Floating Axle King Arthur (FAKA)

The FAKA is a trebuchet design invented in late 2006 by Craig Macomber. It stands for Floating Axle King Arthur. It combines the efficiency of a Floating Arm Trebuchet (FAT) with the increased potential energy of a King Arthur.
Basically, it gets a lot of energy for it's size and weight, and uses it very well, but there are many challenges. A FAKA is a very high end trebuchet form the efficiency perspective. They work great over a huge range of mass ratios. The FAKA is very efficient for its frame height and arm length as well. FAKAs are hard to build, and have have a tendency to go very wrong, and often break things, if the timing is a little off.



This video shows the development and perfection of the FAKA design. It contains lots of video of the first three known FAKA trebuchets. It contains over 50 throws, misfires, or dry fires.

More Videos

Here is a collection of FAKA videos:

Disadvantages of the FAKA

The combination of FAT and KA does have a cost. It's hard to build, and unstable to operate. In the simplest setup, there is nothing designed to absorb energy, and it can not swing back and forth like most trebuchets until it stops. The leftover energy goes into battering the axle trucks into the stops, and the hanger into the upper arm stop. Of course, when properly tuned, there is very little energy left (The FAKA can be tuned for very high efficiency), but before its tuned, or in the event of a misfire things can go quite wrong. Several arm derailments have been caused by this.

My FAKA has misfired once, and it lead to the wheels smashing into the front stop and derailing the arm. This was with 5 pounds of CW. It has derailed several other times, but so far no major damage has occurred.

Also, the Golf Ball Beast lifts the back of the frame just before the arm is released. This is bad because it adds stress to the frame when it re-contacts the ground, and could (And I think it did) derail the axle.1

Design of a FAKA

FAKAs have several parts, most of which are similar to other types of trebuchets.
Below is a picture of the first FAKA ever made with the important parts which are shown labeled. The carriage, which ride the tracks, need to prevent the axle from twisting relative to the frame, if one side gets ahead of the other, it will likely jam, but the counterweight or the arm might also strike the frame. This is why more than one wheel is usally used. The further between them the better they will reduce this twisting.

Secondary Trigger Types

No Secondary Trigger

Medieval-Postal-Service is an example of a FAKA with no secondary trigger. The arm and axle are allow to move as soon as they are pulled/rotated in the forward direction. This means no restraint (such as the upper arm stop) is needed to prevent that axle from rolling forward before the secondary trigger releases the arm. To facilitate this, the counterweight is actually cocked slightly backwards of the main axle, and is triggered by pushing the arm tip towards the back until the weight begins to fall.

On Carriage

J-Buchet has the secondary trigger mounted on the carriage. It prevents the arm from rotating until released. To prevent premature axle roll, they have a small back stop on the tip of the arm. An upper arm stop would also work.

Traditional KA Secondary


In this setup, the triggers are exactly as they are in a King Arthur. The arm tip is held between a backstop and the secondary, which gets released. If the axle were allowed to roll forward before the secondary released, it would pull the arm tip out of the trigger causing a misfire most likely. To prevent thus, a par is installed which prevents the axle from rolling forward until the arm rotates, which only happens after the secondary trigger releases. This bar, called the upper arm stop, can be placed just below the axle (as it is in K'nex FAKA pictured, and in GBB), or much lower (where it could be called a mid arm stop). The upper/mid arm stop is very must hold the carriage or trucks tight against the stop. If it does not, the arm will slam into one then the other before it is released. The stop is also a problem because after the throw the hanger can slam into it quite hard. On a larger FAKA, Golf Ball Beast, it can be hinged so it can get knocked a little bit out of the way.


The triggers for FAKAs are the same as those for King Arthurs, but be aware of the different secondary trigger methods mentioned above.


Adjust the secondary trigger to get the horizontal momentum of the arm assembly to about zero. A earlier secondary release means more forward momentum, and a later secondary. As the mass ratio increases, some backwards inertia, and a later secondary may be favorable. The sling and thus release angle and time tune normally.


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