K'nex Trebuchet





Introduction: K'nex Trebuchet

After finals this past semester my roommate and I decided to build a "big" trebuchet with K'NEX, so that unorganized mess of plastic in my closet could finally be put to good use.

I decided to submit this instructable after the trebuchet was already built, mostly because I wasn't sure how it would turn out.  But I took some pictures and have a few videos so you can get an idea of how it works and how it was put together.

One additional comment before we get started.  I'm really not sure what the range of the trebuchet is, considering I cannot test it outdoors because of the weather.  However, adjusting the range is pretty easy though:  shorter sling yields earlier release; shorter boom tip yields earlier release.

Also, there are plenty of cool ways to rig up a trebuchet, this is just the way my roommate and I did it.  Have fun and be sure to shoot as many projectiles indoors as you can (I recommend practice golf balls).

Step 1: The Five Major Components

1. Boom
2. Frame
3. Counterweight
4. Sling
5. Trigger

Step 2: The Boom & Counterweight

The boom is the first major component we decided to put together.  The size of the boom more or less determined the size of everything else.  The other stuff in the pictures, including the counterweight was considered after the boom was assembled.

In order to keep the boom from shaking apart from the counterweight we tried to add supports where we could.  Ultimately seeking to eliminate twisting of the boom and side to side swaying of the counterweight.  See the pictures for notes on the supports.

The distance the axle must span was also an important consideration:  if too wide, supporting the boom may become difficult, if too narrow, the counterweight may not have enough room to shake side to side.  Ideally the shaking of the counterweight would never be a problem, but in this world, ideal is simply fiction.

Step 3: The Frame

Making the frame is pretty straight forward, but also time consuming.  There are a few frame options:  red columns with grey ties, yellow columns with red ties, or blue columns with yellow ties.  Off the bat I knew I didn't want to use the first method because it would have been too bulky and not rigid enough, and I wouldn't have been able to use the last one cause I am sure I don't have enough pieces.

Step 4: The Trigger

To Tell ya' the truth, I think this little deally is the coolest part.



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    I've akredy made the trencher and it is awesome but I can't actually get it to shoot I have 6 d battery's as a counter weight but I can't get the sling to work right. I followed all your picture and I still doesn't work. Please help

    4 replies

    Glad to hear you're puttin' it together; excellent. Explain what "work right" means...how is the sling currently performing? Where is the ball going?

    It either falls out of the sling or fires downward. In other words the sling realeses to late

    It is going either falling out of the sling or if it makes it up without falling it releases to late and it goes down

    I have a few ideas to try...just before you launch the trebuchet, be sure there is very little slack in the sling and that the projectile is snug in the "pocket"; see if you can make the pocket a little narrower (if it's too bulky, it could get caught), and make sure the sling slides easily off the boom tip (if the sling is getting caught on the tip...it might release late). You may also want to test your pocket design by loading it and swinging it around with your arms. After you've checked those things, and it still releases late, shorten the sling; that should make it release earlier. Good luck! I pulled this from a Google search: http://www.mikesenese.com/DOIT/2010/12/tuning-a-trebuchet/

    I've all ready made the trebuchet

    I've all ready made the trebuchet

    Shoestring, twisty-ties, folded paper napkin...what I had on hand; but light.

    And one more thing how does the trigger work

    Also, keep in mind: this is probably unnecessarily complicated. The goal was to have a setup where I didn't have to pull a linchpin hard and fast; but a pin or a part of a paper clip could have sufficed as a "frictionless" linchpins. So, instead of having a "trigger" and strings, you'd just have the strings, i.e. one secured to the frame, one secured to the boom tip, and one to your smooth linchpin. Or not! Hope this helps.

    You see there's three strings comin' off the trigger mechanism? Check out the first picture in the last step (4). The string on the bottom is secured to the frame, the string on the top is tied onto the bearing which hooks onto the light brown clip of the trigger, and the black string is tied to the grey clip. You see how the grey clip is not quite attached to the blue rod? That way it slips right out with a gentle pull.

    How does the trigger work

    Exactly what information did you want?

    What is the sling made of

    just a hair above 5 feet

    For the sagging part you should of used a knex steel bar from the six foot ferris wheel.