Years ago I purchased a "Roman Stone Thrower" in 28mm. It was a nice metal kit with seven or eight crew. The model seemed to be a medieval catapult with Roman crew. Over the years I would pull it out and work on it a bit, then put it back in the 'TBD pile'.
The stone thrower had wheels, which I have never seen any evidence for on a Roman onager and the crew were a bit chunky (think Michelin tire man).
During my recent painting spree I decided to take a look at the onager again. Sometime in the past I had assembled (mostly) and primed it and the crew. I was unhappy with the wheels... they weren't well cast and they seemed out of place to me, so I removed them. Removing the wheels lowers the height, which makes the crew a bit to tall and the winch 'wheels' sit on the ground. I used some balsa wood to make a frame to raise the onager back to the proper height, added a few details and mounted it on a base.
I decided to not use all the crew, while I am sure an actual onager had more than the five I am using, mounting more on the base made the whole thing look like a Chinese fire drill (chaotic).

