Help needed building an onager

[P. Clodius Secundus]

Ammianus definitely specifies using a mallet/hammer to release the arm, but a latch and lanyard is certainly much safer.

Both Heron and Vitruvius discuss the methods of stretching and pre-tensioning the bundles. There is also an excellent modern article on spring behavior. I'll try to locate it. If you can get access to a copy of Eric Marsden's "Greek and Roman Artillery: Technical Treatises", it is probably the best resource for catapult research.

According to the sources, they had a special machine to stretch the cord as it was wound through the washers. As each wrap was pulled tight it was locked in place with a wedge until the next pass was made. They "tuned" the strands by plucking them to hear the tone.