.60____0.52____1.00
15.75____0.54____0.77
18.90____0.61____0.49
22.05____0.64____0.34
25.20____0.64____0.23
28.35____0.59____0.19
31.50____0.55____0.14
I've tried to upload a picture that overlays the drawn shapes of the longbow and the molly. My relationship with forum interfaces is often a fraught and tenuous one, though.
*EDIT*
Aha! It worked! For funzies, I also uploaded a snip of my bow dimensions calculating spreadsheet.
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How much wider would those designs have to be for 0.1” set?
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The short answer is "quite a bit, but I'm not sure".
It would take some re-figuring. I might have some time tomorrow.
The thing is though that reducing strain means making it wider and thinner. Going in this direction increases the mass per unit of stiffness. So while you might have less set (no set), the performance will suffer by virtue of more of the stored energy being required to accelerate the limbs.
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thats a very good point Aussie, I have made bows with little set ,, but shot slow,, to much mass,,
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The short answer is "quite a bit, but I'm not sure".
It would take some re-figuring. I might have some time tomorrow.
The thing is though that reducing strain means making it wider and thinner. Going in this direction increases the mass per unit of stiffness. So while you might have less set (no set), the performance will suffer by virtue of more of the stored energy being required to accelerate the limbs.
the 63 inch Molly needs to be almost twice it's width.
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I suppose you are quite right, on average. I tried to pick a timber that would be common in the US (almost none here), and I guess I picked a truly exceptional sample.
I'll run the numbers on some other samples and see if we get something more conventional. Closer to average.
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Gonna conduct some necromancy on this old thread:
I found an old project where I followed the dimensions calculated and got pretty much exactly what I wanted. This was a pretty long Molly design, made from boo-backed ironbark. I calculated the dimensions and printed them out 1:1 as templates, then did the same with the projected tiller shape. It didn't take much tillering to get it to spec, and when it was done the draw weight/length were just what I wanted, the tiller shape matched the projection, and the dimensions were pretty much bang on. Just small fractions of an inch here and there.
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Here's another one, in which I make a longbow from Maple, using an average of figures collected by others (Tim Baker's bend test data)
Because I didn't use figures from a sample from this very board, the set was a little higher than I'd have liked, but otherwise it turned out pretty much as designed:
https://ozbow.net/phpBB3/viewtopic.php?f=34&t=17080
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Guys you know I’ve built the same bow design for years because it works well for me.and yes by doing so I’ve played with moving mass to accommodate for set. I was told I was wasting my time. That made me laugh because I was still getting set. But that being said my bows have 1-1/2-3” of reflex in my he last ten inches of the limb. Causing more stress fade to mid limb. So I made that area wider and by adding the mass it cut down on set. If you use your calculations for n a straight limb bow it will be different from say my design. I don’t know how to estimate the added mass needed to prevent the set . Deflex in the handle can balance the stress load more evenly but that’s another can of worms. The more early draw the faster the bow. Think compound for a moment. This is why I think the farther the tips in front of the bow handle with the same set the faster the bow will be! That requires more width fades to mid limb. Short or long bow. Just an opinion.
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That requires more width fades to mid limb. Short or long bow. Just an opinion.
adding width to limit set certiany seems like you can get a good handle on how much extra width it takes to make your design shoot better.
does your width out of the handle run straight for a ways before you begin to taper? if so, have you changed the distance out the limb before the taper starts? or just go a little wider to the same point as before?
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Willie the best bow shots that I have gotten with broadhead arrows was with a straight taper bow 2-3/8” at fades to 1/4” at the tips. I have taken it parallel for 9” then @18” 1-1/4 wide and then to narrow tips. All with the same reflex. This is with Osage . I think the parallel for nine inches is working better but still testing. On a 28” working limb design it’s putting plenty of stress on the limbs especially the inner limbs. I am considering going to 2-3/4 at fades . It might overbuild it though.
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In Archery: The Tecnical Side Hickman showed that the ideal width profile for circular tiller is not straight line taper in width as we consider the classic 'pyramid' bow, but ever so slightly bulged near the flares/fades. I've found, when making pyramid bows with circular tiller, that running the limbs parallel width for 4-5 inches before tapering in straight lines to be a good-enough approximation for ease and speed of layout.
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Badger also mentioned keeping width parallel out of the fades for 2-3" and I have used that as well. It does a great job of evening the stresses out right at the end of the fade into the working limb portion. There tends to be a bit of a stress concentration right at that point and a short parallel section smooths that out.
A second thing not mentioned yet is that the theoretical pyramid shape tapers to a sharp point at the nocks. Since this is impossible in reality the way around it is to layout the pyramid taper to a sharp point, then draw a short parallel width lever section from the nock to where it intercepts the pyramid taper. This can be as narrow as you think you can make it. On my last lam bow I think that lever is about 5/16" wide for something like 6". I was worried about that being fragile and maybe unstable but it has not proven to be so.
If you straight taper to the nocks with some width at the nock then it tends to overstress the middle of the limb a bit and you need to taper the thickness in the outer half of the limbs to balance that off.
Mark
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Absolutely agree with you there. The taper profile needs to converge on a theoretical zero width tip, which in practicality has to have some width, as you mentioned.
This means that the limbs, of tillered perfectly symmetrically, probably won't be exactly the same thickness throughout, but need to taper ever so slightly in the last 1/4 of the limb or so.
I once made a bow with tips just under 1/4" wide. So narrow I cut a single nock in the back of each tip. That was for a Molly rather than a pyramid, but the theory holds.
In fact I think cutting a single nock on the back increases the stability because the point at which the string contacts the cross section of the limb is closer to the neutral plane.
That hypothesis could do with testing, if anyone could be bothered.
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Absolutely agree with you there. The taper profile needs to converge on a theoretical zero width tip, which in practicality has to have some width, as you mentioned.
This means that the limbs, of tillered perfectly symmetrically, probably won't be exactly the same thickness throughout, but need to taper ever so slightly in the last 1/4 of the limb or so.
You are correct on needing to taper the outer limb thickness some if you straight taper to the nock width. You can keep the limbs constant thickness with a narrow lever to the nock, as shown here:
(https://i.imgur.com/gW9SQcH.jpg)
(https://i.imgur.com/B7bBfek.jpg)
(https://i.imgur.com/SjhoQBx.jpg)
It is a bit hard to tell in the picture, but the end of the limb is parallel width for about the last 3.5" or so. It is 5/16" wide for that whole length. This lets the limb width taper follow the theoretical ideal that goes to a point at the nock, then transitions into a slightly stiff tip lever where the limb width goes below the 5/16" width. I added a tip wedge during layup to stiffen the tip a bit more and guarantee it was strong enough. This was the first time I had tried this and I was worried that the spindly narrow tip would be too fragile.
Mark
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Good job on that one! That's quite the gnarly looking arrow pass design you have there. What was the draw weight/length on that one?
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Good job on that one! That's quite the gnarly looking arrow pass design you have there. What was the draw weight/length on that one?
http://www.primitivearcher.com/smf/index.php/topic,70862.msg994666.html#msg994666
Mark
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That requires more width fades to mid limb. Short or long bow. Just an opinion.
adding width to limit set certiany seems like you can get a good handle on how much extra width it takes to make your design shoot better.
does your width out of the handle run straight for a ways before you begin to taper? if so, have you changed the distance out the limb before the taper starts? or just go a little wider to the same point as before?
Willie if we overbuild the first three inches coming out of the fades but run parallel the limbs need to tapered in thickness ? If we overbuild in more overall width thru out the hole limb length but stay the same thickness could this be close to the same spreading the load. Extra mass in the first nine inches out of the fade has little effect on performance in my opinion. Again the added reflex in the last 9” of the limb is throwing a curve in the calculation. I think. Thats why I have been chasing the set with mass from bow to bow. Obviously I build by feel with mathematical measurements from the design in mind as I go. Not smart enough to do it any other way. 🤠🤠
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The thing is though that reducing strain means making it wider and thinner. Going in this direction increases the mass per unit of stiffness.
Aussie,
have you ever kept track of the grams/MPa of completed bows plotted against arrow speed ?
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Do you mean grams of bow mass against the stiffness or stress of the timber against arrow speed? No, I haven't. Could be interesting though.
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Do you mean grams of bow mass against the stiffness or stress of the timber against arrow speed? No, I haven't. Could be interesting though.
grams of mass/MOE as determined by a bend test of the wood.
I guess you could calculate out the actual stress.........
but either way, you would need a bend test of the wood for the next bow to plan acordingly.
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Oh! Yes, I have. When I've done bend tests in the past I've most usually also measured density.
Alas, all my data are on a computer that recently died and I've not yet tried to rescue them. But the data do exist.
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Arvin,
if you overbuilt the entire linb you would lower the actual stress throughout the entire limb, which might incur a mass penalty. I dont think it matters if the first few inches coming out of the fades were thicker or wider so long as its stiffer.
Badger used to leave the inners on the stiffside as he tillered out, and brought the bend in towards the handle more if it looked like he was getting too much set too early. My guess is that near handle set is to be avoided more over set taking further out. Perhaps he is reading along
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Oh! Yes, I have. When I've done bend tests in the past I've most usually also measured density.
I was thinking of both sort of. deriving a mass/moe value and plotting it against some performance metric like arrowspeed
I guess if you have bend tests of the timber used in good performing bows and can still evaluate the working mass, it might be useful
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Ah yes I see where you're going.
I think the warmer rabbit hole would be working strain against mass compared to arrow speed. Yew, for example, has quite a low stiffness but can accommodate a prodigious amount of working strain.
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Badger also mentioned keeping width parallel out of the fades for 2-3" and I have used that as well. It does a great job of evening the stresses out right at the end of the fade into the working limb portion. There tends to be a bit of a stress concentration right at that point and a short parallel section smooths that out.
A second thing not mentioned yet is that the theoretical pyramid shape tapers to a sharp point at the nocks. Since this is impossible in reality the way around it is to layout the pyramid taper to a sharp point, then draw a short parallel width lever section from the nock to where it intercepts the pyramid taper. This can be as narrow as you think you can make it. On my last lam bow I think that lever is about 5/16" wide for something like 6". I was worried about that being fragile and maybe unstable but it has not proven to be so.
If you straight taper to the nocks with some width at the nock then it tends to overstress the middle of the limb a bit and you need to taper the thickness in the outer half of the limbs to balance that off.
Mark
You are hinting at something here but didn't explain all the way through. The perfect pyramid tiller looks like a circle. But we can't build a perfect pyramid for the reasons you described. So if we tiller a non perfect pyramid to look like a perfect one, the bow is not correctly tillered.
Since a pyramid bow is essentially a piked bow, you know that the tiller shifts when you pike the bow. The tips gain more leverage and get stiffer, which pushes the stress towards the inner limbs. You need longer parallels than what you said was suggested. You need about 7 inches per my experiences, as a little loss in length magnifies in stress as the distance increases. 7 inches is a rough number. There is a ratio to explore but I don't know it yet.
In short, it's my strong opinion a pyramid bow shouldn't actually be a pyramid, nor tillered like one. It should start out parallel, then straight taper to stiff tips. Tiller should be stiff tips, that lead to an ellipse. That will allow the short coming of what is possible to be compensated for vs what works on paper.
All that said, a well made pyramid bow will not fail you as Old Self bowman aka Arvin has proven time and time again.
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So if we tiller a non perfect pyramid to look like a perfect one, the bow is not correctly tillered.
You can't get the circular bend with constant limb thickness on a non-perfect pyramid, but you can if you tweak the thickness in the outer half of the limb. I don't know if that should be considered 'not correctly tillered' or not. The red oak lam bow I did with the lever tips was very close to circular tiller with the tips stiff, which to me is very close to an optimal arrangement of mass and strain. This is what Arvin keeps chasing and his bows have shown the results of those efforts.
I haven't found any need for more than the 2-3" parallel out of the fades, but I am sure the longer section you suggest will work fine if the thickness is tapered to suit.
In short, it's my strong opinion a pyramid bow shouldn't actually be a pyramid, nor tillered like one. It should start out parallel, then straight taper to stiff tips.
Well, that certainly isn't a pyramid bow as you describe. It's basically an ASL and will need thickness taper and tiller to suit.
Mark
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So if we tiller a non perfect pyramid to look like a perfect one, the bow is not correctly tillered.
You can't get the circular bend with constant limb thickness on a non-perfect pyramid, but you can if you tweak the thickness in the outer half of the limb. I don't know if that should be considered 'not correctly tillered' or not. The red oak lam bow I did with the lever tips was very close to circular tiller with the tips stiff, which to me is very close to an optimal arrangement of mass and strain. This is what Arvin keeps chasing and his bows have shown the results of those efforts.
I haven't found any need for more than the 2-3" parallel out of the fades, but I am sure the longer section you suggest will work fine if the thickness is tapered to suit.
In short, it's my strong opinion a pyramid bow shouldn't actually be a pyramid, nor tillered like one. It should start out parallel, then straight taper to stiff tips.
Well, that certainly isn't a pyramid bow as you describe. It's basically an ASL and will need thickness taper and tiller to suit.
Mark
I agree. So, my point being that a perfect pyramid can not be built, so modifications must be made. In my opinion, these are the best ones to make. I will add, the 7 inches parallel is not a must. It's not even an accurate description of what's being done there. What's actually happening is the surface area is being increased therefore the amount of stress per square inch is reduced. You can just make the bow wider there and deal with the extra mass mid limb or taper it more agressive towards a straight taper.
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Sleek that’s s what I was saying about making the pyramid wider at the fades. The record broadhead bow took no or very little set. I guess I need to make a straight Pyrimid wider at the fades with no reflex and see where this goes. I do know reflex in the outer limbs causes more strain on the limbs. It has to.
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I agree. So, my point being that a perfect pyramid can not be built, so modifications must be made. In my opinion, these are the best ones to make.
Fair enough. My opinion is that following the theoretical pyramid taper and using levers at the end is the best way to go. I can get circular tiller for all of the working limb with constant thickness and stiff tips (which are desirable, IMO), which is a lot easier to tiller than the ASL profile. Either way will certainly give you a decent bow in the end.
Sleek that’s s what I was saying about making the pyramid wider at the fades.
You are correct on this, Arvin. Extra width on the pyramid shape costs almost nothing in extra limb weight but will help keep set at bay.
Mark
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Asking for a friend... What's 'ASL'? :P
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Asking for a friend... What's 'ASL'? :P
40/male/North Carolina
sorry, I don't know either.
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Asking for a friend... What's 'ASL'? :P
American Style Long bow. The back profile described by sleek is very close to this design.
Mark
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Is it the same thing as an American flatbow?
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I believe ASL stands for American Semi Longbow(which is essentially what Mark said, an American style longbow). Most people today recognize it or refer to them as a Hill Style Longbow(fg backed and faced). It developed from Howard's pre fibre glass design, usually backed with boo, slight stringfollow. John Schulz, RIP used to specialize in these bows, deep handle, longer fades than the usual 2" fades that most people use these days, and narrow limbs rather than wide.
Miller Longbows (taught by Schulz) still make this style of bow, really nice-looking classic style of bow.
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Is it the same thing as an American flatbow?
It may be more the flatbow I was thinking of. Attached is a pdf of the design I had in mind, call it what you will.
Mark
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Attached is a pdf of the design I had in mind, call it what you will.
Interesting find Mark. I guess the original (HHill) wood bows were quite rounded in the belly.
http://www.primitivearcher.com/smf/index.php?topic=45081.0
I can see why a bit of stringfollow was Howards preference as some of his big game hunting was with bows reported to be 115#
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Ok I know someone has done it but let’s say we build a bow with a pyramid on one end and a narrow and thick limb on the other end . Side profile being the same. Would this be no different from building a pretty wild character bow. Just asking before I try it.🤠
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You mean both limbs tillered to the same shape? The narrow deep limb would either not be deep, or it would take horrendous set, I think.
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Ok I know someone has done it but let’s say we build a bow with a pyramid on one end and a narrow and thick limb on the other end . Side profile being the same. Would this be no different from building a pretty wild character bow. Just asking before I try it.🤠
No problem tillering different styles on each limb.
https://bowyersdiary.blogspot.com/2017/03/the-frankenbow.html (https://bowyersdiary.blogspot.com/2017/03/the-frankenbow.html)
Del
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Ok I know someone has done it but let’s say we build a bow with a pyramid on one end and a narrow and thick limb on the other end . Side profile being the same. Would this be no different from building a pretty wild character bow. Just asking before I try it.🤠
That would be interesting. Assuming the wood can stand the strain on the narrow/thick limb then the pyramid side would be well overbuilt because the wider/thinner limb will have lower strain. You could match them better if the pyramid side was also made narrow/thick but tillering both sides to the same bend would be wrong, since the pyramid side wants a circular tiller and the other wants a more elliptical tiller.
I would expect it to have a fair amount of hand shock because of the mismatched limbs not moving in synch with each other. I think you should try it, Arvin. ;D
Mark
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With all that encouragement ya u gave me I don’t see much hope.🤠🤠🤠
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With all that encouragement ya u gave me I don’t see much hope.🤠🤠🤠
Dont despair! Arvin :)
Perhaps if you describe in broader terms what you wish to accomplish, folks could offer suggestions.
for instance, I recall some of the bow design changes you have tried from your "tried and true design" have resulted in bows taking more set and not performing as well. Perhaps a more pratical approach would be a better way to monitor set as the bow gets tillered, rather than examining the theory of why it happened
when you are pushing the performance of wood to the max, and measuring success by yards at Bonneville. skills trump theory, and the winners are fine tuning the best to begin with
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Yes Willie I do believe I need some tillering lessons from a few of the guys on the forum. There are guys better at it than me for sure. I’m good at making them bend even but they end up with more set than I like.
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I think most could learn from you, Arvin. tillering can be an art.
Do you find the set happening in the same places when it ocurs? perhaps if you could write a little about how you monitor set as you tiller out. where you measure. when you measure etc, detail your procedures a bit more for the reader
maybe the set gets worse as you shoot the bow more?
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I think most could learn from you, Arvin. tillering can be an art.
Do you find the set happening in the same places when it ocurs? perhaps if you could write a little about how you monitor set as you tiller out. where you measure. when you measure etc, detail your procedures a bit more for the reader
maybe the set gets worse as you shoot the bow more?
Willie set never happens til I reach 20 inches of draw . Yes some times the more I shoot in a bow more set creeps in. Usually not more tan than 1-1/2” . On flight arrows maybe I should design a 20” draw. But the arrows have to be 22” long. So you might as well go for at least a 24” draw. I floor tiller to about 20 pounds over desired draw weight then go to the long string never going over desired draw weight.at about twenty inches of draw on long string I brace the bow.then go from there.
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I think most could learn from you, Arvin. tillering can be an art.
Do you find the set happening in the same places when it ocurs? perhaps if you could write a little about how you monitor set as you tiller out. where you measure. when you measure etc, detail your procedures a bit more for the reader
maybe the set gets worse as you shoot the bow more?
Willie set never happens til I reach 20 inches of draw . Yes some times the more I shoot in a bow more set creeps in. Usually not more tan than 1-1/2” . On flight arrows maybe I should design a 20” draw. But the arrows have to be 22” long. So you might as well go for at least a 24” draw. I floor tiller to about 20 pounds over desired draw weight then go to the long string never going over desired draw weight.at about twenty inches of draw on long string I brace the bow.then go from there.
What Arvin said. I pretty much do the same thing. I may vary a little depending on type of bow build but pretty well what Arvin says here for alb or pyramid style and recurve long bows.
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I’m good at making them bend even but they end up with more set than I like.
Make one 1/2" wider at the fades than your usual. You pay almost zero limb weight penalty with your pyramid design and the extra width makes a huge difference in the strain on the wood.
tillering can be an art.
Not just 'can be', I'd say it really is in almost all cases.
Mark
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I think most could learn from you, Arvin. tillering can be an art.
Do you find the set happening in the same places when it ocurs? perhaps if you could write a little about how you monitor set as you tiller out. where you measure. when you measure etc, detail your procedures a bit more for the reader
maybe the set gets worse as you shoot the bow more?
Willie set never happens til I reach 20 inches of draw . Yes some times the more I shoot in a bow more set creeps in. Usually not more tan than 1-1/2” . On flight arrows maybe I should design a 20” draw. But the arrows have to be 22” long. So you might as well go for at least a 24” draw. I floor tiller to about 20 pounds over desired draw weight then go to the long string never going over desired draw weight.at about twenty inches of draw on long string I brace the bow.then go from there.
Hoping you can clarify about the "not more than 1-1/2""
would that be an additional 1-1/2" from shooting in over whatever set happened when you finished tillering?
going back to the general question about monitoring set as you tiller:
earlier in the discussion the idea was presented that different limb back profiles should result in different bend profiles (eliptical vs circular etc.) this principle is based on the assumption that all parts of the limb are (or should be) stressed equally.
when the stress is equal throughout the limbs the set should also be equally distributed.
when I started tillering bows I made the typical newby mistakes and overstressd different parts of the limbs creating set in various places. its not hard to pick up one of these bows today and see where the oops occured.
If I were visually inspecting one of your well tillered bows with not much set, I would have a very difficult if not impossible job determining where in the limb set might be more or less than set in another part of the limb, and hence, where one might try to stay a touch wider or thicker next time.
When tillering?
Do you have a predetermined idea or a pattern for what the bend profile should look like?
when you get to 20" and start noticing set.......
are you looking for where the set is occuring and changeing your plan for how the limb bend profile should change?
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Willie I very seldom have over 1-1/2” total set. If so it’s a hunting bow that has been strung for hours on end. Still a great bow just has more Set. As far as following a certain bend profile on my tiller tree no. My tiller tree has my record bow traced out strung. When I get it to match that profile at brace it usually makes a good bow. No matter what the draw weight is. Hundred pounders of the same length take more set. That being said I have only built 100 pounds for flight. They broke records but I think there is room for improvement. As far as where set shows up . I’ve had it thru out the limbs. Fade to mid limbs. My last 9” that has most all of the reflex in it does work out to about 3” from the tips. So I probably over build this area some. You take to much off in this area it will pull out if your not careful in tiller process. I will post a pick of my tiller board but Its posted before. That’s why I am sure some of you guys are better at tiller than me. I’ve tried different methods . One being tiller the bow and take the last couple pounds off at the fades making the working limbs longer at the fade.ive moved the mass around in 67” for some years.
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Pic. I think that was a snake bow . See the shadow in n the left limb about mid limb. If so Bob has that one.
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The second pic is unstrung. The tracing above the braced tracing is of my record bow unstrung. So I would say that the bow unstrung took set fade to mid limb.
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Three record bows . The selfbow in the middle took no set. The one on the right simple composite boo backed Osage took set thru out the limb. Different design though. The one on the left a complex composite boo back boo laminate gemsbok horn belly took set fade to mid limb. All three 67” long
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Simple composite.
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Complex composite
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Sweet lookin bows there Arvin.
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Sweet lookin bows there Arvin.
+1 on that.
Mark
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the record self bow with no set, that design seems hard to improve on....
perhaps it was an exceptional piece of wood that could take more stress
Marks advice to go 1/2"wider at the fades (and therefore a touch thinner when using wood not up to that quality) to reach the same weight goal, sound like a plan that might be fruitful. that is , if you are still replicating the design.
Didnt you do a bow to some perdetermined specs for someone? Alan maybe? how did that experiment go?
I have rambled on hoping others might share how they evaluate or measure set while tilling.
maybe things are slowing down on the forum...............
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Kidder has a good topic here . Lots of folks looking. Good design yields good bows. Yes that was an exceptional piece of Osage I have one maybe two staves that I think are from the same tree. I save those because I’m still learning this skill and keep getting better. Yes I did a computer design bow. The stave was excellent! But still a stave. I ironed it out as best I could. Clamped it to the caul and dry heated it to its design. The dimensions came in with in 5-15 thousands every two inches. Lots of hand sanding. The bow was designed at fifty pounds. It came in at around 40#. It could have been less dense than the data used to design. Moisture content etc. But I learned to watch for thickness change in the design. So if you take into consideration the mass placement in those areas it will gvie you new possibilities. Now Alan also gave me three bend designs from the same computer design. Brace , half draw, and full draw. I know Marc has built from computer design and had success. I will also say this was my first try. So a lot of the draw weight difference could have been on me.i never really got to test the design. Now I’m rambling.🤠
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The bow was designed at fifty pounds. It came in at around 40#. It could have been less dense than the data used to design............ So a lot of the draw weight difference could have been on me...............i never really got to test the design
coming in at 40 could have been from presuming a general strength value for the species instead of doing a bend test on a sample from the same stave? (thats rather simple to do actually, and I wouldnt put that on the bowyer's skill if the preliminary test was not done)
I am more curious about how much additional tillering you had to do after the bow was constructed to thickness.
did you have to make much adjustment to get it bending like the three plots alan supplied?
predicting the desired finished bend profile with a reflex/deflex design should be something a program should be capable of doing
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Loving this topic. Following along trying to learn. Some of it is way over my head
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It was a reflex deflex design.Best I can remember the bow reacted good to the end bend design. I don’t recommend this with a selfbow unless you want to make the bow build a long slow build. Now made from different laminators it would be much easier. Interesting build though. And yes using the dimensions as a guide decreasing the thickness as the design shows starting the tiller at the fades may be a better outcome. Don’t know for sure . I haven’t tried it that way .
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I don’t recommend this with a selfbow unless you want to make the bow build a long slow build. Now made from different laminators it would be much easier.
I agree, Arvin. I made one maple pyramid from a board using software calculated dimensions and it was a significant nuisance to keep measuring thickness with calipers and slowly sneaking up on the final dimensions while tying to keep everything even and correct. Switching to a multi-lam bow design was much, much more straightforward and I had better control of the important dimensions.
Mark