Answer
Most tubes in the frame are concerned loading in the axial direction.
When those tubes are bent, the load path has to change directions.
Per F.5.2.3, a brace is required to partially carry the load, and prevent the tube from deflecting more in the direction of the bend.
The closer the brace is to the plane of the bend, the more effective it can be, and there is a 30 degree rules maximum.
A brace that is perpendicular will have little to no effect.
Per F.6.5.2, Shoulder Harness braces are an exception from F.5.2.3.
A shoulder harness is loaded by the belts holding the driver during deceleration.
On a bent shoulder harness bar, the force puts a moment at the weld to the Main Hoop.
The braces job here is to react that moment.
The closer they are to perpendicular to the plane of the bend, the more effectively they will react the moment.
Therefore, there is a 30 degree minimum angle to the shoulder harness bar.
And running the brace to a triangulated node is also helpful, but not required.
