if (_casterWheelShape == null)
{
    // add caster wheel as a basic sphere shape
    CasterWheelShape = new SphereShape(
        new SphereShapeProperties("rear wheel", 0.001f,
        new Pose(CASTER_WHEEL_POSITION), CASTER_WHEEL_RADIUS));
    CasterWheelShape.State.Name = "Caster wheel";

    // a fixed caster wheel has high friction when moving laterely, but low friction when it moves 
along the
    // body axis its aligned with. We use anisotropic friction to model this
    CasterWheelShape.State.Material = new MaterialProperties("small friction with anisotropy", 0.5f, 0.5f, 1);
    CasterWheelShape.State.Material.Advanced = new MaterialAdvancedProperties();
CasterWheelShape.State.Material.Advanced.AnisotropicDynamicFriction = 0.3f;
CasterWheelShape.State.Material.Advanced.AnisotropicStaticFriction = 0.4f;
CasterWheelShape.State.Material.Advanced.AnisotropyDirection = new Vector3(0, 0, 1);
}

base.State.PhysicsPrimitives.Add(_casterWheelShape);