Introduction: The Knex Trebuchet

Picture of 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

Picture of Connect

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

Step 3: Construct the Base

Picture of Construct the Base

Follow the picture to construct the base

Step 4: Make the Side Supports

Picture of Make the Side Supports

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

Step 5: Consruct the Arm

Picture of Consruct the Arm

Make the throwing arm By following the picture

Step 6: Make the Counterweight

Picture of 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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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

Picture of 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

Picture of 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)

Picture of 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!


ch33sl3y made it! (author)2016-08-09

I need to adjust the sling. ammo falls out almost every time. the one that did go went quite far though.

augustuss (author)2015-08-20


augustuss (author)2015-08-20


brosdv (author)2015-02-04

I thought this only uses K'nex!!!!!!!!!!!!!!!!!

Malcreate (author)2014-11-18

AMAZING!!!!!LOVE the idea!!!

hbartlett1 (author)2011-04-19

do u know how to make one with a rector set? im trying to do so now but not finished yet

knex master 81 (author)2010-11-06

oh yeah second comment

661rhys (author)2008-06-04

Hi there. does anyone know what i could use for a weight for a trebuchet that stands about 5ft high? i previously use 1.5 kg of lead, but because i have moved house recently the tebuchet had to be dismantled and the lead has gone missing. please reply if you have any suggestions.

knex master 81 (author)661rhys2010-11-06

if you can find big rocks you could use those

rawknexstuff (author)661rhys2010-02-13

I made a trebuchet for a school project and it was 12ft tall, and we used about 225 lbs worth of rocks and patio blocks (the patio blocks are about 20 lbs. each) and we broke the school record for distance. So maybe scale that down a bit and you should be good.

GANSTAMAN4 (author)661rhys2009-09-06

well u can use the wheels for a counter weight. i used them once and they work rather well. also you could try to use same smaller batteries.

pindalanderz (author)661rhys2008-08-09

save up your money and buy 1.5 kg of silver. that way you have a good investment worth about 960 USD right now ( about 500 british pounds) that you can use as a weight for your trebuchet.

good idea, lol

Me and my friend built one a couple years ago ours wasnt as big as yours but it was close. To get a tennis ball to go a meter we used a brick.

sam and the beanstalk (author)2007-10-29


stopanator (author)2009-09-16

i made and it it so cool! i cut my old 3 year old shirt dident fit my mom almost threw out but i took it i used a rock for weight

13remco13 (author)2008-10-31

it looks good! I have a question:is there an alternative for the counterweight and the piece of cloth?

GANSTAMAN4 (author)13remco132009-09-06

u can use smaller batteries for the weight, but u need cloth for the sling

Darbysboy1 (author)2007-12-08

flio191 you dope it says that you need 50 blues!!!

I have already corrected that

ok i hadn't realized that

Darbysboy1 (author)Darbysboy12007-12-09

i had a lot of trouble with your trebuchet because when i would fire it would just swing the cloth cause the marble fell out right behind the thing

Darbysboy1 (author)Darbysboy12007-12-09

so i decided to mod it to prevent that from happening (but i think i got a "little" carried away because i ended up with a prototype of a catapult) now i guess the problem can't be mine unless i rebuild the trebuchet. ehehehehe :D please tell me should i post it (here's some pics)

lol! that's awesome! how far does it launch? Yours looks really cool :) If you do post it ( I hope you do) I will definitely build it. It looks like you are using rubber bands for propulsion, am I right?

i need either more rubber bands, newer ones, or the wide, blue ones like you find in the store. because im using rubber bands that i just find on the ground, theyre not in very good condition. right now it only shoots about 2 1/2 feet.

snipeyouout (author)Darbysboy12008-02-18

my prototype trebuchetshoots about 10 feet

GANSTAMAN4 (author)snipeyouout2009-09-06

wow dude that's so cool

shmuki (author)Darbysboy12008-01-02

that looks awesome! put it on so i can build it plz.

Darbysboy1 (author)shmuki2008-01-03

sorry dude, thats a negative. I would post it but two hours after I posted my last comment, I put too many rubber bands on it and it broke. oh sorry, I meant that there were pieces flying everywhere!!!!!!!! I just gave up.

GANSTAMAN4 (author)Darbysboy12009-09-06

isn't it more of a catapault?

matrix828 (author)Darbysboy12008-05-25

post it!!

make it again and post it!!!!

da best (author)Darbysboy12008-01-19

i built it from your pics its good +1

zuulthedestroyer (author)da best2008-02-12

can u post it den?

erikos kostarikos (author)2009-04-28

i like it very much. i thought about it to make a trebuchet 2, but i dindnt know how. thanks for the ideas

vandeegraaffsrock!! (author)2009-04-17

this sucks balls it does not work at all it sucks

stormz76 (author)2009-04-09

i put 2 d's and 2 c's and a rubberband on the arm doubled it up and it works great. i couldnt get it to work but then i put 1 marble in and it went a good 20 feet

CHRISTYTWISTY (author)2009-03-11

hey how many blues u need 4 dis ps looks great dooo

duker (author)2008-11-23

I think you should post it.

Ltcheesecracker (author)duker2008-12-17

it is posted nooblet

duker (author)2008-11-23

I think it is a great idea!!!!!!!!!!!!!!!!!!!1

duker (author)2008-11-23

I think it is a great idea!!!!!!!!!!!!!!!!!!!!!!!!

Yang46 (author)2008-06-12

can you make a mini version of the catapult you posted?

pls (author)2008-04-16

i like the idea, i may try an make one as well.

chugs78 (author)2008-04-09

it only needs 46 or something blues

agg man 2 (author)2008-01-24

nice job but i had a little trouble firing b/c the marble fell

da best (author)2008-01-19

the frame is a bit to week it snaped on me but good 1+

SFHandyman (author)2007-08-25

Nice Machine. I've never played with knex. People make some impressive projects with it. I had an erector set as a kid. I wish I still had it.

I made a quick, easy, no tools, no cutting, no drilling, no lumber, no hardware, no measuring, no plans, super easy Trebuchet.

Quick-n-Dirty Trebuchet

I published it last night but it isn't showing up yet. Maybe it isn't accessible yet, but check it out when it shows up. Here are the video links from Google, they are up and running now.

Trebuchet Video 1
Trebuchet Video 2

Shot with my still camera so the quality isn't that good. The first one shows a close shot of the machine firing. Second video shows the view of the projectiles hitting about 14 feet away. It throws 12-15 feet consistently.

I tried a heavier weight and my projectile hit the wall 20' away. It's my biggest room, so I can't test the full range here.

It's not pretty but it's sure easy. It's also one of those projects a 10 year old can make without wasting their allowance. It's made out of 9 chopsticks, 2 toothpicks, 2 rubber bands, 3 paper clips, a weight, and masking tape. (I know 10 year olds aren't supposed to join but they can still read - I would have been all over this site when I was 10).

Quick-n-Dirty Trebuchet

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