I don’t know much about Unreal (just got here!), but from a physics point of view it sounds like your centre of gravity is below the ground plane - can you check its position (rigid body) relative to the wheel colliders?
I don’t know much about Unreal (just got here!), but from a physics point of view it sounds like your centre of gravity is below the ground plane - can you check its position (rigid body) relative to the wheel colliders?