Instructables
Picture of Desktop trebuchet from bicycle frame
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!
 
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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.
T3hpw9ag3 6 years ago
yeah, everyone has room on their DESK for a DESKTOP trebuchet of this size
Visitor (author)  T3hpw9ag3 6 years ago
You obviously need a better job. :)
nah, im 14. besides, this thing is a freakin behemoth. nobody has room on their desk for this, let alone office codes of conduct would prohibit such devices.
Visitor (author)  T3hpw9ag3 5 years ago
Our office code mandates that fire shall be fought with fire.
so by what you just said, in event of a fire, use a flamethrower? lol xD
by the way, it seems neat, just a bit misnamed. oh , and lets see how long we can make this reply tree!
It shall go on forever
 yes it must!
 It must never end
Yay, im helping!
This was a thing of beauty but it has now fallen to ruin and has been forgotten
arb1ter4 years ago
can someone convert measurments plz
kingbirdy4 years ago
you do realize this is a trebuchet, don't you?
criggie5 years ago
I'm 75% finished a similar trebuchet - but mine's made with the front forks rather than the back. Forks were removed from the headtube and wedged into a slab of wood as a base (can fit wheels later if the damn thing actually works) I also cleaned and used the hub from the front wheel - cut off all the spokes from the rim and unthreadded from the hub. (saved for some future use) Biggest problem I have currently is attaching the boom to the hub.... currently its off-centered which will probably throw the whole action out of whack. Also, the end of my boom is off-set so the purse comes alongside the shaft, rather than through the forks. Also need to source some kind of weight which will fit through the forks properly - there's not a lot of room for error.
John Smith7 years ago
You could always make a heavy lead weight by using the technique in my instructable
Visitor (author)  John Smith7 years ago
That is going to need a lot of solder. One can use other metals too. I have a smaller trebuchet where I used two large sacrifical zing pieces (those used to protect steel boats from corrosion).
Uranium is probably the heaviest metal out there. I hear there a storage problem, so you might get some dirt cheap!
Vertigo6667 years ago
where d'you find the weights? of course, i could use sometthing lying aroud that happens to be heavy enough.... a large electric motor?
just go to our local tire place ask for 2kg of old tier lead alloy u then melt down in a old kitchen pot and cast into foil try like u get fore one serve of lasagna
Visitor (author)  Vertigo6667 years ago
I bought the weights from a diving shop.
chromedonut5 years ago
This is great. I've always liked hurling machines AND repurposing junk around the garage. This does both with one SWOOP!
I'm going to make one tonight because I've got nothing else to do.
Well done! Perhaps I missed it in the Instructable but what kind of range are you getting? The pictures are very good, I think, but a video for a project like this is a must! I've built several small trebs as well as one large one but never considered the use of a bicycle frame; I think it's quite creative.
Visitor (author)  StepsoftheSun7 years ago
I have not fired it outside yet, so I do not know the maximum range. It shoots a golf ball sized (soft) ball across our office 8 meters with a very flat trajectory. Should do 15 meters outside.
add weels and you git better range
noahw7 years ago
Medieval office warfare - sounds like the right kind of work environment. Nice job!
trebuchet037 years ago
Wow! What an amazingly awesome idea! I'll have to think a little bit and see if I can convert this idea into a FAT type.
How would you create the tracks for a floating arm treb if you're sticking with parts from a bike?
That's what's going tot take some thinking ;) I have an idea for tracks, I'm just trying to come up with a way to make the weights slide properly :)
Jezza Bear7 years ago
Very nice. I was thinking whilst I read this was that if I had a broken bicycle, I will possibly have a unused spindle from the wheel, If I was lucky the quick release type. That would make a great axle. Good job