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"Cum catapultae proscriptae erunt tum soli proscripti catapultas habebunt."

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Trebuchet Construction Hints and Tips

This website has been up since 1996 and I continue to get regular requests from readers for help in building their trebuchets. It didn't take very long to begin to see patterns in the questions and this update to my website should help you build your machine.

Q: Do you have detailed, step-by-step instructions?

A: No. Something you should keep in mind is that I built all of my trebs without plans. I had a picture and made some guesses about lengths and just started cutting and drilling. The plans were drawn later.

Even so, here are some basic hints:

  • Start at the bottom and work your way up. Build the base first. Then add the vertical piece for the throwing arm. Add the diagonal supports, the arm and the arm cap.
  • Pre-drill the holes, especially when building the smaller treb. If you don't pre-drill, the wood may split.

Q: Should I use #10 flathead screws or #12 round head screws?

A: Who cares? This isn't rocket science. Use whatever will hold the wood together. While the smaller screws are less likely to split the wood, slightly larger screws may hold tighter. But, this is a small model. The extra strength probably isn't necessary. Use whatever you have. You could use nails, dove-tail joints, fitted pegs, wire or cord. Whatever works.

Q: How long is the (insert piece here)? The plans don't have a measurement noted

A: The drawings are to scale so with the measurements noted your should be able to calculate the unnoted leangth using a simple proportion. But, again, precise measurements aren't necessary.

Q: My treb throws too high/low. What's wrong?

A: The sling adjustment is the trickest part of the trebuchet and the key is in the little hook on the end of the arm.

As the sling swings out, it attempts to go straight with the arm. There comes a moment when the projectile is traveling at a 45 degree tangent to the arc of the arm and, at that moment the sling should slip off the hook.

If your machine is throwing like a baseball or not releasing at all, the sling is slipping off too late. Straighten the hook a little so that it slips off sooner.

If the machine is throwing with a very high arc, straight up or even backwards, the sling is slipping off the hook too soon. Add a little more bend to make it slip a little later.

Q: I make a very small adjustment in the hook but can't get the perfect throw.

A: The movement of a pendulum is based on the length. A long pendulum moves slower. The same is true for the trebuchet sling. A short sling will swing out faster and small adjustments in the release hook angle will make a big difference because of the shorter amount of time available.

Use a longer sling and small adjustments in the release hook will make small adjustments in the release angle allowing one to fine tune the range.

The length of the sling also has an effect on the release angle and thus the range but it's usually easier to adjust the hook than to change the length of the sling.

Q: My machine tries to flip itself when it throws.

A: The counterweight may be too heavy.

There are two basic concepts of trebuchet. A fixed counterweight and a free counterweight. The machine I have on my website are free counterweights. The swinging bucket allows the weight to fall more vertically and is thus more efficient in converting that vertical motion into the rotational motion of the throw.

Fixed counterweight trebs do not have a bucket. They just have a heavy weight on the end. The motion of the counterweight falling tries to pull the treb over. A counter to this is to put the trebuchet on wheels. So, as the counterweight falls, the treb rolls forward, allowing the counterweigtht to fall more vertically.

There is a new design for a treb which has the fulcrum on wheels while the counterweight drops absolutely straight down on a track. It is the most efficient way of doing it but such a design was beyond the engineering technology of ancient times.

Q: Why aren't there plans for "Bagelchucker"?

A: Simply stated, "Bagelchucker" was not a very good design. When my original design, called "Eli" didn't get built for various reasons, I fell back on a design I could throw together quickly. It's based on two tripods and, in actuality, when you pull down on the one end, it over-balances and tries to fall over on top of you. That's part of the reason I was wearing a helmet. If you want to build a traction treb, I advise visiting the Grey Company's Traction Treb site and build something like "Jake".

Q: What does "Cum catapultae proscriptae erunt tum soli proscripti catapultas habebunt." mean?

A: In Latin, it says, "When catapults are outlawed, only outlaws will have catapults." -- Revised: 27 May 2002
Copyright © 2002 Kevin A. Geiselman