How I created a desktop trebuchet (also known as a counter weight catapult) from the rear fork of a bicycle.

List of parts:

One unserviceable bicycle frame (rear fork must be good).
4kg of lead (two 2kg diving weights).
10mm bolt or threaded rod and nuts.
12mm metal pipe (10mm inner diameter).
5mm screws and nuts.
Wood.
Scrap metal.
Thick cloth (a pair of old jeans, perhaps).
Old leather belt.
Some fishing equipment (they are really practical).
Wood glue.

You will also need basic wood and metal working tools, like a drill, drill bits, hack saw, file, etc. The bicycle frame is the only mandatory part. You can improvise pretty much all the other parts. My parts don't actually match the list exactly, because I used all kinds of leftover stuff I already had.

Keep in mind that a trebuchet with 4kg weight is really not a toy for small children. The sling release pin is practically a metal blade that reaches a very high velocity. It will tear skin badly. When firing, keep in mind that the ammo will probably be thrown to the exact opposite direction than where you aimed. So make sure everyone stands next to the trebuchet (not in front of it, not behind it). USE AT OWN RISK!

Step 1: Frame and Axle

My first step was to cut the rear fork form the bicycle and construct the axle. I cut the chain stays as close to the bottom bracket as possible and performed a similar operation on the seat stays. This leaves some room for shortening and trimming the stays later.

The axle consists of a threaded rod, a metal spacer pipe, washers and nuts. My spacer is 55mm wide. The width of the spacer determines the width of the throwing arm. For this throwing arm design the spacer should be slightly narrower than the weight. Note that the chain side fork has an odd kink that is supposed to give room for the freewheel (a really odd 70's design). Your forks will not have those and you may need to add a few nuts inside the forks to give more space for the weight. So leave the axle a bit longer so you can experiment. It is easier to remove the excess width later than it is to add more.

If you look closely you will notice that I used an 8mm rod and 10mm pipe for spacer. This is not as good as 10mm rod and 12mm spacer because the drop outs in the fork are designed for a 10mm axle.

Step 2: Throwing Arm

Next I created the throwing arm. I took two 50cm wood strips (1 cm thich, 2cm wide) and planed a taper to the ends so that I could form an A-shaped form.

First I drilled the axle holes. This is an important step since it finalizes the length of the throwing arm and dictates the optimum size for the ammo. My throwing arm is 50 cm long. The axle hole is 36cm from the arm tip and the weight fasteners are placed 13cm fro mthe axle. 36/13 gives a 2.7 ratio for the arm, which is probably a bit too small. When measuring the hole, keep in mind that the weight must be able to swing below the axle without hitting the base. Again, remember that it is easier to make something shorter than longer.

After drilling the axle hole I inserted the spacer into the holes (the spacer sit quite tightly in the holes) and glued the tapered ends. The spacer kept the holes aligned properly. After the glue had dried (or cured, since I used epoxy) I glued in the horizontal struts. After the gluing was finished I drilled a hole for the sling release pin axle to the arm end. Then I took a narrow saw and cut a slot along the glue joint for the sling release pin itself.

In the picture can see th finished trebuchet, but don't jump ahead yet. I will next instruct you to trim the chain and seat stays and build the wooden base.

Step 3: Trim Frame and Build Base

After the throwing arm I built the base for the trebuchet. The base keeps the chain and seat stays from moving, provides a launching strip for the sling and fastenings for the trigger. Before I could build the base I had to trim the chain and seat stays to correct length.

Since the trebuchet I was going to compete against had 30cm axle height, I was going for 30cm too. I don't think I mentioned that this trebuchet is intended for inter-office warfare, did I? This trebuchet will increase my range of fire and give me an upper hand against my boss' trebuchet.

I set up my frame like in the picture, using the axle without the throwing arm. This is not optimal, since it is very difficult to get the drop outs aligned parallel to each other. I measured 35cm from the center of the axle along the outer side of each stay and cut the stays diagonally. It was not very accurate, since the other drop out is bent, but it worked Ok.

After trimming I drilled holes for the base, fastened the crosswise struts and glued the launch strip for the sling. You could use for example a metal sheet bent to low U-shape for the launch strip, but I think this approach is a bit more rigid.

Step 4: Weight

The last big step was building the weight. I purchased two lead diving weights and hung it to the throwing arm with a piece of leather belt and metal plates cut from the same bicycle chainguard as the sling release pin.

I cut the belt and attached the metal plates to it, estimated the height of the weight from the bottom to the upper fastening holes and subtracted that amount from the distance between the axle and the base. Then I subtracted a few centimeters (just to be safe) to get the distance between the weight fastening holes and the axle.

You absolutely DO NOT want to MELT LEAD! http://en.wikipedia.org/wiki/Lead#Health_effects

Step 5: Sling Release Pin and Trigger

By now all "heavy" work is done and the rest is just tinkering. The first small part is the sling release pin.

I made the sling release pin from a discarded bicycle chainguard. All it needs is a smooth pin and holes for the trigger pin and sling fastener. The ring used to fasten the sling to the pin is normally used for attaching fishing hooks to lures. Fishing equipment is really good for all kinds of things.

After the sling release pin I built the trigger. The trigger design is simple, a metal ring, attached to a wire, is placed over the screw that holds the sling release pin in place. To fire the trebuchet, pull the ring off the screw and the throwing arm is released. Fishing equipment is again used extensively here.

The sling release pin can be adjusted by loosening the nut a bit and tilting the pin. The more the pin points forward, the later the sling is released and the lower the trajectory will be. You need to adjust the pin angle every time you change the ammo weight, sling length or weights.

Step 6: Sling

I made the sling pouch by cutting a key strap into four pieces of equal length and sewing them together. My method was quite tedious to do without a sewing machine. The sling pouch is pretty thich in the ends, so don't break your sewing machine or fingers. You should improvise here again.

I sew the strap pieces so that in the middle they are side by side overlapping only slightly and in the ends the pieces are completely overlapped. This creates a nice bag in which the ammo sits snugly when firing.

The pouch is attached to the throwing arm with two strings. The other string is fastened to the smaller trigger pin hole. The other pin has a metal ring that is placed on the sling release pin. The strings should be measured so that the sling is as long as the throwing arm. Making it longer will usually be inefficient. If you fire heavier ammo, you will have to make the sling shorter.

Step 7: Improving the Design

I have now described how I built a trebuchet. I wish that you do not follow my instructions blindly but think for yourself, IMPROVE them and write an instructable from your work. I found Instructables after I had finished the trebuchet so therefore this instructable is not as good as it could be. I apologise for the bad pictures. It is 21:30 now, the sun sets before I get home from work and I do not have proper lighting for taking pictures.

I have few ideas for improving the trebuchet. I will do them if my boss manages to build a better trebuchet (medieval arms race at work, brilliant). I'll disclose them and challenge you to implement them.

1. The axle could and should be made of a front wheel hub of a bicycle. I would use metal pipe for the arm and support it with bicycle spokes. It would look great and work like charm.

2. Adding wheels to the trebuchet improves it's efficiency. The whole frame moves forward when it is fired. This means that the axle and the throwing arm moves too giving the ammo more velocity. I already have small wooden wheels waiting.

3. The sidewise wood strips on the base could be replaced with pieces from the main tubes from the bicycle frame. Drill holes to the tube through which the seat and chain stays can go through.

4. The launching strip could be made of a slightly flattened main tube that is cut open lengthwise. A mountain bike frame with an oversize diagonal tube would oh, suit you, sir! All in all, the amount of bicycle frame reuse could be improved

5. The throwing arm contains a lot of excess wood. The wood strips could be tapered to make it lighter. After all, every gram that is removed from the throwing arm goes right into the ammo. This could be "micro management," though.

6. Make the sling using old derailleur or brake cable and old bicycle tire.

This is no rocket science. They built trebuchets in the 12th century. How hard can it be?