A f2k trebuchet has an arm that rides on wheels, which roll forward on rails, while the weight bar running through the arm falls down in a vertical groove. (Watch plenty of YouTube videos to understand the motion of this design) The wheels start off above the rails, and the arm is initially supported by a second set of wheels (or one wide wheel) on a stationary axle fixed between the two halves of the frame near the rear end of the rails. The wheels on the arm hit the rails and immediately begin rolling forward, and the arm tip moves in an elliptical path.This is supposed to be a very smooth, quiet transition and the actual impact is very low.
Here's a YouTube video of our trebuchet:
This design was for a school ASME (American Society of Mechanical Engineers) Club competition. The parameters for the contest were as follows: Design a trebuchet that launches pumpkins (about basketball size). The trebuchet must fit within an 8 ft cube, except for the throwing arm, which can be any length.
Our design was very stout and held around 720lbs in counterweight, not including the weight bar and arm itself.
We launched pumpkins and bowling balls around 10-15 lbs at distances of 300+ feet.
Caution: This particular design is a very time intensive project (expect 60+ man hours) and requires some metal fabrication, such as cutting, welding, and drilling.