Chassis Strengthening adds rigidity to the chassis.

Chassis Bracing Several modifications may be done to the 240sx to strengthen the chassis.

Strut Bars Strut bars are designed to make a monocoque or unibody chassis design stronger and much more rigid than from how it was built in the factory. Most strut bars are commonly found on a Mac Pherson strut system chassis car. Strut bars prevent what is known as chassis flex from occurring and allowing the monocoque or unibody chassis from twisting or creating torsion due to horizontal movement in the strut towers. Strengthening the strut towers allows the chassis to stabilize under harder and stronger G forces making it much more superior to the common factory suspension.

Fender Braces These stiffen up the front by providing a strong link between the bulkhead and just past the firewall. This can help increase overall chassis rigidity making your front end stiffer, and turn in, the steering response is better. However, there have been reports that these can dial in understeer.

Tension Rod Braces The tension rod mounts tend to be thin and flex substantially, so a form of reinforcement in that area produces noticeable results. Using a Nismo power brace will replace the mounts with a permanently welded bar and plate reinforcements. There are also other replacements that include adjustable bars which don't replace the mounts. Examples such as the Powertrix tension rod brace. Additional T/c rod brace solutions include Tanabe, Nismo, C-Tune, Cusco, etc. These bars are a guaranteed performance increaser. There is some ambiguity about the over/under steer tendencies so an adjustable bar can be useful there.

C-Pillar Bars The s13 hatch in particular has a fair amount of flex in the c-pillar area due to the extensive size of the rear opening. A viable upgrade would be a c-pillar reinforcement bar that bolts to the rear seatbelt mounts. It should be noted that the mounts on the s13 Zenki ('89-'90) are more towards the glass and the s13 Chuki ('91-'94) hatchback seatbelt mounts are parallel. A solid metal bar may be economically produced by measuring the distance between the 2 points and drilling mounting holes for this model.

Note: These bracings are produced as well for coupes, but in those instances tend to mainly just be cosmetic since its not a weak point for them.

Lower Rear Tie Bar Connects rear lower control arm to the center of the rear sub-frame to help stiffen the chassis. They are a good thing, but the difference is very minute. It is minor and considered more of a cosmetic part than a functional one. This should be last on your list if at all.

Misc. Chassis Bracing Other options such as ladder bars, drivetrain tunnel bracing, lower shock tie bars, and floor bars are also available, but do not produce as much of a noticeable gain as the parts mentioned above. These aren't quite popular as they tend to require custom fabbing instead of them being a straight bolt-on affair.

Roll Cage

Keep in mind that if you are going to add a cage you need to pay attention to the type of material and construction (I.E. cold rolled steel, chromoly, etc) If you are serious enough to put in a cage you probably compete with your car in different classes. Keep in mind that there are minimum requirements to compete in certain classes. A cage suitable for your local drag track might not be acceptable as a GT3 class car, or an SCCA competition.

Custom Made

A custom built roll cage is usually built to fill a requirement in a specific racing class. Roll cage guidelines for different classes can be found here (Insert Links)

Prefab

Several companies produce roll cages for the 240sx such as Autopower and Cusco. You can get a cage as simple as a 4 point and as complicated as 8 points premade (as many as you want when custom)

Roll Cage Reviews