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!

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.