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    Compound twinbow pistol (posts vs pulleys)

    Mooki
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    Compound twinbow pistol (posts vs pulleys) - Page 2 Empty Compound twinbow pistol (posts vs pulleys)

    Post by Mooki on Fri Mar 01, 2019 7:18 am

    First topic message reminder :

    Cheers,

    I'be been thinking, on-and-off for the last 35 years or so, about building a small crossbow, guess this was triggered by a souvenir xbow from San Marino ages ago...
    Anyway, the direction my mind is currently wandering is sort of inspired by the "Twinbow".
    Being the first build I naturally started off with the fun parts without having a clear picture of the final design.
    I used some discarded kitchen drawer handles I had lying around, they act as the stock and arrow rest, with a classic roller nut as release.
    Tried it out with some rubber bands and everything seems to work fine (with that comparatively low load).

    I'll assume the twinbow is familiar enough - since my draw is pretty long for such a short bow I'd like to add some kind of compound system - however since pulleys are a bit pricey for decent ones I was thinking of doing a POC using posts instead of pulleys.
    Any ideas as to how much this will affect performance? (hence the thread title)

    Build so far: Total length 32cm, probably using 30cm prods from spring steel 2.0x35mm (tapered ends to 15mm), riser fixed in the middle approx where the trigger is.
    Compound twinbow pistol (posts vs pulleys) - Page 2 Dsc_0410

    Am I way off in my thinking below, about 24cm draw giving roughly 4cm flex per prod end?
    Prods almost straight at rest, middle post/pulley on the fixed riser. My thinking is that string would be approx 165cm total, draw 24cm should "consume" 50cm forcing the posts closer, so at each "crossing" the string needs to be (50/6=8 ) cm shorter, = 4cm per prod end. Good/Bad thinking on my part?

    If so, it should suffice fixing the prods approx 6cm out from the body, keeping the width at 17cm.
    However, that might be bad due to "compound angle" being too steep - thoughts?
    BTW, that's the brass posts in the pic, threaded through with M4.
    Compound twinbow pistol (posts vs pulleys) - Page 2 Dsc_1010

    Any thoughts or feedback welcome //M


    Last edited by Mooki on Thu Mar 21, 2019 6:00 am; edited 2 times in total (Reason for editing : Edited title to better reflect contents)
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    c sitas
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    Post by c sitas on Tue Oct 29, 2019 8:19 am

    Mooki, about twenty strands to make a cable. Twist it good to compact the strands. Should only have to serve where the string nocks. Also ,that's where you join the cable. If you serve good and tight about 8 to 10 inch overlap should work.I just now seee your drawing. I thought you were continuose wrap. Terminal posts would make it easier to do.
    Mooki
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    Post by Mooki on Wed Oct 30, 2019 4:32 am

    Thanks for your time and effort  cheers

    Yupp, a continuous loop would've been nice, but I think a "classic" cable type with loops at each end should work - if I can come up with a decent attachment to the body (in the rear), a post or similar.

    Hmm, 20 strands of B50 equivalent?
    I was thinking 18 strands of #2.0 Dyneema, roughly half the diameter and strength of B50, should meet the recommended strength for a 35-45lbs x-bow - but might be a bit small in diameter...

    My rough outline for making the cable was originally:
    2 poles, 9 complete turns => 18 strands.
    Set tension to 90 lbs and serve the loops.
    Twist 50 turns (66" cable length), increase tension to 250 lbs over night.
    Release tension, close the loops with serving.
    Set tension to 90 lbs and measure length, adjust with twists.
    Serve the middle, tight as I can.
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    Post by c sitas on Wed Oct 30, 2019 5:00 pm

    Mooki, one thing to remember with using two seperate cable anchors is, both sides have to be matched perfect. It's no different than cableing a two wheeled bow. The pully wraps have to be tune perfectly identical for good performance.Your setup sounds right on, and don't forget you can adjust the length some by twisting.

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