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Master Of Junior Members
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Discussion Starter #1
That's mine first crossbow :)
Currently it does not have some power because of rubber band. So I'm asking for some advice what to put on front, but the condition is that it has to be spring or some rubber band. I made front part easy mountable so I can have more extensions.

Thanks in advice :)
 

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Master Of Junior Members
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16 Posts
Discussion Starter #3 (Edited)
Thanks :)
Tera tube, I just google it and I find it very good for that, tube-like and strong, gotta try it :D
and, gold will be just fine ;)
 

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Master Of Junior Members
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Discussion Starter #6
Update :)
Put some new rubber bands, I can say it's way more stronger than before. I also ordered new stronger to test the limits.
 

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Master Of Junior Members
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Discussion Starter #7 (Edited)
Here is just test shot of crossbow, I shoot a file in cardboard box and it went all trough it and hit in the wall which was about 10 meters behind. Video is like someone record it with potato , but you get the point. :) and frame rate is 240 fps.
[ame="http://www.youtube.com/watch?v=GhqOxKe2_v4"]http://www.youtube.com/watch?v=GhqOxKe2_v4[/ame]


Today I also made one relative normal arrow,they have no some aesthetic, just for test and if they show up like good then I'll made more ( I stole feathers from chicken,hope it wont get mad) . I will post a video as soon I test it and measure speed and calculate given energy.

 

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Master Of Junior Members
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16 Posts
Discussion Starter #8
Physical side

Today I made some measurements, results are next:

Draw force is 28 kg (or 270N ), for that force I have to stretch rubber for 37 cm until it reaches the trigger. From that you can calculate elastic coefficient of rubber just by equation F/l,or 270N/0.37m, you get about 730N/m.

I need to calculate this to calculate arrow speed, I planned to do it with camera but I don't have so fast camera. Equation for speed with elastic coefficient is l* root from ( k/m) , where m is the mass of arrow (in my case is 27g,or 0.027kg). Here I get some nice results, speed was about 61 m/s , or 220 km/h +- 5 km/h because of some energy loss on friction and pulleys.

In energy that would be 50J ,( Ek= (mv^2)/2.

[ame="http://www.youtube.com/watch?v=5dwpUmVJh2E"]http://www.youtube.com/watch?v=5dwpUmVJh2E[/ame]
 

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Master Of Junior Members
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Discussion Starter #10
Thanks! :-D
That is just beta version, it's only to see bow how it behaves and it's also my first crossbow :)
 

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Master Of Junior Members
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16 Posts
Discussion Starter #11 (Edited)
balista crossbow

I have some problems with bands and connections with string, a couple of shoots and it brakes.. so, I came up with new design for which I think that the bands will last longer and it could be more powerful, it's also powered with thera bands.

What do you think guys, do you maybe have some recommendations or advice before I start with making one?
And the wood is oak, so it should be quite strong.
 

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weird kid in the corner
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237 Posts
This is awesome! I mean...Really cool! Thanks for sharing!


Sent from my iPad using Slingshot Forum
 

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Naturalist
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You could receive a lot more draw lenght - and that way more speed - if you change the way your rubber is placed in.
Left your actual version, right the one with the extended draw lenght:


Yo don't even need to change your build except for two little hooks beside your trigger - easy to add them, i think ;)
Hope i can help you a bit with this!

Greetings, Haecksler!
 

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Registered
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You could receive a lot more draw lenght - and that way more speed - if you change the way your rubber is placed in.
Left your actual version, right the one with the extended draw lenght:


Yo don't even need to change your build except for two little hooks beside your trigger - easy to add them, i think ;)
Hope i can help you a bit with this!

Greetings, Haecksler!
To improve, drastically shorten horizontal bottom piece.

Increasing width of fork is your enemy.
 

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Master Of Junior Members
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16 Posts
Discussion Starter #15
I recently tried right version of your sketch and it is better, maybe I didn't get much more on draw weight, but at least my rubber didn't go left rigt when pulled.

The forks are wide because at firs I was planning to put some spring between pulleys but it wasn't functioning like that, so I took a rubber which I had on my slingshot.

And about connecting rubber with cord, it will work with hooks, but they are at least twice heavier than currently connector, with the fact that I have to use both for that kind of connection.

I think that it's possible to get more draw weight with my sketched design, but not with same amount of rubbers like on the one I have. In front I can put a lot more rubbers to increase draw weight, which is not the case with that which have pulleys because to much rubbers can jump of the pulley. Currently, I'm working on strength, not efficiency :)

Thanks guys :-D
 

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Master Of Junior Members
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Discussion Starter #16
Just having some fun (off topic )

Meanwhile, while I'm thinking what to do next with my crossbow, I made that cannon with my friends, the bullet is dishtowel with 5 bars of pressure on its ass. I think you'll like it :D
[ame="http://www.youtube.com/watch?v=sZ_5b3pQHfM"]http://www.youtube.com/watch?v=sZ_5b3pQHfM[/ame]
 
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