A-Chassis truck bouncing around?

(None of the models I’m using are mine! I’m just using them for fun! I also assumed scripting support was the right topic, if it isn’t I’m sorry. Might fit better in building support.)
I’m trying to make an A-Chassis fire truck.

What’s weird is, whenever I start the game the truck just starts bouncing around.
video: (Sorry for the bad quality! :frowning: )
1.wmv (2.5 MB)

I have searched it up everywhere I can and it seems that no one else has had this problem.