Vehicle Movement Component - Wheels "fall" through floor - rotated in debug view

I set up a vehicle with the vehicle movement component.

When I press play the vehicle sinks into the floor. I think it has nothing to do with my physics collision model but with the orientation of the wheels.

When I run ‘showDebug vehicle’ in the console then I see that the wheels lie flat on the floor. The model is exported from blender in an fbx file. The body and the wheels are one mesh with the wheel-vertices assigned to the wheel-bone. The orientation is x-forward and z-up.

Do you have any suggestions?