LEGION XXIV TREBUCHET PAGE
History and Details on Trebuchets
The Commander's Trebuchet in Action
Updated July 29, 2009
The Trebuchet did not exist to be used by the Roman Legions, although the Onager catapult developed by the Romans, equipped with a sling, foreshadowed the mechanics and action of the Trebuchet of the Middle Ages. This one-20th scale reconstruction of a 40 foot tall machine, weighs a total of fifteen pounds with ten pounds of #9 lead shot in the "counterpoise" or counterweight and can throw a 1.5 ounce golf ball 40 feet ! The throwing arm is 17 inches long from the fulcrum pivot and five inches from the fulcrum back to the counterweight suspension point, giving a power ratio of 3.5 to 1. The machine stands 31 inches tall "at rest" with the throwing arm in the vertical position and the counterpoise hanging beneath it.
Front and Right Side Rear and Left Side
Above - The Trebuchet is "cocked" and ready to be discharged. Note that the throwing arm points away from the target and is "cocked" to the rear of the machine. In the Left View, the sling containing the projectile is resting at the front of the tray with the sling lines leading back to the release prong at the end of the throwing arm, which is being restrained by the trigger pawl in the Right View. The tray or trough guides the sling and prevents it from becoming caught up in the framework of the machine.
Close-Up of Trigger and Sling Tray The Trebuchet "At Rest"
LEFT Close-Up View of the trigger pawl, at the rear of the trebuchet, showing the sling release ring engaged on the prong on the end of the throwing arm. The sling and projectile are resting in the tray at the front of the machine. RIGHT The Trebuchet "At Rest" with the throwing arm in a vertical position with the counterpoise weight hanging under it and the sling hanging from the upper end of the arm. This is the attitude of the machine when not is use and prior to being cocked, loaded and discharged.
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THE TREBUCHET IN ACTION WITH A 1.5 OUNCE GOLF BALL
LEFT - A split "second" after "release", the sling and projectile have been dragged backward off the tray as the counterweight descends lifting the machine's throwing arm, and RIGHT - causing the sling and projectile to accelerate into an arc toward the discharge point. It only takes about a "second" for this small trebuchet to accelerate and throw the projectile. Full size machines would expend up to five "seconds" to accelerate and release a projectile weighing up to 1000 pounds and then cast it several hundred feet.
LEFT - The sling and projectile is nearing the apex of its arc, as the counterweight approaches bottom dead center, when the free end of the sling line will slip off the prong at the end of the arm, releasing the projectile to fly down range. RIGHT - The projectile is seen moving off to the left after the ring on the free end of the sling line has slipped off the release prong, opening the sling pouch to discharge the projectile on its way. The throwing action is similar to an "overhand" baseball pitch.
The machine returns toward the "at rest" position, as the empty sling flails about.
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TREBUCHET HISTORY AND DETAILS
The GRAVITY - COUNTERWEIGHT TREBUCHET was the most powerful and efficient non-explosive mechanical weapon ever developed by man and was literally the heavy siege artillery of the Middle Ages during the 14th (1300's) into the 16th (1500's) Centuries. It was developed with but one purpose in mind - to attack and demolish walls and fortifications. It is thought that a small type of trebuchet may have been developed in China as early as 300BC. The concept of the larger "gravity" powered trebuchet originated in China in the 1100's and came to Europe in the 1200's. Machines up to 50 feet tall at rest, with counterweights of many tons are known to have been deployed in various sieges. Modern full-size recreations have been used to throw 400 pound upright pianos, and even a Volkswagen Beetle automobile. They could be very accurate and were capable of impacting the same place on a wall time after time, after being adjusted or "tuned" to cast projectiles of a certain weight over a specific distance. The term Trebuchet comes from the Old French word "Trabucher, meaning "to overturn" or "to fall-over". In the English of its day, it was known as a "trip-gate" or a "trap-gate". A stone from it was "trapped" at the target from which we get our modern sport of "trap" shooting at an object thrown by a machine. The Trebuchet's seemingly simple design belies the elegant and sophisticated engineering that made the Trebuchet probably the most efficient mechanical artillery ever designed.
Large Trebuchet of 1385 with swinging counterweight - Osprey New Vanguard #58 Medieval Siege Weapons (1) Illustration by Sam Thompson www.ospreypublishing.com
Gravity powered "Counterpoise" trebuchets were massive and difficult to transport and were frequently constructed "on site" at the siege location and pretty much stayed where they had been built. Some of the largest machines used "human squirrel cages" or animal powered windlass devices to pull down the Virga (throwing arm), thereby lifting the counterweight, to cock the weapon for discharge. The "rate of discharge" was slow; but each shot could be devastating. The earliest gravity trebuchets had a counterweight fixed rigidly on the rear of the virga arm. While simpler in design, the centrifugal force of the descending mass of the counterweight; (which wants to fall straight down instead of in an arc), tends to upset or move the machine forward and then backward as it swings. The placing of wheels on this style of trebuchet allowed it to move forward and backward with the swinging action of the counterweight; but also could leave the machine in a different position from which it had been discharged; thus requiring effort and time for the "treb" to be brought "back on station" and re-aimed for the next shot. The use of a "free-swinging" counterpoise allows the counterweight to descend in a vertical falling motion and imparts a more efficient and powerful force to the throwing arm. The "engine" also remains more stable and stays in place and "on station"; permitting a saving of effort and time in not having to "re-stage" the machine for its next shot. Also, the swinging counterpoise dampens the swinging action of the throwing arm after discharge, while an arm with a fixed weight will continue to swing like a pendulum for some time after discharge and due to its heavy mass, is more difficult to bring to "rest". Most of the great trebuchets of history were probably of the swinging counterpoise design.
The TRACTION TREBUCHET was a smaller, shorter range and more portable machine that was "human powered", and therefore, was much easier to construct and move from place to place and siege to siege. Instead of a massive counterweight, the "Traction Treb" employed a group of people pulling down on ropes attached to the rear or short end of the Virga (throwing arm) to provide the weapon's throwing power. The mechanical action was the same; but the weight of the projectiles was much less and the rate of discharge was much faster, up to several shots a minute, as opposed to several minutes to "tens" of minutes per shot with a large counterpoise type machine.
Traction Trebuchet of 1115AD being powered by 20+ women. This machine has a spoon instead of the usual sling on the end of the throwing arm. - Osprey New Vanguard #58 Medieval Siege Weapons (1) Illustration by Sam Thompson www.ospreypublishing.com
The smallest Traction Trebs could be powered by the weight and pulling strength of one person using a single rope; but most were designed and sized to utilize from 20 to 100 men and/or women, generally two per rope, using their combined weight and pulling strength to power the weapon. These Teams would frequently be local non-combatants (women, children, older men) assisting in the siege or in the defense of their town. Traction Trebuchets had a range of from 200 to well over 300 feet (60 to 100+ meters) casting weights up to 130 pounds (60kg).
The Romans used the "Beam & Sling" concept of the Trebuchet on many of their torsion-powered Onager catapults, also termed "Mangonels" (see Onager on our Weapons Page); but it was the Chinese, in the 3rd to 5th Centuries BC, who devised the idea of mounting a large beam and sling on a tall wooden frame and utilizing human energy to power it. Traction Trebuchet type weapons reached the middle-east by the 7th Century AD and into Europe some time later. The Trebuchet was made largely obsolete by the development of large caliber gunpowder weapons in the 1500's - The first of which (cannon barrels cast on the field of battle) were utilized by the Ottoman Turks in the siege of Constantinople in 1453. The last recorded use of a large siege trebuchet was in 1521; when the Spanish "conquistador" Hernando Cortez besieged Tenochtitlan, now Mexico City, in his campaign to subjugate the Aztec Empire.
The mechanics and action of a trebuchet appear to be simple; but they are actually quite complex and involved. The trajectory is largely determined by the projectile's weight and the bending or angle of the release prong at the end of the arm or "verge". A change of the projectile's weight will alter the trajectory unless an adjustment of the prong angle or sling length is made. The range is a factor of the mass of the counterweight, verge/arm length and the sling line length along with trajectory adjustments.
The basic mechanics of a trebuchet are:
Shorter sling lines allows the arm to swing through a greater arc before the sling can catch up to it, meaning a faster sling rotation around the end of the arm in order to catch up, which results in an earlier release yielding a higher trajectory.
Longer sling lines mean slower sling rotation and later release and a flatter trajectory.
A less hooked or straighter prong at the end of the arm yields an earlier release from the prong and a higher trajectory.
A prong more hooked or forward pointing causes a later release from the prong and a flatter, lower trajectory.
A counterweight bucket twice as long, twice as wide and twice as deep will contain "eight" times the volume and weight of material of the same density and mass.
A heavier projectile, with other factors unchanged, pulls the sling free of the prong sooner and results in an earlier release and higher trajectory.
A lighter projectile results in a later release from the prong and a flatter, lower trajectory.
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