Leonardo's crossbow designs are a series of shooting weapon schematics designed by Leonardo da Vinci that are in the Codex Atlanticus. One version, a self-spanning infantry weapon called the Rapid Fire Crossbow (Balestra Veloce in Italian), is found on sheets 143r, 153r, and 155r. The other is the Giant Crossbow (Balestra Gigante in Italian) design intended to be a mounted siege weapon found on sheet 149a in the Codex.
The creation of the large design is linked to Ludovico Sforza, an Italian prince in the Renaissance era. Given the constant warfare in the Italian Peninsula at this time, he wanted to expand and advance both his military and the territory he governed in the Milan region. To do so, he wanted to update the current treatise on military engineering by Roberto Valturio. Leonardo responded by writing Sforza a letter that included a number of innovative machine designs with one of them being the Giant Crossbow. Leonardo also highlighted in the letter his expertise in engineering; most likely having known that Sforza was wanting to hire military engineers at the time.
While there is no exact date for the illustrations of Leonardo's crossbows, they are generally believed to be drafted between 1483 and the early 1490s. Many scholars generally agree that Leonardo completed the drawings in the manuscript in Milan, but there is debate as to why he originally went there. Some believe he came to Milan in search of work as a painter and then he got news of Sforza's military desires upon arriving. Others suggest that the initial reason he came directly to Milan was to work for Sforza.
Design
The crossbow as a weapon had been around long before Leonardo's designs. However, his designs made the weapon more advanced and why Leonardo's crossbows stands out. If a crossbow is designed with a narrower shaft and a tapered bolt, which adjusts the nocking of arrows, it greatly improves the airflow of the bow and the drag on arrows. This allows the crossbow to operate much more efficiently and have a more precise aim. These ideas were present in Leonardo's designs and were developed independently of any other influences.
The mathematics that Leonardo utilized to construct his crossbow designs were far advanced despite having some now known inaccuracies with today's current knowledge of geometrics and design. Nonetheless, Leonardo was "the first modern engineer to attempt to apply the geometrical mathematics of the laws of motion to the design of machines." The other mathematical marvel that is noted in Leonardo's designs of the crossbows is the proportional techniques that he utilized in every aspect of the designs.
Mechanically, the Rapid Fire Crossbow's tiller was split into two stacked wooden pieces held together at the front by a hinge and held in place at the back by a spring-loaded latch. Sandwiched inside the tiller, a pair of interconnected folding levers attach the lower half of the tiller to a sliding plank sitting on the top half of the tiller between the two metal prods, similar in mechanical concept to the Gastraphetes, that contained the rolling nut and spring-loaded sear at the back of the plank. Meanwhile, the bottom half of the tiller contained the trigger. Upon pressing a switch on the side of the tiller to release the latch, the user can then fold the bottom half of the tiller down. At the same time, the interconnected levers then push the sliding plank forward until the drawstring is caught by the rolling nut, which is held secure by the sear's spring tension. Upon pushing the bottom half of the tiller back into place, the latch locks the upper and lower halves of the tiller back together. After a bolt is loaded on the crossbow and aimed, the trigger is then pulled to compress the sear's spring and enable the rolling nut to release the drawstring to propel the bolt. Such a design removed the need for external spanning tools that an arbalist had to carry and simplified the arming process of the crossbow.