Introduction: The Knex Trebuchet

This instructable will teach you how to construct a large, almost authentic looking knex trebuchet. This trebuchet can launch marbles about 25 feet with a good launch. I had built this about a year ago, and just took it out of the basement. Loaded it with a marble, pulled the launch pin, and to my surprise, almost broke a window. Later, i loaded it up with airsoft pellets, and launched it at my sisters door. The trebuchet stands about 2 feet, 4 inches tall. It is very simple to construct, and takes about 30 min. The movie should show you a short clip of the trebuchet launching.


I am not liable for anything stupid you may do with this trebuchet....If you injure or kill anybody with it, don't come crying to me. Just be smart

PS: this is my first instructable, so tell me if it is good, and what improvements i could make to it.

PSS: sorry about the low quality pictures, But you should still be able to get information from them.

UPDATE: i just built a duplicate at my friends house using this instructable...I think i miscounted a few pieces, but if you have your knex bucket near you, this shouldn't be a problem. Also, you may need to tinker with the string length, counterweight weight, and other small stuff...It still works fine though!

Step 1: Materials List

Knex stuff:

(all measurements may not be accurate)


Blue x 50
Yellow x 24
Orange/red x 6
Grey x 1


Yellow x 9
White x 12
Light Grey x 2
Dark Grey x 2
Red x 18
Blue x !0
Purple x 10

PS: Red can be used in place of orange, and the other way around

Other stuff:

Some sort of tape ( I used scotch tape..the clear stuff...because i was sort of in a fine!)
two D cell batteries
a bit of spare cloth

Step 2: Connect

This is really easy....Just connect the purple connectors to the blue connectors.

Step 3: Construct the Base

Follow the picture to construct the base

Step 4: Make the Side Supports

Make the side supports by following the picture...DO THIS 2X

Step 5: Consruct the Arm

Make the throwing arm By following the picture

Step 6: Make the Counterweight

Construct the counterweight (power of the trebuchet) by following the picture.

NOTE: SOME D-CELL BATTERIES ARE DIFFERENT WEIGHTS, i used energizer, which are pretty darn heavy!

Step 7: Constucting the Axle

Pretty much self explanatory..follow the picture!

Step 8: Cut Out the Strings

Cut out 4 strings. (one for the counterweight, two for the sling, and one for the trigger arm)

1st string:
about 7 inches long

(this will be called your first string)

2nd and 3rd string:
about 10 inches long

(these strings should be same length, and will be called your 2nd and 3rd strings)

4th string:
As far away as you want to be from the trebuchet when firing!

(this string should be pretty long, and will be called you 4th string)

Step 9: Make the Sling

Here comes the trickiest part of the trebuchet...making the sling.

Cut out a piece of cloth in an oval shape, that is about 2.5 inches wide and 5.5 inches long

once you do that, make two holes, one in each end.

Tie your 2nd and 3rd cut strings to each end.

Then tie a orange connector to the end of one of the strings.

Whichever string you didn't tie the orange connector onto, put a loop in the end...(one that doesn't slip and is about as big around as a pencil)

Step 10: Put Some Stuff Together

In this step, you will stick the side to the base and the axle in the sides.

Step 11: Stick Some Stuff to the Arm!

In this step, you will connect the counterweight and sling to the arm.

First, take the first string you cut, and tie that to the white connector on the short side of the arm.

Tie a Dark Grey connector to the end of the string you just tied to the arm.

Connect that connector to the blue rod sticking out of the top of the counterweight.

Next, connect the orange connector on one end of the sling to the long side of the arm.

( make sure all your knots are tight, so things don't fall off! )

Step 12: Put the Arm on the Trebuchet

In this step, you will put the arm on the trebuchet...PAY ATTENTION...this is pretty important

Put the axle through the white connector close to the center of the arm. Put your light Grey pieces on either side of the white connector to make sure it doesn't wobble. MAKE SURE IT IS CENTERED.

When you have put the arm on the trebuchet, the counter weight should swing down to the base, and not touch the bottom...If it does, adjust your knots.

Step 13: Making the Firing Pin

In this step, you will make the firing pin

Put Dark gray connectors on both sides of a red or orange piece.

Then, tie a string to one of the Dark gray connectors.

put the firing pin though the top yellow connector on one of the side supports of the trebuchet..Make sure the firing pin is on the side that will hold the long arm when it comes down( the side that the top of the arm is leaning towards in most cases)

Step 14: Loading and Firing (the Really Fun Part)

You are done with the trebuchet!!(now for the fun part....hehehe)

This may be a little tricky, but i will do my best to explain!

1. Pull the arm down to the firing pin

2. put the firing pin over the arm to keep it from firing while you are loading it

3. pull the sling back a bit, so the loop is by the orange connector

4. put the loop in the end of the orange connector

6.pull the pouch taught, and make sure there are no loops...the loop side should be on the bottom, and the side tied to the arm should be on the top.

7. Put your projectile in the pouch

7.5. Make sure there are no animals, humans, or breakable objects in the way

8. Back up, take hold of the firing pin sting, pull, AND FIRE!!!

9. watch your missile take a satisfying arc across the sky.

Step 15: Other Ammo

There is other ammo you can use in your trebuchet

i sometimes use

1. marbles (a nice heavy projectile)

2. Lego heads (You can launch multiple ones for a LOTR battle of Minas Tirith effect)

3. Airsoft pellets (Great spread shot!)

4. Anything that is a reasonable weight and fits in the pouch!