SCCA GCR
18.3.3. Side Protection - Open and Closed Automobiles
A. The minimum side protection shall consist of a side tube connecting
the front and rear hoops across both the door openings.
Additionally, there shall also be either a diagonal tube from the
front hoop to the rear hoop bisecting the door opening below the
horizontal side tube, or not less than two (2) horizontal side tubes.
Additional tubing may be added. NASCAR-style door bars are
recommended.
B. In automobiles with full roll cage installations including side bars,
interior door panels may be altered, replaced, or removed. When
door panels are removed, all sharp edges or projections shall be
protected.
Full SCCA Roll Bar Section here:
http://sports.racer.net/docs/rules/2002/sect_18_roll_cage_and_chassis_2002_gcr.pdf