The heart of the cage's structure is a 1.75″diameter, .120″ wall main hoop, diagonal brace, and harness bar. The main hoop and diagonal brace's function is to protect the driver from roof collapse in case of a flip or roll over. However, it cannot do this effectively without additional lateral support.
Equally sized rear legs known as “kickers,” coupled with upper and lower X's help displace and distribute loads on the main hoop to other portions of the cage structure and uni-body frame. In the same sense, A-pillar bars extend forward, perpendicular to the main hoop, terminating next to pedals.
Additional tube bracing and “taco gussets” triangulate the main hoop, A-pillar, and roof spreader bar to strengthen the different junctions. In addition, dimple dyed, steel gussets tie the roll cage to the uni-body structure on the A and B pillars to stiffen, and ultimately strengthen, the chassis and cage assembly.
The A-pillar bars are tied together with a roof spreader bar and a diagonal roof bar which triangulates the roof area. Combined, the A-pillar bars, roof spreader, and diagonal roof bar outline the stiff envelope of safety that encapsulate and protect the driver.
Since the S13 chassis has such a low slung windshield and A-pillar angle, additional bars were welded in place to provide the A pillar bars additional support against collapse. Also note the tube gussets that triangulate the main hoop and A pillar bar. The main point here is to ensure that the main hoop and A-pillar do not collapse if the car were to land on its roof during a high speed crash.
The SCTA rule book states that the roll cage structure must protect the driver from all sides. This is a commonly overlooked or under interpreted rule, as the roll cage should not only provide side protection, but also protection below the driver's rear end. Note how the hump in the floor board for the catalytic converter had to be clearanced to accomodate the butt protection bar.