- Terrain is imported but is basically a flat plane with a mesh collider
- A train locomotive chassis (rigid body: box with wheel colliders) sits on the terrain, collides correctly. It sits on the terrain due to gravity.
- As soon as I put the “cab” and “engine” on the loco (imported mesh with meshg collider, part of the same rigid body), the whole thing pulls strongly in one direction until the loco tips over or has a seizure, rocking back and forth repeatedly.
- The imported mesh is symmetrical and is about the same width and length as the chassis.
- Still tips if I give rigid body a mass of 1 and wheel colliders a mass of 10, 100, etc.
- Scale is 1uu = 1mm, so gravity is -9810 in y-axis.
- The terrain is normal to y-axis.
Where is this other force coming from? What is going on?