Theory

The trebuchet, unlike the mangonel or ballista, uses gravity to give the projectile speed. On the basic level, the trebucet works on the see-saw principle--a counterweight brings down one end of a lever, sending the other side, the throwing arm, into the air and launching the projectile. Several innovations have dramatically improved the range of the siege weapon.

A sling, David and Goliath style, attaches to the end of the throwing arm. One end is fixed on, and the other end is looped on a rod that is angled near-parallel to the main arm. Once the sling is moving fast enough, the inertia from the projectile pulls the looped end off the rod. The sling then opens, sending the payload flying thru the air. The angle of the rod can be adjusted to change the flight path higher or lower.

Another refinement is to make the counterweight fall straight down as opposed to travelling in an arc. This increases the acceleration of the throwing arm and doesn't rock the frame around as much. A few methods have been developed to accomplish this.

	Wheels When a fixed counterweight trebuchet is fired, the falling weight shakes the whole unit, and in the case of some smaller designs, actually tips the machine over. For stabilization, wheels can be added to allow the trebuchet to move where it needs to. A significant increase in range also results--the projectile is launched up to 33% farther. Wheeled trebuchets boast the highest range for a given size because the counterweight can be raised higher (allowing it to fall farther). It is best suited for small designs, however as the size increases, finding areas where the unit can role becomes difficult. It also becomes an engineering feat to construct a frame that can withstand the motion.
	Swinging Counterweight A simple, scalable, effective method is to have a swinging counterweight. The weight is held on to the arm by a rod or rope and is therefore able to fall in the most natural path. Almost all of the massive trebuchets are built in this way.
	Floating Arm In both other designs (and for catapults), there is a point on the throwing arm that does not move, at least with respect to the frame. This is not the case with the floating arm trebuchet. In the FAT, the swivel point is mounted on wheels rather than the frame. The arm rolls along the top of the frame on a horizontal track; the motion allows the counterweight to fall straight down, and gives the projectile additional forward momentum. The FAT is a only recent trebuchet design, but it is the most efficient in terms of using available energy usage.

The range and size of the projectile are determined in part by the amount of energy available. This in turn depends the mass of the counterweight and the distance that it falls, since energy equals force times distance (E=F•d). Therfore, if either the counterweight mass or the height it falls were increased two fold, the available energy would double.