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    How to know you went too heavy

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    Post by actionbow on Mon Jun 08, 2015 7:45 am

    Laminate recurve. 31" nock to nock. I left it at around 230lb @ 14". It was shooting like a beast, crazy hard. I was just getting the red dot sighted in. About 60 shots in blam!

    https://docs.google.com/file/d/0B4Dp009Fgn7KaGRwVE4zaEVjMjQ/edit?usp=docslist_api
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    Post by kenh on Mon Jun 08, 2015 8:09 am

    Too bad -- nice looking prod.   Looks like the fades were too steep.  It broke right where the fade ended.  I don't think I'd have made the "riser" so thick...
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    Post by actionbow on Mon Jun 08, 2015 9:22 am

    You are half right. Mostly I left too much at the tips. I made the fade work too hard. Sometimes it works out...other times...
    problem with a laminated bow is you can't change your mind after you glue it up. I got greedy. I was shooting at around 340 fps, it's kind of addictive.

    This is why I wear safety glasses religiously.
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    Post by c sitas on Mon Jun 08, 2015 11:49 am

    You know speed is nice. What you were shooting is about an average for most compound crossbows. Possible you weren't taking enough energy out of the bow on the shot. Maybe the bolt is to light. That could act almost like a dry fire. The results would be the same. I've both seen and heard this happen. It is quite unnerving,eh.
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    Post by actionbow on Mon Jun 08, 2015 11:56 am

    If I can get "average" compound bow speeds from a home built recurve I will be more than satisfied.

    shooting 440 grain bolts. I just needed to lighten the mid-limb/tip area more.
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    Post by c sitas on Mon Jun 08, 2015 1:39 pm

    How many pounds are you pulling?
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    Post by actionbow on Mon Jun 08, 2015 2:16 pm

    230# @14"
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    Post by Alan Case on Mon Jun 08, 2015 2:37 pm

    Did the limb fail as the bow was drawn, or did it fail on the shot?

    It was a nice looking effort. It looks like the glass may have failed in compression on the belly side.  What is the thickness stack used along the working limb?  Does the limb continue to gradually taper from the fades to the tip, or does the thickness remain constant?  What is the thickness of the glass laminations?  It is asking a lot of the materials to get the high draw weight at the 14-15" draw length, and with such a short prod length, but it I believe it should be possible.
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    Post by actionbow on Mon Jun 08, 2015 2:41 pm

    Ahh...good questions.

    It failed on draw.

    Normally I will laminate the riser between wood lams but this time I tried it behind. I think you are exactly right, the belly glass failed right at that fade.

    It's an extreme bow so I am not surprised it failed. I learned a couple things and I have already bought the lumber and made some adjustments to the fades in the press.

    Great observations everyone. This is helping.
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    Post by c sitas on Mon Jun 08, 2015 4:53 pm

    Giver Action; this kind of thing is really fun. Both for the builder and the watchers.
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    Post by Gnome on Tue Jun 09, 2015 5:31 pm

    An impressive experiment, Actionbow- I'm especially impressed that no arbalists were injured in the production of this display of skill. Seriously, it's cool to see somebody pushing the envelope with a laminate recurve prod. Hope to see the next iteration soon!
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    Post by actionbow on Tue Jun 09, 2015 5:46 pm

    I'll press it this weekend.
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    Post by twedzel on Tue Jun 09, 2015 9:55 pm

    Alan, Excalibur's limbs probably have less working limb than actionbows failed prod, and they draw 220lbs@14"ish draw length and 290lbs@ 16"or17"ish draw length. They are all fiberglass but it proves that it can be done.

    I was just putting together one of my first prods with a riser almost identical to this one thick with short fades. I did this in hopes of avoiding hinging off the fades and getting more working limb. Anyways it popped off when I was testing the wood. But I really liked how the wood alone was bending so I have decided to leave it off and see what happens after laying up the fiberglass (kind of like a bendy handle bow but in prod form). I wouldn't be surprised if the very slight bend in the middle will loosen everything up after binding it in place. But some part of me is hoping that the binding block will reinforce the center section enough to minimize movement.
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    Post by actionbow on Tue Jun 09, 2015 10:55 pm

    There is a big difference between all fiber limbs made in a  commercial compression mold and what I do.

    I'm not worried. 60 shots is enough to convince me it is possible.

    I'll post chrony vids when I nail it. Fingers crossed for 350 fps.
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    Post by Alan Case on Wed Jun 10, 2015 12:56 am

    Actionbow,
    I am pretty sure you are on the path to making it work. I bet the failed limbs did shoot well while they lasted.  I look forward to see how your next ones turn out!

    I am building and testing similar draw weight glass & wood composite bow limbs for flight archery (distance shooting). I would actually be happy with 60 shots, but I push them pretty hard.  The arrows are tiny at around 110-120 grains, so each shot is almost like a dry fire. 

    Twedzel,
    I don't have any experience with the Excaliber crossbow limbs.  Are they solid glass?  It seems like they would be pretty heavy, but they seem to perform pretty well regardless.
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    Post by hullutiedemies on Wed Jun 10, 2015 4:24 am

    Karpowicz was getting 350 fps with Turkish horn bows
    http://www.atarn.org/islamic/Performance/performance_table.htm
    But energy efficiency was only 50%.

    What was the efficiency here ?
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    Post by twedzel on Wed Jun 10, 2015 9:03 am

    Actionbow

    Its probably not as different as you think. This link gives a good overview of the process for compound limbs. The process of making crossbow limbs will be much the same. Also consider the outer 10% of the limb is taking 90% of the force. If you are using Bo-tuff then you are using the same stuff that most of the major manufacturers are using in the areas of your limbs that taking the most strain. Don't get me wrong, I also think this is getting to the edges of what the design is capable of, but we agree it is doable.

    Alan

    I am pretty sure Excalibur limbs are solid glass, but I also do not have first hand experience with them either. I did a fair bit of research on them for design inspiration. The added weight of all glass vs glass and wood core would be much more of a factor in a longer limb. You can use heavier materials in shorter limbs shooting heavier arrows. Its the same reason why steel is almost unheard of as a longbow material but workable for a crossbow prod.


    Last edited by twedzel on Wed Jun 10, 2015 4:58 pm; edited 2 times in total (Reason for editing : man I shouldn't try writing in the morning)
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    Post by actionbow on Wed Jun 10, 2015 3:26 pm

    Twedzel I typed a long response pointing out the differences but in the end it basically proved you right.

    They have R and D  and production tolerances I can't approach but I also have a attitude toward risk and a desire to do things that a large company doesn't. 

    Also, I'm determined. Money isn't my motivation. Excellence is.
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    Post by actionbow on Wed Jun 10, 2015 3:29 pm

    My idea is that with wood lam cores I can hopefully get the best of both worlds. The speed of wood and the durability and draw length of glass. Maybe, just maybe my penchant for deep recurve shapes and the beauty of wood will result in something wonderful and deadly.
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    Post by c sitas on Wed Jun 10, 2015 4:11 pm

    Well Action; at any rate , your workmanship is second to none. The thing looked beautiful  before it went south.You definitely  have a rare  talent and a masterful eye. Good job
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    Post by twedzel on Wed Jun 10, 2015 5:04 pm

    actionbow wrote:Maybe, just maybe my penchant for deep recurve shapes and the beauty of wood will result in something wonderful and deadly.
    I like your thinking!
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    Post by Alan Case on Thu Jun 11, 2015 2:15 pm

    Hullutedemies,
    50% efficiency is pretty good for an all-natural material bow that is shooting such a light weight flight arrow. The efficiency really drops with lighter arrows and faster speeds. Also, as Twedzel pointed out, at very high speeds, longer hand shot bows are at a disadvantage compared to short & heavy draw weight crossbow designs. 


    I there is some pretty good test data on the Arrow Trade Magazine site. Excaliber crossbows have efficiencies in the low-to-mid 70% range for arrow speeds ranging from 320-380 fps. That is pretty good for a solid glass limb. The Middleton is right up there too. I would expect a well made laminated glass/wood prod should do even better. 


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    Post by hullutiedemies on Sat Jun 13, 2015 1:27 am

    Alan Case wrote:

    I there is some pretty good test data on the Arrow Trade Magazine site. Excaliber crossbows have efficiencies in the low-to-mid 70% range for arrow speeds ranging from 320-380 fps. 
    Found it - 348 fps was the 73% bolt
    http://arrowtrademagazine.com/articles_pdfs/jan_13/Jan13-MatrixCrossbowTest.pdf

    So Matrix should cast 470 fps with 50% efficiency-
    and this is why one should allways record the energy efficiency. Because it immediately tells the potential maximum performance of a bow.
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    Post by Alan Case on Wed Jun 17, 2015 11:21 am

    I really like how the arrow trade magazine conducts their tests.  They are very thorough and measure the stored energy and arrow kinetic energy for different weight arrows.  They even use two chronographs.  I also figure around 470-480 fps for the Matrix 380 at 50% efficiency (using the virtual mass model) although the arrow would only be around 155 grains in order to achieve this.  It isn't bad performance for solid compression molded glass limbs.  On the other hand, a determined prod builder should be able to exceed this performance level by a good margin using laminated wood & glass construction. 

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    Post by hullutiedemies on Thu Jun 18, 2015 1:47 am

    155 grains is about 10 grams . I consider that heavy. If I want to go fast I use couple gram kebab-sticks with my inswinger compounds. My "heavy" target bolts weight about 10g.

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