King Arthur (KA)

Super propped trebuchet where the arm is locked to the hanger when cocked with the arm pointing almost straight down, and the hanger almost straight up. The hanger is released first and when it gets to a certain point it pulls the secondary trigger releasing the arm. Invented by Chris Gerow who built the original King Arthur (Lots of great information on his site) pictured on the right. Also see the Floating Axle King Arthur


Power Source



  • Long drop for maximum energy
  • Small counter weights can yield very large distances
  • Very tunable


  • Requires some extra complexity
  • Can be hard to cock


There are two triggers for KAs, the primary and the secondary. The Primary holds the hanger inline with the arm initially. It usually consists of a rod through the arm and hanger. With the design, the trigger load is inversely proportional to it's distance from the hanger axle. The secondary trigger is much more complicated. It has to release the arm (allow it to rotate forward) at just the right time, and prevent if from rotating backwards. It is usually released by a line attached to an extension of the hanger (beyond the hanger axle). In the pictures below of mini-ka, it is a thin blue thread. This line is usually attached to a rod (like a nail) that holds the secondary closed.

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Firing Process

See the videos on the Mini KA page for timing and context to go with the descriptions and pictures below.



The KA starts cocked with the arm pointing down and counterweight at the top.


The arm and counterweight locked together by the Primary trigger.


A string, rope or cable is attached the end of the hanger. It connects at the far end from the counterweight, past the axle. It should to behind the axle (away from the throwing direction. It connects to the secondary trigger (see below), and here is called the secondary trigger line. It is a thin blue thread in the pictures.


The arm tip held in place by the secondary trigger, which can be released when the secondary trigger line (see above) is pulled. Once of the simplest secondary trigger designs (pictured), consists of an arm backstop (can be a block behind the arm, or as shown a slot which the Firing Pin fits in.), and a swinging stop that holds the arm against the back stop. The swinging stop can be held in place by having a rod pass through a hole in it, as well as some material above and or below it (the small nail pictured goes through the swinging stop into the base). This tod then simple needs to be pulled out by the secondary trigger line to release.

Primary Release


Once the KA is properly cocked and the projectile is loaded, the primary trigger can be released. This allows the hanger to rotate forward. In the pictured KA, it is done by pulling the yellow string which removes a nail that passes through the arm and hanger.

Secondary Release


Once the hanger gets to about 90 degrees forward, it should pull on the secondary trigger line (requires careful adjustments). This allows the arm to rotate forward. After the secondary is release, it functions just like a hinged counterweight trebuchet.

At Rest



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