The medieval trebuchet is a missile-throwing device, with a heavy counterweight on the short end of a beam that pivots about an axle fixed to a supporting frame. A sling is attached to the long end, and when properly "tuned" by adjusting its length, it will release the missile at exactly the moment the counterweight is directly under the axle, and will bring it to a halt by taking all the kinetic energy developed. Disregarding friction, there is a 100% transfer. It's been observed that trebuchets throw further if the frame is on wheels, though the reasons given by constructors are a bit vague. I can see that that backward swing of the counterweight will exert a forward pull on the frame, which if allowed to roll forward will add to the propulsive force on the missile. On the other hand, if the machine is fixed to the ground, then instead of transferring some of its KE to the frame, the counterweight holds onto it and transfers it to the missile via the beam alone. Provided the beam is brought to a halt, there is still a theoretical 100% transfer of KE to the missile, or so it seems to me. What I mean is, I can't see where energy is being lost when the machine is fixed. It must be, since practical experiments show the wheeled-base machine to throw further, but I can't see why. What's the solution?