Building a trebuchet is less complicated than it might first seem. Sure, there are specific formulas and software programs that can crank out optimal performance trebuchet designs. However, I know builders who can estimate dimensions, cut out and then assemble their trebuchet hurling machines quickly without the aid of high-tech software analysis. Their trebuchets also perform with amazing distance, accuracy, and consistency. In this short article we will examine key elements to build in the proper sequence and in the proper scale [or relationship] to other components of the final working machine. Don’t be intimidated, it’s really not rocket science.
For most trebuchet enthusiasts, the fun of “hurling” is also in the designing, constructing and modifying of these amazing models. So let’s take a look at the key elements of building a trebuchet.
The base of a trebuchet must support the glide track, tower frame, and swing arm lever. The trebuchet base is typically attached to the glide track [board] upon which the sling harness will slide. There is no absolute formula for designing a trebuchet base. In fact, they come in a myriad of sizes, both in width and length. However, I have found that reliability and consistency is produced when the length of the base is 75-80% of the length of a swing arm (lever). Another good rule of thumb is that the width of the base should be about one-third of its’ length.
The base should be designed with the tower width and counterweight size in mind, since the size of the weight or box will need to swing through without interference. If the attached weight or box is wider than the support [tower] frame it will not pass down and through as it should…unimpaired.
Glide Track [board]:
This is the surface upon which the sling harness slides after the trebuchet trigger is activated. In most cases, the glide path is about three-fourths (3/4) the length of the base. When the swing arm lever is released the attached sling is drawn along the glide path and then whipped upward in an arc path until the cradle holding a projectile opens and releases it.
Tower Support Frame:
When building a trebuchet the tower frame is generally about 3/4 the length of the base. For instance, if the base measures 40 inches, a good tower frame height would be 32 inches. Again, these are approximations. The width between the tower primary support beams will depend greatly upon the type and size of weight planned for use. You will need to decide what size and the weight of an object to throw, and then select the appropriate weight(s) to use. Counterweights should be “approximately” 75-100 times the weight of the projectile.
Swing Arm Lever:
The primary swing board is commonly referred to as a swing arm lever, but some will call it a long arm lever or long arm beam. The swing arm lever works well when it is 1.4 – 1.6 times the base length. So, if the base board length is 3 feet, the swing arm lever could range from 50 inches to 58 inches. This will depend upon the builder and certain other adjustments that can be made, but I would tend to lean to the shorter side…probably choosing a 54 inch length (1.5 x).
The counterweight will ultimately determine the height and/or distance that a projectile will potentially fly. As indicated above, a counterweight that is 80-100 times heavier than the projectile will generally produce optimal results. However, understand that all other features of the trebuchet can serve to enhance or diminish overall performance. For instance, if the swing arm is too short or too long the trebuchet results may be limited.
The sling is attached to the long end of the swing arm lever. It is connected by rope, string or cable to a cradle or pouch holding the object to be “hurled”. The sling harness length should be approximately 3/4 of the glide board. The sling harness should be designed to open and release the object in the cradle or pouch when it reaches the precipice of the launch path. This will depend upon whether one of the sling ropes is attached by a prong or in a slot cut at the end of the swing arm lever. Each trebuchet may have a slightly different release design.
Although not precise calculations used by all trebuchet builders, these primary features of a trebuchet will be useful for any novice trebuchet enthusiast. Understanding the essentail design components should also help you with the building a trebuchet basics.