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    A question about dimensions

    Kristjan
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    A question about dimensions Empty A question about dimensions

    Post by Kristjan on Thu Jul 08, 2010 4:17 am

    Hello, this is the newest addition to the forum speaking. I emailed a person, whom you might know as ''Basileus'', asking about a crossbows basic dimensions, which i didin't find in his epic tutorial -> (LINKZ) . He in response asked me to post the question onto the forums, so poeple could find it via google and thus could solve their problems.

    I was hoping to get some some help at a crossbows basic dimensions and recommended sizes (width, with the bow aswell as without it, the lenght, the height etc). Being completely new to this kind of thing, I believe its good for a newbie to have
    allready tested designs, and not try to create an abomination by themselves.

    So, fellow arbalists, how big is your ''crossbow'' A question about dimensions Icon_razz .

    (posted the same thing onto general discussion accidentally, but doubt it will get many responses there)
    Ivo
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    A question about dimensions Empty Re: A question about dimensions

    Post by Ivo on Thu Jul 08, 2010 1:42 pm

    Hello Kristjan,

    All in all it is a very general question and will greatly depend on the materials available...So how "big" of a crossbow are you thinking of? They come in all sizes! A question about dimensions 951629

    PS: Check out http://alcheminc.com/crossbow.html for some basic plans and crossbow components. Also check out http://www.thecrossbowmansden.com/Home.html and the "Crossbow Books" and "Medieval Crossbows: Photos, Drawings, Diagrams" topics...that should definitely help a bit. A question about dimensions Icon_smile



    A question about dimensions Untitled
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    ~ "I don't have any special talents. I'm only passionately curious."
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    Kristjan
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    A question about dimensions Empty Re: A question about dimensions

    Post by Kristjan on Fri Jul 09, 2010 3:20 pm

    Thank you for the links. Though i have another question aswell, but making a new topic seems pointless even if its not to do with dimensions.

    So, can the bow part of a crossbow be made of an old cars' leaf spring ? Couldn't decades of use could make it too brittle or w/e ? Wouldn't want the end of my bow to snap off at first testing and come flying torwards my head.
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    A question about dimensions Empty Re: A question about dimensions

    Post by basileus on Sat Jul 10, 2010 9:17 am

    Kristjan wrote:So, can the bow part of a crossbow be made of an old
    cars' leaf spring ? Couldn't decades of use could make it too brittle
    or w/e ? Wouldn't want the end of my bow to snap off at first testing
    and come flying torwards my head.

    Old leaf springs should be as good as new ones, unless they have rusted badly. The dimensions of your bow will probably be limited by the holes in the leaf: make sure you place the center hole into exact middle of the bow and the other (two) holes outside of the bow. I suggest attaching the bow with bow irons and using a thick stirrup which supports the weaker middle part (with the hole):

    http://users.utu.fi/sjsepp/paja/making_bow_irons/making_bow_irons.html

    You can use other types of bow attachments, but bow irons work great even with strongest steel bows. If you use other type of bow attachments, leave the middle of the bow wider than normally, so that area around the hole does not jeopardize the durability of the bow.

    The dimensions of the bow depends highly on the thickness of the leaf and the desired draw weight. If you're aiming at ~150 pounds I suggest designs similar to "Alpha v2" and "Delta" on this page:

    http://users.utu.fi/sjsepp/paja/designing_crossbows/stats/bolt_velocities.html

    Both have a very long draw length, but Alpha v2 is more comfortable to cock because the string angle (and hence stack) is smaller. Both are roughly in 150 pound class. If your leaf is over 6mm (~1/4") thick, you have to make the bow longer and/or narrower at the center. For example, the "Gamma" design on above page requires the use of a wippe cocking lever. When the design is ready, just follow the procedure described here to cut it into a bow:

    http://users.utu.fi/sjsepp/paja/making_a_steel_prod/making_a_pyramid_profile_steel_prod.html

    If you share the dimensions of your leaf spring I'm sure we can design a bow that suits your needs. Please include the locations of the holes.The other dimensions of the crossbow are highly dependent on the bow, so that needs to be designed first.
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    A question about dimensions Empty Re: A question about dimensions

    Post by Ivo on Wed Jul 14, 2010 1:16 am

    One fellow (one of many A question about dimensions Icon_smile ) made this scary little guy using a leaf spring...and it was a "mean" little machine.

    http://forum.arbalet.info/viewtopic.php?t=8250&highlight=andriy

    Ivo A question about dimensions 951629



    A question about dimensions Untitled
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    Riot
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    A question about dimensions Empty Hey Basileus

    Post by Riot on Mon Aug 02, 2010 6:54 am

    Cheers for your posts all, esp the links Basileus!

    Ive recently got my greedy little hands on two shorter length of spring steel that I would like to make into a prod (no doubt i will wreck one of them in the process).

    I would ultimatly like to make something in the heavier draw weights 200-300lbs as will be using a crank to help n loading (im lazy and love electric come-a-long's).

    The leaf spring is from a light automotive trailer. Dimensions as follows. its rather deflexed....
    Length (end to end): 560mm
    Length (end to end along the back) : 600mm
    Width: 45mm
    Thickness: 6.2mm
    current deflexed brace height: 90mm (straight line tip to tip with a length of string)

    Or...... if someone knows the conversion formula to scale up and to scales down STEEL leaf spring material of the same thickness and center widths... please feel free to help A question about dimensions Icon_biggrin

    Cheers
    Riot
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    Post by Ivo on Wed Aug 04, 2010 1:30 am

    Hello Riot,

    I'm not sure about this since I have little experience with leafsprings, but it sounds like it will be a little more than 200-300lb...say around 200kg...haha..may be more. A question about dimensions 722249



    A question about dimensions Untitled
    * *
    ~ "I don't have any special talents. I'm only passionately curious."
    * * *
    ~ "All Genius is Simple"
    * *
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    A question about dimensions Empty Re: A question about dimensions

    Post by basileus on Wed Aug 04, 2010 7:22 am

    Riot wrote:Cheers for your posts all, esp the links Basileus!

    Ive recently got my greedy little hands on two shorter length of spring steel that I would like to make into a prod (no doubt i will wreck one of them in the process).

    I would ultimatly like to make something in the heavier draw weights 200-300lbs as will be using a crank to help n loading (im lazy and love electric come-a-long's).

    The leaf spring is from a light automotive trailer. Dimensions as follows. its rather deflexed....
    Length (end to end): 560mm
    Length (end to end along the back) : 600mm
    Width: 45mm
    Thickness: 6.2mm
    current deflexed brace height: 90mm (straight line tip to tip with a length of string)

    Or...... if someone knows the conversion formula to scale up and to scales down STEEL leaf spring material of the same thickness and center widths... please feel free to help A question about dimensions Icon_biggrin

    Cheers
    Riot

    Ok, so you'll definitely be needing some cocking device for this... As a comparison I got one crossbow made from 6,2mm leaf: it's 900mm long (along the back), 45mm wide at the center and around 7mm wide at the tips. It slightly (~60mm) deflexed and pulls around 150 pounds at 40cm draw (bow's belly to nut). Given that your leaf is only 600mm long expect higher draw weight and much shorter draw.

    I'd cut the leaf into following dimensions:

    • 37mm wide at the center
    • 7mm wide at the tips
    • 600mm long (along the back)
    • 8mm wide nocks/ears at the tips (or more, if string loops are thick)
    Try to place the tips as high as possible, so that you don't have to cant the bow (in it's stock) any more than you absolutely have to.

    Even though 7mm at the tips sounds too little, it's not. It should be 0mm if we wanted to stress the bow equally at every part, but that's obviously not possible. If you leave the tips too wide, the center of bow will be overstressed for no good reason. Also, the dry-fire speed of the bow will be low. With heavy bolts you might not notice a difference, though.

    One word of warning about using very thin tips... make sure the width taper is very even all the way up to the tips. I managed to bend the tip of one of my steel bows with an accidental misfire coupled with too strong linen string. Of course the bow bent at the exact point which was left slightly too narrow (~0,5mm) compared to surrounding areas.

    I also warmly recommend making a wippe cocking lever - it's fast to use and (relatively) simple to make:

    • http://users.utu.fi/sjsepp/paja/making_a_wippe/making_a_wippe.html
    Unfortunately I have not had time to finish the above article although my wippe now works perfectly.

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