orientation issues. This occurs a lot in games. Notice how elevators in the facility do the same thing when moving up or down. roblox physics relies heavily on parts “connecting” physics with each other. The catapults main body is a separated body from the world, so it has its own physics. The car also have its own physics too, however the catapult is more dominant. The tires try connecting physics with the catapult and the cars body tries connecting with the tires, this loop causes the cars body to bounce up bringing the tires. gravity pulls it down and then the loop starts again. to sum this up
tires connect to catapults physics
car body connects to tires.
body jumps from connection.
gravity pulls it back down.
tires connect again thus starting the loop of tweakination.