Primitive Archer
Main Discussion Area => Bows => Flight Bows => Topic started by: willie on December 07, 2024, 06:09:30 pm
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Arvin mentioned some specs for a new osage flight bow. :) and as I am learning to work with Virtualbow along with learning about what makes a bow good for flight, I thought it would make for a "design along" to post as I go. Comments and questions and insights welcome.
Arvins design:
Single piece selfbow
62" NTN
50# @ 25"
7" brace height
1.5" flipped tips
8" between fades
Virtualbow can be downloaded from https://www.virtualbow.org/
https://www.virtualbow.org/files/user-manual/ explains the program in detail
When Virtualbow first opens, it presents with a multi pane window. The upper right pane shows the bow in Back view, side view, single limb view or with both limbs, as selected by the buttons in the lower part of the pane. One can also zoom or rotate. The lower right pane will show plots when Items are highlighted/ selected in the left sidepane.
Below are some typical views when working with the program functions.
units_moe.jpg Two features are demonstrated in the first screenshot.
1. Selecting the "options" dropdown in the upper left pops up the units dialog where you can choose your default units, either metric (SI) or (US).
2. In the upper half of the left hand sidebar, by highlighting materiels > new materiels, the lower section in the side bar presents text entry fields to set Rho (materiel density) and E (stiffness). Having the correct Rho value is not required for simple "static" design but will need to be set properly when generating "dynamic" outputs.
The stiffness of the materiel is neccessary for knowing the draweight of a design. It is set on the E line by clicking on and resetting the desired MOE (modulus of elasticity). It is has been unchanged in the example below, as it is close to the book value for osage. Actual bend testing of a ripping from the stave will determine a closer value and will be done before the Arvins design is finalized.
MOE is explained in more detail at https://www.wood-database.com/wood-articles/modulus-of-elasticity/ and book values for various woods can be found by searching in the database. From the wood database, one might find an potential Elastic Modulus or MOE of 1,350,000 lbf/in2. Clicking the existing E in the sidebar allows one to enter 1,350,000 directly. The program will display in scientific notation once the value is entered.
The values from the database are obtained by averaging across many samples of various densities at 12% MC. If one has a primo stave with good rings at a 6% MC and applies heat treatment on the belly, then using a book value in the program could likely indicate the need for a wider stave then neccesary.
dimensions.jpg brace height and draw length are set in the left hand sidebar by
highlighting "dimensions". Enter your preferences in the lower sidebar.
thickness.jpg Virtualbow is optimized for designing laminate bows. Each laminate materiel can be defined as a "layer" with its own MOE and its own thicknesses.
In this designalong, Arvins selfbow being from a single materiel, the thickness profile will include the handle, fades and limbs all in a single layer
Highlight layer > newlayer. A table will appear in the lower sidebar. Entries in the lower table for a 31" limb are explained line by line as follows.
1. 0 is center of bow. 2 specs the depth of handle at center of bow in inches.
2. In the left hand column of this table, the distance out from the center of bow is given in percentage. making the percentage entries in the tables to need some calculation. Keeping in mind that we are designing a 62" bow, and each limb will be 31", the second row of the table is calculated thus. For a 4-5/8" handle on the bow, the half length would be 2-5/16" on each limb. Divide 2-5/16"(2.31) by 31" to get .075 which is entered as 7.5 in the left box and the right box keeps the handle depth at 2".
13% at .87" thick is an arbitrary point I chose for Arvins proposed handle ramp. It helps set the slope and shape of the ramp where the handle thichness transitions into the limb.
Choose a percentage that works best for you if needed. Raise or lower the thickness (.87") in the second box with the small arrows that appear when you double click inside the box.
The 13 in the first box on the line can also be raised or lowered also with up/down arrows in it's box. Shape the ramp to your liking by adjusting both values.
22% is where the thickness taper becomes fully working.
Again, divide the distance out on the limb that you desire the limb to start fully working by the overall limb length. 6.8" / 31" = .22 or 22 in the table when entered as a percentage. .42" in the second box is a thickness you estimate for the beginning of the limb thickness taper.
The last row defines the the trial thickness at the nock. 100% of .31"
The actual limb length of 31" will be specified innthe table and plot whan the "profile" line is highlighted as explained in the next post below.
f1width(1).jpg Illustrates setting limb widths. Highlighting "width" in the sidebar brings up a width plot and tables in the lowers sidebar for setting the width values in inches. Again, points along the limb are again shown as percentages of the total limb length. Arvins design calls for a 1.5" wide handle, 8" (13%) from the widest part of the fades to the" widest part of the fades, then straight for just under 7" (35%), then into a willow leaf taper to 5/16" at the nock. Arvin mentioned 4" straight section before tapering, and that can be done also. A straight taper can be had too. It just makes for small changes of the location of where the maximum stress occurs in the limb.
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Defining the limb length and side profile
Virtualbow can use multiple segments to build out a limb. One can define each segment seperatly, both by length and shape. Once all segments of the limb are specified, the overall profile and total length of the limb are viewable. Segments can be straight sections, arcs, spirals or splines. Lengths entered in the lower sidebar tables and shown in the plot graphs are in inches.
length1.jpg 19" straight limb
Select profile > line in the left sidebar. The entry in the lower section of the sidebar has been set to a new length of 19"
length2.jpg 12" spiral section added to the 19" straight section.
To add a spiral segment, go to the upper left corner next to "comments" and click on the left-most of four boxes and select"spiral" in the dropdown. Under the highlighted "profile" in the sidebar, a spiral entry will be added and a table shows in the lower sidebar.
The first row defines the spiral segment as 12" long.
The second row defines a longer radius as the spiral slowly begins it's bend.
The third row defines a much shorter radius that ends the spiral.
Arvin wants a 1.5" flipped tip, so playing with the values of the two radii creates what I think his caul produces. Left clicking on the plot area will bring up a context menu that lets one select a photo to overlay the plot on when building out the segments.
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Basic program outputs.
If you have gotten this far, its time to look at some basic outputs Virtualbow can generate. Clicking on the left hand green arrow at the top will generate various tables and graphs for your design.
Virtualbow uses the stiffness of the materiel entered earlier to calculate the draweight of your design. If you can brace and pull a 2x4 stud to 36", you are a better man than I, and Virtualbow will output some fantastic draweight. However, the program does not attempt to inform you if your design will actually work out in pratice.
All materiels have a stress limit. In the case of a self bow we generally want to avoid stress above the level that causes set, and will need to obtain that info some other way.
f1width(2).jpg shows some basic characteristics of our design as numerical values. Of note is 51+# of pull @ our design draw of 25" and occuring at a maximum stress of 16,407 pounds per square inch. or roughly 16.5K psi.
f1width(3).jpg selecting the shape tab displays the bend shape at brace height. Not the slider along the bottom that allows the output to display limb shape at any drawlength up to our stated full draw.
f1width(4).jpg the same design shown at full draw
Selecting the stress tab brings up a plot showing the stresses along the limb from the handle at the left to the tip at the right.
Moving the slider to full draw will let you see where along the limb the maximum stresses occur and the amount of stress which can be read on the vertical scale at the left.
f1width(8).jpg shows the distribution and maximum stress at full draw. not sure how that smily crept into the file name
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Arvin has some pretty good ideas about limb width profiles. :)
Different design limbs have their stress distrubuted in different parts of the limb.
Virtualbow allows us to try different thicknesses, profiles and widths to allow us to anyalyze where different conditions occurs.
Does the width or thickness profile matter when shooting flight? What should we be looking for, presuming we can adjust thicknesses approiately?
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If I understand you correctly there is no width requirement in flight rules.
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This is going to be interesting. I see all this information and understand some. I will look at the pictures real close! The challenge for the designers is not only the design but getting the builder to understand it. 🤠🤠🤠request for information ( RFI) will be present I’m sure.🤠🤠
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If I understand you correctly there is no width requirement in flight rules.
No width requirements that I know of, but I have not looked at the rules lately. Your stave might limit the width tho, and dictate some design decisions.
Most people have a back profile in mind and cut the width to a pyramid, semi-pyramid or more of a straight limb when they set out to build the bow and then tiller to bend profile by removing thickness.
Edit: setting widths has been added to the first post..
With this program we can define a back shape and adjust the thicknesses to make it work, or vice versa, define a thickness taper and adjust a back proifile to suit. I can post examples of any other design offered.
With Virtualbow, once a profile, either a back shape or a thickness taper is tried, the other can be adjusted and the resulting stress plots drawieghts and bend profiles can be generated by the program.
So long as the stresses shown in the stress plots do not exceed the limits of the materiels, any configuration of limb can be built without set.
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Something i didnt see mentioned is are the tips stiff or working? If stiff, how long is the stiff portion?
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Something i didnt see mentioned is are the tips stiff or working? If stiff, how long is the stiff portion?
Arvin said he wanted the last 3-4 inches to not bend. I dont see much bending in the screenshots just now added to reply2
should they be stiffer?
To help see the tip stiffness, I modified the design to let the progran show a straight tip. They are kept at the same stiffness. The bow lost 3.5 lbs when the tips were straightened, so the inner limb was thickened slightly to bring the bow back up to the same draweight .
The maximum stress without the flipped tips is, 8% lower than with. The 8% reduction will also allow us to draw the bow 2 inches more if one wants to bring the bow up to the original stress level. Straightening the tips makes a noticable change in the force draw curve. It shows as a lower "hump" in the early draw portion, and a slightly noticable difference all the way out to 25"
f1width(9).jpg shows the force draw curve with 1.5" recurved tips.
f1width(14).jpg shows the force draw curve for the modified limb without the recurved tips.
f1width(12).jpg is the bend shape of the straightened limb,
Note: I have edited this post and some posts above for completeness
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How wide will it be out the fade, and at the last part of the bending limb before it gets stiff?
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sleek, I have added a screenshot and instructions for widths in the first post. The yellow bow design is used to generate the stress curves and bend shape posted in reply 2,
one can have the backshape either more pryamid or parrellel requiring less width, and by changing thicknesses, still be able to control where the limb will bend and consequently, be stressed the most.
bending limb before it gets stiff
the transition from bending to stiff is gradual with most all bows, unless one sets out to deliberatly design a lever bow.
A bigger question is how should the best part of the limb work? should one try to keep most of the bend toward the inners? towards the mids? keep the inners and the mids working as much as possible? etc.
the screenshots shows the stress plot of a limb that works
mostly in the inner
inner and mid equally
mostly mid
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I ran my formula with the numbers you gave me and the dimensions are exactly what they need to be for the poundage.
I have opinions on the profile of a bow, but being uncertain of their accuracy, ill only say what I firmly believe to be true. The more you stress the inner limb, the more torque you get, the more you bend the outer limb, the more horsepower you get. The limbs profile AND length needs to be built according to those goals. Longer gives more torque shorter more hp. Which, is a large reason my shirt bows have a hard time keeping up with Arvins long bows in shooting broadhead, but also why his long bows struggle against my short in flight.
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I ran my formula with the numbers you gave me and the dimensions are exactly what they need to be for the poundage.
I have opinions on the profile of a bow, but being uncertain of their accuracy, ill only say what I firmly believe to be true. The more you stress the inner limb, the more torque you get, the more you bend the outer limb, the more horsepower you get. The limbs profile AND length needs to be built according to those goals. Longer gives more torque shorter more hp. Which, is a large reason my shirt bows have a hard time keeping up with Arvins long bows in shooting broadhead, but also why his long bows struggle against my short in flight.
If agree Kevin.
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I ran my formula with the numbers you gave me and the dimensions are exactly what they need to be for the poundage.
I have opinions on the profile of a bow, but being uncertain of their accuracy, ill only say what I firmly believe to be true. The more you stress the inner limb, the more torque you get, the more you bend the outer limb, the more horsepower you get. The limbs profile AND length needs to be built according to those goals. Longer gives more torque shorter more hp. Which, is a large reason my shirt bows have a hard time keeping up with Arvins long bows in shooting broadhead, but also why his long bows struggle against my short in flight.
I agree about an agressive side profile stressing the limbs more, especially recurves that are getting the tips out ahead of the handle.
From playing with Virtualbow with various reflex and deflex arrangements, I am seeing more energy being stored in the inner halfs of the limbs and not as much as I had previously thought being added by bending the outers more. That said I have just been playing with 62" bows and havent really spent a lot of time with longers designs - yet.
below are two graphs available in virtualbow that show the amount of curvature in the limb:
the first at brace and the second at full draw. you can move the slider and watch the curvature change as you "draw" the bow.
Looking at the stress graphs sort of tells you the same thing, Maybe the dedicated curvature graphs with its exaggerated scale helps some visualize better. I can see the usefullness for someone who mainly builds glass laminates and tillers by tapering lams before glueup and doesnt often do a lot of sanding and trying to change the bend by eye after the fact
Using the shape view to see the changes in curvature after making a tweak to your model is not that easy, (for me anyways), especially if there is a lot of reflex or deflex
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🤠🤠🤠just a guess but less mass in thickness he fades from ramps to mid limb. Thin the tapper
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this quote from Badger is from another thread, but I have found his observations about wider limbs
interesting
I did some tests a few years ago on exceptionally wide osage bows. The results kind of disagreed with my own mass theory. I built several osage flat bows with stiff handles at about 2" wide to 2 1/4 inches wide. To my surprise the mass came in slightly lower than the 1 1/2" bows. I use to build all my 50# osage bows at about 1 1/8 wide to about 1 1/4 wide. I found I got less set when going wider and I also was adding mass but they were still better performers because of less set. When I bumped them up even wider I fully expected to see a substantial increase in mass but it didn't happen. The bows came out much thinner than I would have expected also. The only way this is possible is if at 1 1/2" wide I was still doing more damage to the wood than I thought I was.
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I can't claim to have done all the work that Badger has (I don't know if anyone can), but his claim that set is exceedingly damaging to performance does seem to make sense to me. That implies avoiding set is more important than extracting the last 1% out of the wood.
Based on all the modelling I have done, a wide pyramid profile bow that uses levers at the tips will offer the best performance from the wood along with being the most forgiving in terms of extra weight not hurting performance as much as other designs. It strains all the wood equally, keeps the moving mass as low as possible and if you overbuild it by a few % the extra mass is mostly near the grip and doesn't hurt performance as much. Reflex may be worthwhile, because it increases early draw weight and that is good for performance as well, but you would need to look at the differences between reflexing the whole limb versus recurved tips.
Mark
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Ok let’s talk shock! Shock kills speed in my opinion. Please chine in here if you’ve built plenty bows with reflex thru out the length of the bow vs putting the be reflex in the last 10-12”. I feel more shock in the reflex thru out the length of the bow. It’s interesting what you observed in your computer designs Mark. Kevin and I feel there is a whip like action in the bow limbs come forward. With the reflex in the last 10-12” may effect how the whip action takes place . Just my brain thinking.🤠🤠🤠
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a wide pyramid profile bow that uses levers at the tips
I hate quoting myself, but I realized this may be ambiguous. The pyramid profile I am talking about is not a straight taper from fades to the nock, the sides of the bow are convex and curved outwards a bit from straight. The levers happen because the ideal pyramid profile tapers to a sharp point at the nocks, but this is physically impossible because we need material for the nocks. So the profile shape is drawn to a sharp point at the nock, then a width for the nock is chosen and that width carries in towards the grip until it crosses the profile shape. If that is unclear I can make a sketch to illustrate better.
I feel more shock in the reflex thru out the length of the bow.
That's an interesting observation, Arvin. If you look at fibreglass recurves the limb bend tends to have the nocks move more vertically than horizontally. This seems to reduce shock and vibration. Some of the FG bowyers think this is because the limb mass is moving vertically, so the forces tend to cancel out between the top and bottom limb and it helps stop the limbs more quickly.
I have no idea if their theory is correct or not, but when you only reflex the tips it does the same thing to the limb bend and tip path as the FG recurves, just to a lesser extent.
Mark
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Is this why Howard Hill bows are shocky. Some had a slight reflex thru out the bow. The smoothest shooting glass longbows i e shot had some deflex in the handle fade section and slightly recurved . That design holds the fight records also. I think it’s hard to compare wood to fiberglass in design though . This pyramid design in wood is more compatible than glass . I did this design in a glass bow and it was full of shock. Don’t ask me why. I don’t know.
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this quote from Badger is from another thread, but I have found his observations about wider limbs
interesting
I did some tests a few years ago on exceptionally wide osage bows. The results kind of disagreed with my own mass theory. I built several osage flat bows with stiff handles at about 2" wide to 2 1/4 inches wide. To my surprise the mass came in slightly lower than the 1 1/2" bows. I use to build all my 50# osage bows at about 1 1/8 wide to about 1 1/4 wide. I found I got less set when going wider and I also was adding mass but they were still better performers because of less set. When I bumped them up even wider I fully expected to see a substantial increase in mass but it didn't happen. The bows came out much thinner than I would have expected also. The only way this is possible is if at 1 1/2" wide I was still doing more damage to the wood than I thought I was.
Mass itself has very little to do with a bow in my opinion. Of course where mass is located is important, but mass is a function of, not a design feature of a bow.
Mass is only a biproduct of the bows width and thickness. Its width determines the stress it can take and its thickness determines how much it can bend before taking set. The combination of those two individual design parameters are what gives a bow its mass. Applied incorrectly, you can get a bow the correct mass bit not the correct draw weight the mass is supposed to give. The entire idea of chasing mass is not going to get anyone anywhere.
The stress of compression is spread out iver surface area, the stress of tension likewise. You need the correct surface area for the draw weight of the bow, and the correct thickness for the bend radius. The stresses are not distributed across the mass.
So the question is, how wide and long does a bow need to be to not take set. The formula I have been working on is an attempt to solve that issue.Im about 80% certain i have it figured out, but without more bows of many different wood species built, i wont be 100%. Its a simple formula The specific gravity is used along with the draw weight and from there you get how many sq inches of working limb you need. Soon as I know i am not putting out bad info, ill make a phone app that lets you put in your desired bow stats and it will tell you what you need to build.
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How long should a bow be 1” wide for half limb the limb then a even taper from there. Just curious Kevin .
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Ill run that when I get off work. What poundage do you want?
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Fifty pound and my normal reflex in the end. 8”!stiff handle
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This pyramid design in wood is more compatible than glass . I did this design in a glass bow and it was full of shock. Don’t ask me why. I don’t know.
you have a build of your osage 67" pyramid design in glass?
I think it would be quite informative to see a pic of that glass bow at full draw on the tiller tree.
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It had so much shock I gave it away!🤠🤠🤠
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It had so much shock I gave it away!🤠🤠🤠
lol, I had a shakespeare 66" that left it's home on the bow rack the same way.
The smoothest shooting glass longbows i e shot had some deflex in the handle fade section and slightly recurved . That design holds the fight records also. I think it’s hard to compare wood to fiberglass in design though .
maybe a direct comparison could be hard. but the underlying reasons should be similar. too much flexibility in the outers where the limb return speed is higher and thus has more momentum when the string stops the limb?
can you comment on the deflexed recurved you recently built? is it damper in the hand or more so that your straight backed 67/68 inchers? I assume you have had a chance to shoot some light arrows from it recently.
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Fifty pound and my normal reflex in the end. 8”!stiff handle
Assuming no stiff tips, the entire bow bends, You taper to .25 nocks.
The bow limbs will be parallel, one inch wide for 25.875 inches
Then you start the taper from one inch wide to .25 for 13.79"
Thats 39.665 inches long limb, double that and add your 8 inch handle you get a long bow of 87.33 inches total.
I imagine that bow would throw a heavy arrow very well and be well suited for a very long draw, which would allow for higher than typical speeds. How fast is a pure guess based on what the end draw length would be.
Now I wonder what virtual bow says about your hypothetical bow Arvin? Im curios to check my work against that. It would be fun to have a bow build off competition between my formula and the virtual bows design.
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Kevin there was some mention of extremely long bows in bowyers bible 1 I think . I might go back and review that section. Kevin Ive got BJ fixing to build some narrow flight bows . If it can be done with no set at 25” by him it might Poke a hole in your theory. That we will have to watch. Brian can tiller a bow.
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Kevin there was some mention of extremely long bows in bowyers bible 1 I think . I might go back and review that section. Kevin Ive got BJ fixing to build some narrow flight bows . If it can be done with no set at 25” by him it might Poke a hole in your theory. That we will have to watch. Brian can tiller a bow.
Im often excited to be shown im wrong. It gives me an opportunity to learn more and more quickly.
Which theory are you refering too? Im full of them lol.
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Sq inches in the back of the working limbs for no set. You said you wanted more hands on proof . Just trying to help you out.🤠🤠🤠
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Oh yeah, Id like to see that tested and tried to be proven wrong. Got to have honest trial by fire. If Im right it will stand to scrutiny, if nkt, i need to know.
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Sq inches in the back of the working limbs for no set. You said you wanted more hands on proof . Just trying to help you out.🤠🤠🤠
Whats the dimensions of the bow he is building. Ill try and calculate how much set i think he will take.
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Now I wonder what virtual bow says about your hypothetical bow Arvin? Im curios to check my work against that. It would be fun to have a bow build off competition between my formula and the virtual bows design.
Virtualbow doesnt "say" much, it's more of a visualizer for your inputs, but I would be happy to plug in your design if thats what you are asking. can you supply a thickness description?
1. assuming a straight taper, I would need the thickness at the point where the handle dip ends and the full bend begins along with the distance of that point from the center of the bow. And the thickness at the nock.
2. if the taper is not straight, then additional info will help. Its easy to change the thicknesses once the end of the dips are located to see different results.
should I assume the same recurve at the tips as in Arvins model above?
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I have no thickness description unfortunately. The thickness of the bow is determined by how stiff the wood is, and that I cant calculate. So, i leave it to the bowyer to determine as they tiller it, and as there is no draw length given by Arvin, one could only guess. That said, the draw length and thickness are not important to my equation, because they arent a factor in set and how much surface area a bow requires to be a certain draw weight. You can assume a 28 inch draw if you like to fill the requirements of virtual bow program.
The design i ran for Arvin is based on working surface area alone. the stiff tips, the recurve etc are not relevant, as that is again, a factor of tye bend radius, determined by the bows thickness.
If there is a way you can plug in generic numbers or just leave them as variables to be determined by the numbers I gave above, it would be nice to see.
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Now I wonder what virtual bow says about your hypothetical bow Arvin? Im curios to check my work against that. It would be fun to have a bow build off competition between my formula and the virtual bows design.
Virtualbow doesnt "say" much, it's more of a visualizer for your inputs, but I would be happy to plug in your design if thats what you are asking. can you supply a thickness description?
1. assuming a straight taper, I would need the thickness at the point where the handle dip ends and the full bend begins along with the distance of that point from the center of the bow. And the thickness at the nock.
2. if the taper is not straight, then additional info will help. Its easy to change the thicknesses once the end of the dips are located to see different results.
should I assume the same recurve at the tips as in Arvins model above?
Basically ai leave the thickness to be determined by the final bend radius the bow has, as you tiller it toward your draw length, never drawing past your desired draw weight. So, thats a fancy way of saying... idk?
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So, thats a fancy way of saying... idk?
yeah, Thats the traditional way of tillering. If you wanted the program to help you could measure a bow you have, or since you eyeball your bend while you tiller, would comparing two bends in a screenshot of the program help?
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Its my opinion that if this hypothetical skinny long bow were to be built, it would be tillered so that the strain is even across the whole bow. If the program can tell you what tiller would look like with even stress, choose that tiller, and the draw length, then the thickness should sort itself out?
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Ok 🤠🤠🤠it would look like a yummy bow. Never mind.
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Im sure it would perform better. And the tiller would not resemble one. Just the length and width would be reminiscent of a yumi.
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87.3 ntn 1" wide
I estimated some values for a thickness taper that produces equal stress for as far out along the limb as I could. you cant take equal stress all the way out to the tip. The resulting bend profile or shape plot is as expected for equal stress. Stress and strain go hand in hand.
Virtualbow is just a visualizer. The program does not calculate any limb dimensions for you.
It does predict stress and bend radiuses for whatever you input as widths and thicknesses, and will calculate the draweight once you supply a MOE for stiffness. and
It is up to the user to watch that the stresses do go too high and cause unwanted set, and the user will need to make allowances for any limb crossections other than rectangular.
woodbears spreadsheet works differently as you need to plug in estimates of the materiel stiffness and set a factor for limb crossection (round to retangular), a strain factor to hopefully prevent set, and an elasicity factor if you are working with a highly or lowly elastic wood, and some values for relative stress should you want stiff tips or a stiffer handle rather than a circular bendy handled bow.
Next you try various thickness tapers, and the spreadsheet will calculate the approiate widths to suit your givens. If you plug an impratical design into the spreadsheet it will simply show you that you are asking too much of the wood by outputting 6"wide limbs sections or some such.
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I have been trying to read all the information and it is very interesting. I haven’t really had enough time to do it justice but wanted you to know I am watching. (-P
Really neat project. I have to admit it’s probably a little over my head, but I understand the basic mechanics.
Like Arvin said the pictures are nice to study too. I like the idea of the project. I will be watching
Bjrogg
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Well I locked myself off PA thank you Marie!!! She helps not so smart guys like me get back in!!! I have made progress . This is a shorter version of my longbow design. Here is where I’m at.
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Glad to see you made it back. :) I can't wait to see how this one turns out. I've always liked 62-64" longbows. :OK
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Well after typing for 20 min I lost everything . 🤠🤠🤠I’ll try again.ok This is shorter version of my longbow design. 62” at this point. The stave had a pretty high crown in it. Here is width dimensions every four inches from center of bow. Pic
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Arvin, since I own and shoot your bows...when you say it is a shorter version, you don't mean that you just bent it to your old form and then cut the ends off... You made a new form that allowed for the same deflex/reflex design, but in a shorter version?
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I used my fancy new contour gauge that Willie gifted me. Now if I can learn how to use it. I pushed it down at every 4” and traced the contour on to my 1” graph paper. The edges on the end of the crown was the width every 4”. You can see the crown getting smaller as I went out the limb. I don’t know how that ends up in the computer design. Engineer problem.🤠I just have to build it!! I’ve got the bow straight and the edges real close to my thinking. This is better than building 50 bows to find out where the high stress areas are.SET that ugly happening.
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No bob I used the same form but I might add more reflex if the computer says I can get away with it.
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I'm not one of the experts like yourself...but it would seem to me that "the tips have to be in front of the riser". how old school is that? :)
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They are Bob .
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It looks like a hunting bow already. :OK
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Bob it’s for flight. It will have a 25-26” draw from back of the handle and shoot a 23 inch flight arrow. The reason I shorten the design was I wanted to shoot a shorter arrow with less drag. That’s why I shortened the bow. Willie wanted to play with virtual bow and I’ve got Allen and Kevin adding there two cents in. It’s a fun project. Allen leans toward spreading the load more by deflexing the handle but hard head Arvin thinks otherwise.🤠🤠so I’m thinking he’s setting back watching hard headed Arvin.🤠🤠I’m sure we will be amazed if it turns out. Laugh at Arvin if it does not. That’s all good!🤠🤠🤠
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I used my fancy new contour gauge that Willie gifted me. Now if I can learn how to use it. I pushed it down at every 4” and traced the contour on to my 1” graph paper. The edges on the end of the crown was the width every 4”. You can see the crown getting smaller as I went out the limb. I don’t know how that ends up in the computer design. Engineer problem.🤠I just have to build it!! I’ve got the bow straight and the edges real close to my thinking. This is better than building 50 bows to find out where the high stress areas are.SET that ugly happening.
this crown is gotta be accounted for some how and I have got an idea, although it might take a couple of days and be after the new year.
Could you find something like a paint can or a kitchen pan or whatever to set the gague with to a smooth radius and then trying various smooth radiuses to fit the countour gage to the crown. Try your gague on the stave in as many places as possible please.
Am I seeing in the pic of the graph paper, that the crown is a little flatter on one side of the stave than the other?
I got the 62" design pretty much plotted out in Virtualbow, and only have to make adjustments to the thicknesses depending on what we figure out to do about the crown.
will post some pics in a while.
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Bob it’s for flight. It will have a 25-26” draw from back of the handle and shoot a 23 inch flight arrow. The reason I shorten the design was I wanted to shoot a shorter arrow with less drag. That’s why I shortened the bow. Willie wanted to play with virtual bow and I’ve got Allen and Kevin adding there two cents in. It’s a fun project. Allen leans toward spreading the load more by deflexing the handle but hard head Arvin thinks otherwise.🤠🤠so I’m thinking he’s setting back watching hard headed Arvin.🤠🤠I’m sure we will be amazed if it turns out. Laugh at Arvin if it does not. That’s all good!🤠🤠🤠
I have been following along on this new Flight bow of yours... I do like the profile with wide limbs and small tips...all reflexed. I have always thought that a bow shaped like this would be a top notch hunting bow, and I have used it on hickory bows in the past. If yours works out, I will have to try it with osage as well. Thanks for doing the work for us!
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Here is a end drop off the stave.
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this crown is gotta be accounted for some how and I have got an idea, although it might take a couple of days and be after the new year.
Here is a end drop off the stave.
Willie, that is easily dealt with in autocad using the section properties calculator.
Arvin, assuming you are using the outer surface as the back I need to know how wide it is at the outer ring and how much crown there is. Measure crown by putting a straight edge across the outside edges of that outer ring and then measure to the highest part of the curve.
Mark
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Willie, that is easily dealt with in autocad using the section properties calculator.
Arvin, assuming you are using the outer surface as the back I need to know how wide it is at the outer ring and how much crown there is. Measure crown by putting a straight edge across the outside edges of that outer ring and then measure to the highest part of the curve.
Mark
Hi Mark,
if you dont mind trying the calculator in autocad that would be nice.
The limb crossection at the fade the time looks like a segment of a circle who's radius is 3.5" on top of a rectangle, both elements being 2.6" wide. (my sketch here shows the heigth of the segment to be about .28" and the "equivelent' rectangular section from Virtualbow is 2.6" wide by .42" thick, but that thickness is still tentative and may not work further down the limb as there is a gentle thickness taper in spite of the bow being a pryamid back, thus It would be interesting to see if that composite section can be compressed in the vertical direction without the radius of gyration changing.
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Arvin, heres what we got so far. Comments from others are welcome
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more...
hmm the projected arrow velocity with 190gr arrow seems low. Should we look to tweak it up, Arvin?
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Arvin, heres what we got so far. Comments from others are welcome
so you went from carrying the 2.6" to 35% of the limb length to a Pyramid? Is it more efficient?
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Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?
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How fast is it with a 450gr arrow? Also with a 150 gr?
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hmm the projected arrow velocity with 190gr arrow seems low. Should we look to tweak it up, Arvin?
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Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?
A bow can store all the energy it wants, statically. Its dynamic efficiency is what puts speed to the arrow. It seems that the dynamic efficiency and static efficiency are on opposite sides of a sea saw. Its hard to make them play well with each other.
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Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?
you must be looking at the 109 energy storage factor on the charateristics page?
According to the manual
• Energy storage factor: This value indicates how good the shape of the draw curve is in
terms of energy storage. It is defined as the energy stored by the bow in relation to the
energy that would have been stored with a linear draw curve. The energy storage
factor increases the "more convex" the shape of the draw curve is.
• Factor < 1: The draw curve stores less energy than a linear draw curve
• Factor = 1: The draw curve stores as much energy as a linear draw curve
• Factor > 1: The draw curve stores more energy than a linear draw curve
a way of quantifying the "hump" in the force draw curve I think
so you went from carrying the 2.6" to 35% of the limb length to a Pyramid? Is it more efficient?
I am not sure why the original plan included the straight section. Maybe as a precaution to handle the extra stress trpically induced by a recurved tip?
I asked arvin if we should compare two designs with and without the straight section. The elimination of the straight section seemed to reduce stress a small amount if the ramp down in thickness just past the fade was made a bit less abrupt, and it definately makes for a less complicated limb taper for the builder.
about the arrow speed:
Stressing the limb more will make the arrow go faster, all other things being equal, stressing the limb less will avoid the possibility of set or unseen damage from compression. Determining a working stress for the materiel thats not too high or low seems to be the trick. but determining what that ideal stress level is, comes from experience and the quality of the wood.
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How fast is it with a 450gr arrow? Also with a 150 gr?
150 gr 237 fps
450 gr 178 fps
the 50# draweight is at 25" from the back of the bow and with a brace height of 7"
when the bow is built we plan to offer feedback to Stefan, the developer of the program. Hopefully he will comment about other observations of the accuracy of dynamic functions.
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How fast is it with a 450gr arrow? Also with a 150 gr?
150 gr 237 fps
450 gr 178 fps
the 50# draweight is at 25" from the back of the bow and with a brace height of 7"
when the bow is built we plan to offer feedback to Stefan, the developer of the program. Hopefully he will feed back with some other observations concerning the accuracy of some of the dynamic functions.
Make it a 26 inch draw from the back of the bow if you can. 450 grains us a benchmark I use, and if it doesnt hit 185fps minimum i wont shoot flight with it. But I also pull 26. So I need 26 to see a good comparison.
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Hi Mark,
if you dont mind trying the calculator in autocad that would be nice.
The limb crossection at the fade the time looks like a segment of a circle who's radius is 3.5" on top of a rectangle, both elements being 2.6" wide. (my sketch here shows the heigth of the segment to be about .28" and the "equivelent' rectangular section from Virtualbow is 2.6" wide by .42" thick, but that thickness is still tentative and may not work further down the limb as there is a gentle thickness taper in spite of the bow being a pryamid back, thus It would be interesting to see if that composite section can be compressed in the vertical direction without the radius of gyration changing.
I will try to get a look at this in the next couple days and let you know.
Mark
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What spine do we need on a 24” 150gr arrow for this bow. Flight arrow. Also if we where to shorten the working part of the Limb to 24” from center of bow. I think I use a 5”-6” brace or 6-1/2 -7” from back of bow.where did you measure brace from? I think in this scenario measure to back of the bow will be best.
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the program inputs brace from the back at the handle to the string, So with a 2" thick handle, 5" "inside to inside"
I will try to anwser the part of the question that I think you were asking me, maybe the arrow part was for sleek?
Also if we where to shorten the working part of the Limb to 24” from center of bow.
Shortening the working part of the limb means some other part of the limb has to work harder if you want to keep the the draweight at 50#. Kinda like playing whack-a-mole if you are concerned about stresses getting too high someplace else.
right now, the limb is bending significantly 5" out from center and is fully working 6" out from center.
the limb starts getting thicker 22 inches out from center but is still working.
the first screenshot below shows the stress at brace, and the red line is drawn at the maximum stress (just under 10,000 PSI as read on the scale on the left hand side.
the same line is shown in the second screenshot of the limb at full draw. The maximum stress has risen to 16,000 PSI and the area above the red line represents some of the area where the limb is doing the work that will be returned to the arrow upon release. If I could over lay one pic on the other you would see a better representation.
The green line shows how much the stress has risen out at 24 inches from the center, but the limb is only an inch wide there, so the work contributed at 24 inches out will be just over a third of the work done at the fade which is 2.6 times wider.
Normally, when one tillers a bow and starts to see set or loose early draw weight, one just reduces the weight goal and calls it another 35 pounder for the ladies.
what do you think about if you can come in at 50# without set, or said another way, the bow is basically overbuilt.....
putting some more hook into the recurves and taking a few scrapes to get back don to 50#, untill the arrow speed comes up?
I dunno how many times you can rebend the tips but you are our master osage bender......
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Willie I have not reached the full recurve design and yes I can heat it up a few more times to get there. I’m waiting to see the final design thickness to proceed in that recurve section. Right now we have 56 in thickness 26” from center of bow . that is the thinnest part of the limb.
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that thickness should be enough, maybe too much ,but I would not do any thickness reductions anytime soon.
the program has no way to account for any loss or addition of stiffness if we heat bend the wood or heat treat to set a bend. being too thick out there in the non bending section is not going to change the limb design elsewhere on the limb appreciably, so leaving it "as is" will be good.
maybe you typically set the bend with a scorching heat treat with most bows?, but maybe not doing that right away would be a good idea if you want to increase the bend later in the build.
taking any excess mass off the tip to reach a final highest possible arrow speed should be the last thing done, after (or before?) the hotter? heat treat to set the final bend.
just to get ahead of ourselves, I will see what a 3" hook does to the design as it stands >:D
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to anwser Arvins earlier question about what happens if we make the outers work less and become more like levers, I adjusted the model. and might have to change my earlier opinion.
Directly below is the stress plot shown earlier with more bendy outers, and below it is the stress plot for the same design with stiffer outers. the stress increased 1/2 of 1% but the arrow speed went up from 222 to 236. quite an improvement for a modest increase in stress.
The difference is not easy to see at a glance, but a careful inspection arount the 24" mark will show less stress (and bending) and a straighter slope to the nock.
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bending the recurve to 3" behind the handle from the 1.5" in the previous improved design raised the stress and increased the poundage from 50.9 to 55.9. But Arvin needs to stay at or below 50# to stay in the class.
Taking a few scrapes to bring it down to 50.1# and yields a bow with 10% more stress than the 1.5" recurve and the arrowspeed increases from 236 to ummmm... 229.
there could be any number of explanations. off the top of my head is
I am not doing something consistently with my models
I need to input a more realistic wood density
something with the new string angle is not helping
the tips are too thick and hence too heavy.
below is the stress plot for the 3" recurve and the arrow velocity during the powerstroke for both recurves.
the 190 gr arrow shot with the smaller recurve seems to leave the string sooner and comes up to speed quicker.
note how at the point of the highest velocity the curve seems to go flatter. this would be consistent with the observation that its hard to eek the last few fps out of a light arrow, and there are considerations that the program does not account for in addition to hoping the program for arrow velocity dynamics is accurate at the edge of the envelope. interesting to play with tho.
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Willie try a 150 gr arrow though the new design please.Willie I’m curious about if the design has a specific need for a certain weight of arrow to have the most speed from the bow. It would be a pain probably to do but try 140-200 gr arrows through the design . Any design. Do it in 10 gr increments to see at what weight the arrow needs to be the most efficient. Please explain to me in simple terms if this would or would not be useful.
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I’ve been using my trusty contour gauge and I have all 4” increments the same on both ends of the bow. Well the one right at the fades have a slight difference. All the rest match. So if I can get the thickness from each 4” increment right we should be good to go.
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Any design. Do it in 10 gr increments
Please explain to me in simple terms if this would or would not be useful.
I will plug in some numbers and see what happens, it's easy enough to do. It might be useful to see if the program makes sense to experienced shooters.
Expecting the dynamic assumptions of the program to work out in real life might be reasonable for a typically stressed bow with a typical arrow at 9 or 10 GPP.
At 3.5 GPP with a highly stresses bow, the program may or may not be as capable.
There are two different parts of Virtualbow outputs, dynamic (moving) and static (stationary).
Dynamic looks at limb return speeds and consequent arrow speeds
Static predicts draweight and bend shapes.
As for the static bending part of the program, I have a little more confidence, as the maths behind bending beams is more established. So long as the bow acts like a beam, I believe doing something like making the bow longer or shorter, thicker or wider etc. should be relatively consistent.
"so long as the bow acts like a beam" might be pratical at lower stresses, but we are looking to push the stress in the bow until just the point where some damage to the materiel is justified so long as it yields better performance, but just a bit more stress causes diminishing performance. This is more of a grey area that could cause difficulties for the dynamic predictions.
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I have all 4” increments the same on both ends of the bow.
you have been re-shaping the crown to the same "pattern"? if so can you reconfirm the "best fit" of the pattern to the diameter of something?
the number crunchers might be a while longer to work out a set of thicknesses.
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"so long as the bow acts like a beam" might be pratical at lower stresses, but we are looking to push the stress in the bow until just the point where some damage to the materiel is justified so long as it yields better performance, but just a bit more stress causes diminishing performance. This is more of a grey area that could cause difficulties for the dynamic predictions.
willie,
The Euler beam equation holds even at bow limb amounts of deflection as long as shear deflection in the material is small, which it is with the woods we use. I assume all of the various bow design software uses the Euler equation, because the more general Linear Elasticity equation is much harder and more cumbersome to solve and doesn't seem to be necessary.
I can't say how VB does its dynamic analysis, so I don't know if running on the ragged edge of failure affects it or not.
Mark
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Mark, I presume flight shooters like highly stressed bows in order to get right at 50# pull with as little mass in the limbs as possible. From what I understand the better performing bows dont last very long before arrowspeeds with the light arrows drop off, so some degradation of the materiel is acceptable. maybe not enough to be seen as set or permanent deformation tho. Maybe a flight shooter could weigh in on what retired bows look like
A non flight shooter probally doesnt get too concerned if a bow begins to loose draw weight at a given draw length, but if I understand correctly, what some flight shooters do is shoot longer arrows at increasing drawlengths in order to keep at the max 50# pull allowed in class.
This seems to require a design to operate in a narrow range of stress, and with the variabliity of bow woods, this stress level seems hard to predict with conventional materiel testing beforehand.
From what I hear, when a flight bow starts to take set it is past it's prime, so that rules out whatching for set as you tiller a viable method for tillering flight bows.
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arvin, I tested a dozen different 50 # designs for arrow speed.
when scaling down your 28"draw record bow to 25"draw significant speed was lost
increasing reflex and straining the bow harder made for more speed, but unless the basic design changes radically, you are bound to start getting set before gaining much arrow speed back
here are the force draw numbers and plot for the most recent 62" design basically as the previous threads show should you decide to keep the design as modeled above
in lb
7.0 0.0
8.0 4.6
9.0 9.0
10.0 12.2
11.0 15.1
12.1 18.1
13.0 20.6
14.0 22.9
15.0 25.2
16.1 27.6
17.0 29.9
18.0 32.1
19.0 34.3
20.0 36.9
21.0 39.3
22.0 41.8
23.1 44.7
24.0 47.3
25.0 50.1
here is the force draw curve plot for a visual
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one way to account for the crown in the stave is to compare similar succesful bows as modeled in the program.
As the crown becomes less of the thickness of the limb, as one progresses out the limb,
a slight taper in thickness reduction is called for. about .01 for every 2 inches out to 24" from center.
a similar increase in thickness from 24" out to the tip would accomplish the stiffnees called for in the outer limbs.
for example, for a limb that would most likely come in heavier than 50#, but might be a thickness profile to work from as one reduces according to the draw curve posted above...
0 2.00
2 2.00
2.5 1.62
4 0.68
5 0.58
6 0.57
8 0.56
10 0.54
12 0.53
14 0.52
16 0.51
18 0.50
20 0.49
22 0.48
24 0.47
26 0.48
28 0.50
30 0.52
31 0.53
once the profile is established as like the numbers above, one would want to remove wood evenyl throughout the full length of the limb in order to keep the relative thickness profile the same.