A working LEGO Ballista that can fire a 2x4 brick up to 15 feet.

The frame itself is very simple to build, but threading and tensioning the strings is a bit more complicated. Hopefully the instructions are clear enough in this regard. I also briefly show how to thread and tighten the springs in the video.

It's important that both strings be twisted to have about the same amount of tension. If they are too unbalanced, then one arm will be much 'stronger' than the other and it will not operate properly. If you encounter this case, just unwind the 'weaker' spring and re-insert the piece at the base of the arm from a different direction (front/side/back) so that you can twist the string another 1/4 or 1/2 a revolution to match the other one.
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Name Ballista
Designer JKBrickworks
Designed 2015
Inventory 119 parts
Theme Castle
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