Joint for a wheel rolling on the x-y plane of the world frame intended for an idealized wheelset. To meet this objective, the wheel always runs upright and enables no slip in the longitudinal direction of the wheel/ground contact.
On the contrary, the wheel can optionally slip in the lateral direction which is reasonable for the wheelset where just one of the wheels should be laterally constrained.
The frame frame_a is placed in the intersection of the wheel spin axis with the wheel middle plane and rotates with the wheel itself. A wheel body collecting the mass and inertia should be connected to this frame.
To work properly, the gravity acceleration vector g of the world must point in the negative z-axis, i.e.
inner Modelica.Mechanics.MultiBody.World world(n={0,0,-1});