is there a bug on vehicle systerm when i setup a 6 wheel car

hello there!
i want to make a control on a 6 wheels mooncar and i found if just add 2 more wheel in ““VehicleMovement”” it will fall inthe ground it seems thereis no collision
and i delete 2 wheel itis fine as the tutorials do and itis easyly setup become a game

but how the 6 wheels set up is comeout so many bugs?

can some one catchup with it?

and how can make the arms rote when move on the rock?