Ancient Engineering
The Atomic Bomb of the Middle Ages
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The Original Floating Arm Trebuchet!

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Here it is in the cocked position. The counterweights are steel plates that sit inside two wooden boxes, one on either side. The fulcrum is the wheel on top of two tracks.

Here's a picture from the back (the boxes are empty because the steel plates are missing in this photo):

What keeps the wheels on the track? Very narrow tolerances. The CW boxes and the channels ensure that the end of the arm stays centered, and there is only a 1/4 inch clearance on either side of the arm between the tracks.

When the trigger is pulled, the CW drops straight down in the channels, pushing the arm backwards on the wheels and simultaneously rotating it.

Once the weight has passed through the middle, it begins to jerk the arm forward while continuing to rotate it. Notice how the axle is direcly over the counterweight at the bottom of the drop.

In a dry-fire situation, there is no stall or wobble at all and the momentum of the arm carries it through, lifting the CW a bit in the process, and then it all comes back. A normal oscillation.

When firing a 10-12 lb load however, the pull of the projectile on the arm tends to stall it in exactly the vertical position, leaving zero residual motion in the machine. A perfectly tuned machine!

Here's the front of the machine showing the trigger assembly (which happens to be sprung open in this photo). The trigger is at the top, between the two CW boxes.

Here's a close-up of the trigger in the set position. It is two pieces of angle iron. One piece supports the end of the arm, and the other piece supports the first piece. A loose clamp holds them together. Pull the clamp out, and the trap-door falls open releasing the arm.

(to see an animation of the trigger in action, go to )


  • Arm: 3 feet from CW to axle, 6.5 feet from axle to pin.
  • Height: 5 feet to track, 8 feet to top of channels.
  • Wheels: 10 inches
  • Sling: 7 feet long.
  • Total counterweight: 470 lbs.
  • Total drop: approx. 5.5 feet.
  • Range: slightly better than 300 feet using 10-12 lb projectiles.

To see some shots of this thing in action, check out the Sherman Oaks Skeet Club and the videos link from the home page!
What's the point?
As you probably know, trebuchets traditionally consist of an arm resting on an axle, which rests high on a base structure. The arm of the trebuchet is like an off-center see-saw with a huge counterweight on the short end and a sling attachment on the long end.

This arrangement worked great for the last 700 years or so, but recently people have begun to study the mathematical efficiency of trebuchets and are attempting to make them even more efficient. In this quest, there are great debates about what should work best. This design is just one more experiment in that spirit.

But what do we mean by "efficiency"? It's fairly easy to calculate the energy generated by a dropped weight. It only depends on the amount of weight and the distance dropped. You can measure the height difference of the weight on a treb between the cocked and un-cocked resting positions, and from that know how much energy you have to work with. Then calculate the maximum possible distance that much energy can propel a projectile of a certain mass (assuming a perfect trajectory). It's all very easy, but misleading.

The problem is that the ratio of the short arm to the long arm makes a big difference. Since gravity always works at a constant acceleration (and gravity is the only source of power for us) then people tend to amplify this acceleration by making the short arm a small fraction of the long arm. If the short arm is 2 feet long, and the long arm is 10 feet, then the velocity of the tip of the long arm is 5 times that of the short arm. But, then the counterweight can only drop a maximum of 4 feet! (from 2 feet above the axle to 2 feet below it. In practice it would actually drop much less than this) You could get much more power by shifting the ratio of the arm and letting the counterweight drop further, but this reduces the amplification effect. Basically what happens is that the machine becomes less efficient, but due to the extra power from the longer drop, it can throw the projectile further. With the same weight, a LESS "efficient" machine can potentially throw FURTHER!.

So, what to do. One of the problems with a longer drop is that the counterweight revolves around the pivot point of the axle. Some of the energy is used in pushing the weight out and back horizontally as it moves around the axle. This can be solved by hanging the weight from the end of the arm, and propping it up so that when it falls, it tends to fall straighter. But this creates a problem at the bottom of the drop. Since the counterweight will be far out in front of the axle, it hits the bottom of its drop while the arm is still leaning towards the back. This causes the arm to stall, and if it happens too early it will rob energy from the projectile. So with a fixed axle machine, the longer the counterweight drops, the earlier the arm will stall, and energy is lost.

It seemed to me that a good machine would let the counterweight fall absolutely vertically, the way gravity works best. The solution- Rather than swing the counterweight around the axle, let the axle move out of the way so the counterweight can fall straight down. Then the axle should move BACK so that the counterweight doesn't stall the arm.

This machine does that by letting the axle roll freely on a track, and keeping the counterweight in a vertical channel. As the counterweight falls, it pushes the arm away, then jerks it back with the full force of the weights dropping. The axle passes OVER the weights, and the momentum of the arm carries through and pulls the weights back up a tiny bit before sliding back to the rest position. Since there is no stall, the ratio of short arm to long arm can be much smaller. In this case it's about 1 to 2 instead of the more common 1 to 5 or 1 to 4. I lose some amplification, but I more than make up for it with increased power from a longer drop. It's a very efficient machine.

Tell your friends!


Interesting Notes

Some Trebuchet History:

From the 13th century writing: "Itinerarium Peregrinorum et Gesta Regis Ricardi"

In June and July of 1191, Richard the Lionheart (the Duke of Normandy) laid siege to the city of Acre as part of the medieval Crusades.

The Duke concentrated on constructing siege machines and placing trebuchets [petrariae - literally, stone hurler] in suitable places. He arranged for these to shoot continually day and night. He had one excellent one which he called "Bad Neighbor" [Malvoisine]. Its continual bombardment partly destroyed the main city wall and shattered the Cursed Tower. On one side the Templars' trebuchet wreaked impressive devastation, while the Hospitallers trebuchet also never ceased hurling, to the terror of the Turks.

Besides these, there was a trebuchet that had been constructed at general expense, which they called "God's Stone-Thrower". A priest, a man of great probity, always stood next to it preaching and collecting money for its continual repair and for hiring people to gather the stones for its ammunition. This machine at last demolished the wall next to the Cursed Tower for around two perches' Length [11 yards or 10 meters].

The count of Flanders had had a choice trebuchet, which King Richard had after his death, as well as another trebuchet which was not so good. These two constantly bombarded the tower next to a gate which the Turks frequently used, until the tower was half-demolished. Besides these, King Richard had two new ones made with remarkable workmanship and material which would hit the intended target no matter how far off it was. . . . He also had two mangonels [traction trebuchets] prepared. One of these was so swift and violent that its shots reached the inner streets of the city meat market.

King Richard's trebuchets hurled constantly by day and night. It can be firmly stated that one of them killed twelve men with a single stone. That stone was sent for Saladin to see, with messengers who said that the diabolical king of England had brought from Messina, a city he had captured, sea flint and the smoothest stones to punish the Saracens. Nothing could withstand their blows; everything was crushed or reduced to dust.