NOVA Online | Secrets of Lost Empires | Medieval Siege

Destroy the Castle > How Your Trebuchet Works
Wheels/No Wheels

When modern engineers first examined medieval drawings of trebuchets, they couldn't help but notice that they often had wheels.

Their first assumption was that the wheels were added to roll the weapon around the landscape and to help aim it, which was probably true in smaller machines. As modern engineers began to reconstruct these monster machines, however, they came to realize that they were too heavy to roll on medieval roads, much less battlefields.

They also noticed something else: These rolling behemoths fire appreciably farther than their stuck-in-the-mud cousins. Modern engineers suspect that their medieval counterparts may have observed the same thing and stayed with the wheeled design as they built the machines larger.

Model rocking in action

Why is a trebuchet that rolls forward and back again more stable and powerful than its cousin? Well, one of the design flaws of the wheel-less trebuchet is its tendency to rear up into the air as its arm begins to spin and then slam back down. This monstrous lurch can destroy the trebuchet rather than the castle wall.

Rolling

Rolling wheels prevent the crushing lurch. Instead, energy is more smoothly channeled into the trebuchet's arm and missile.

Diagram of forces

Wheels add power as the trebuchet rolls forward. Like a pitcher who leans toward home plate as he hurls, the forward motion adds velocity to the pitched weight. The rolling back and forth of the wheeled counterweight also allows it to fall in a straighter line, the most efficient way for a counterweight to respond to the force of gravity.

Firing trebuchet