The diagrams below show how a Trebuchet and similar items of ‘leverage artillery’ worked. The diagrams have been simplified and do not include methods of preparing the trebuchet for firing, or of holding and release methods.
Basically a Trebuchet consisted of a large wooden arm (black in the diagrams) pivoted close to one end. The pivot was connected to a wooden framework (brown in the diagrams). A large weight was suspended from, or attached to, the end the beam nearest the pivot, and a sling to the end of the longer piece of the beam. Only one end of the sling was firmly attached. The free end was slipped over a spigot. When fired, the free end of the sling would slip off the spigot thus releasing the shot.
To load the Trebuchet (Insert - top left of page), the sling end of the beam would have to be wound down and locked in a release mechanism. The sling would be laid out under the beam in a long channel on the floor under the beam, and a shot placed in a pouch in the middle of the sling. The free end of the sling would then be placed onto a spigot. The Trebuchet was then ready for firing. The channel was there to guide the sling and shot in a straight line on firing, and to prevent the sling from becoming entangled in the Trebuchet’s framework.
The release mechanism has been operated and the large weight is exerting a downward force. This raises the other end of the arm, and the sling with the shot is being pulled along the channel and is
leaving the frame assembly. If the sling was too long, the shot would hit the ground and would scrape along it, possibly swinging to one side. The Trebuchet would not fire correctly. This could mean that the shot would cause a hazard to the operators or other members of the besieging force if the Trebuchet
fires at an angle.
The shot has now been swung clear of the frame and is swinging in an outwards and upwards arc. The downwards force exerted by the weight is swinging the arm at an increasing speed. The shot at the end of its sling will continue rotate until the free end of the sling slides off the spigot releasing the shot to carry on to the target.
The use of a sling gave a greater range than if the shot was simply placed in a tray at the end of the arm. The principle is the same as if you threw a ball, and then used a sling to do the same thing. The momentum gained using a sling sends the ball a much greater distance. The Australian
Aborigine Woomera (spear thrower) works similarly to increase the distance of a throw.
The free end of the sling has slipped of the hook or spigot, releasing the shot from its pouch. The shot now continues to its target, and once the Trebuchet's arm comes to a rest it can be readied to fire again.
Several factors affect the range and trajectory of a Trebuchet. The greater the weight attached to the arm, the faster the beam will be pulled down, and the greater the range. A heavy shot would not travel as far as a lighter shot, but clearly would have a more devastating effect. It would also pull the free end of the sling off the hook earlier than a lighter shot. This would result in a higher trajectory, thus affecting range. A shorter sling would rotate faster than a longer one and would consequently release the shot earlier and on a higher trajectory.
If the spigot (right) was in line with the beam, the free end of the sling would be released earlier than it would have been if the spigot was hooked or angled down as shown. The earlier the release of the shot, the higher
the trajectory. A late release would result in a flatter trajectory. The artillery men would have been well aware of the dangers to them of a too early or late release of shot.
As can be imagined firing a Trebuchet called for more than muscle power to to haul down the beam and load it with a shot. The medieval artillery-man would need to know the effects of the weight of shot, the angle of the spigot, and the length of the sling on the range of his particular Trebuchet.
There were disadvantages to using Trebuchets during a siege, as they did not have the range necessary to keep them away from archers. A Trebuchet or Mangonel could send a 25Lb (11.5Kg) stone some 175metres with considerable accuracy, but a longbow easily outranged them. As the French found to their cost at Crécy and Agincourt, an English archer with his longbow could send 15 arrows a minute to over 275 metres range. 1000 archers could fire 15,000 arrows in one minute, so it literally rained arrows on the enemy.