Introduction: 2 Bicycle Trebuchets - Forks and Frame

Good morning from Cambodia.

This Instructable is a guide to building 2 simple traction trebuchets from a single bicycle.

One trebuchet uses the front forks and front wheel - the other uses the frame and bottom bracket.

With careful dismantling, use, and reassembly - there's no real reason why you couldn't use the bicycle for its intended purpose to go home again after you've slaughtered the enemy.

As with any mechanical devices, care is needed at all times when building, testing and operating them.
If you make these trebuchets, PLEASE BE CAREFUL in all aspects of their use.

This project arose from practical English language classes conducted in Phnom Penh. The students in those classes wrote the following "how to" guide after building and using their trebuchets.
No grammar books were harmed during the production of this Instructable.

That's enough from me - over to the students... see step 1

Step 1: You Will Need

Here are the tools that we need

a hacksaw
a screwdriver
a measuring tape
a knife
wire clippers
a socket set

T notes -
small spanners and sockets were used to tighten up the heavy-duty jubilee clips

Here are the materials that we need

a bicycle frame
bicycle forks
a bicycle wheel
2 metal poles (3m each)
ropes (12m)
8 metal stakes
2 bamboo poles (2.5m)
an aluminium bar (2.5m)
a metal nail
rubber strip
duct tape
jubilee clips
strong cord (6m)
old jeans (to make the sling pouch)

T notes -
 the rubber strip was cut from a heavy tyre inner-tube and was around 3m long and 1 inch wide
 the aluminium bar was a box section found locally and used to try to lighten the throwing arm
 the metal nail was used as an adjustable pin at the tip of the throwing arm

Time needed

One weekend


We need only $10 for each trebuchet, (if we already have a bicycle).

Step 2: Forks Trebuchet

A - This metal pole is 3 metres long... hammer it 1 metre into the ground.

B - The forks fit into the metal pole

C - Guy lines are 3 metres long

D - Here you need to tie the bamboo with the inner tube. Tie in circular motions and tie at both sides tightly. Make sure this is strong.

E - The handle is around 1 metre long from the centre of the wheel

F - Here is a metal nail fastened with a rubber washer, a jubilee clip, and covered with sticky tape

G - The sling pouch is made from an old pair of jeans

H - Pull hard here to fire

T notes - 
A - Cut/drill a hole in the ground with a separate length of pipe, then drop the 3m pole into place.

B - The forks are held inside the pole by the guy lines... make sure the inside diameter of the pole doesn't allow more than a millimeter or two of movement of the forks and that the forks are long enough to be secure within the pole.

C - Use reasonably large pipes or solid bars for the pegs here - tent pegs won't hold it.

D - Fasten the bamboo pole closely to the centre of rotation of the bike wheel. Tie the inner tube tightly around the rim, then progress across the wheel securing the spokes to the pole with the inner tube. Tie the inner tube off tightly at the "opposite" rim and return to the start point in the same way. This must be a very secure fixture.

E - From the wheel axle, position the bamboo with around 1 metre to one side and 1.5 metres to the other side.

F - The throwing angle is set by bending/adjusting a soft metal nail that's attached to the tip of the throwing arm. A nail angle continuing parallel from the arm gives a high throwing angle ... bending the pin upward moves the release point of the sling to a later time - meaning the trebuchet will throw further forward with more upward angle of the nail. Getting this right takes some time and is a trial and error procedure.

G - Cut an oval shape from an old pair of jeans, hem the edge of the material with tape, and add reinforcement across the pouch with a chopstick to stop the pouch folding inward and holding back the projectile.

H - A strong, smooth pull on the handle launches the projectile.
During testing - ensure a wide, clear area around the trebuchet and carefully watch the throwing angle... the projectile may be launched straight up.

Step 3: Frame Trebuchet

Picture 1

1 - Rubber strips stop the arm slipping

2 - Fasten the arm to the pedal crank using heavy jubilee clips

T notes
This is an unfinished joint. Make sure that the clips are tight, use packing/shims to get the right shape.

Picture 2

A - Make a hole in the ground for 1m and put the metal poles inside

B - Drop the seat post onto the metal pole

C - Tie the ropes from the frame to the ground

D - Test as many times as you like - that's it

T notes
A - Hammer a 120cm length of scaffold pole 1 metre into the ground, remove it and empty out the soil, then replace it in the hole. Drop the main Trebuchet pole into the buried scaffold pole.

B - The main trebuchet pole fits inside the seat post of the bicycle frame - make sure it's well seated and all the way inside.

C - Use reasonably large pipes or solid bars for the pegs here - tent pegs won't hold it.

D - When testing the trebuchet, ensure there's a wide, clear area all around and that you watch the release angle - it may fire straight up.

Picture 3

A - The throwing pin. Change the angle of this pin to change the angle of the throw.

B - Fasten on the pin with a jubilee clip. First put rubber strip around it to keep it still then put on the clip. Cover everything with duct tape. Bend up the pin and make sure it can release the sling.

C - This part of the string needs to stay still. Don't let it move.

D - The throwing arm.

Picture 4

A - These strings need to have the same length from the arm.

B - The string needs to be a circle to put on the pin

C - You need to balance the sling pouch

T notes
Picture 4
The strings of the sling need to be fastened to the arm closely together and they need to be the same length, as when the arm moves upward the projectile can be rolled out of the pouch. 

Step 4: Results

On competition day, both trebuchets threw a 330ml bottle of water around 55 metres.

However, the forks trebuchet was the stronger on the day as a problem developed with the frame trebuchet's throwing arm.

During testing, we used a bamboo arm with the frame trebuchet, but found it to be heavy and cumbersome in its movement. We replaced the bamboo arm with an aluminium box section, which unfortunately was not strong enough to carry the forces generated. 

Included is a photo of the frame trebuchet in action with a bamboo arm during development. It was early evening... apologies for the picture quality.

Step 5: Lessons Learned

Both designs worked equally well, with a materials failure stopping one of them.

For the arms, use a 1" diameter bamboo pole - any bigger is too heavy.

Make absolutely sure that that all arms are very well attached to the bicycle parts. We had no problems in this regard, but it needs to be said.

Longer is better with the guy lines... don't place them too close to the trebuchet pole.

More power at the front of the trebuchet gives more power to the throw, but also - the application of a gunner's strength is important. It must be a strong, clean, even pull on the handle and the gunner must follow through with that pull until the projectile releases, (you can feel it).

To a degree, the trebuchets can be aimed. Due to stretch in the guy lines the direction of throw can be changed by the gunner. It's not a wide range of motion, but it's possible.

Safety Safety Safety.

Make sure that everyone on site is awake and aware at all times, and announce each throw with a countdown
Again, we had no problems in this regard, but again... it needs to be said.

Enjoy, and be safe.

Redneck Contest

Participated in the
Redneck Contest