Help needed building an onager

[P. Clodius Secundus]

Robert,
I don't know which style of onager your drawing depicts, but I imagine it follows one of the three traditional designs that differ mostly in how the buffer that stops the arm is attached. Most build either the Payne-Gallwey with it's vertical four-legged railroad style buffer frame, or the Schramm type with a tripod buffer frame. Both are equally wrong since there is no evidence that the onager ever had a buffer frame to begin with.
The oldest and most historically accurate interpretation, designed for Napoloeon III by DeReffye, had no buffer frame. It used a wooden ramp to substitute for the pile of turf blocks that is supposed to hold the buffer. I've worked with all three types and from my experience the DeReffye is the best. We had one that was originally a Schramm-type. Every time we shot it either the arm or the frame would break. Once I'd convinced it's builder to ditch the buffer frame, extend the arm, and let the it go forward like Ammianus said, the range nearly doubled and the machine stopped trying to demolish itself. While researching the problem for him I realized that the arm should actually be bent. This solves nearly all the problems that many see as inconsistencies in the text.