Originally posted by Foggy45
short version:
Some people like them and swear by them, others buy a rear strut brace and say that’s fine.
The higher up the C-pillar the seat belt is mounted the more a chassis bar may help. (Think civic hatchback)
if your budget allows, and you want every possible point covered, then get one, or get a cage, or at least a multipoint rear brace.
However if you want to buy just one, I’ll say again; get a strut brace!

Stress Bar Theory Educational Clicky: http://www.e30m3project.com/e30m3performance/myths/Strutbar_Theory/strut_bar_theory.htm



for those that want to read:
Granted any stand alone bar in the car that connects the two parallel sides will inherently add strength to the chassis.

The benefits are a question of where you put it, what you’re trying to improve, and cost ratio.

Also this is a HUGE fighting point on other automotive forums. the civic hatchback guys swear they work, the Silvia crowd argued too much about whether the benefit was there, the 350Z people say nope, and nobody on the Supra forums would talk to me

The 2G has a much stiffer design of the rear "C-Pillar" then the Civic Hatchback that most of these bars were originally intended for.
On THOSE cars they DO work well, but that’s because they have no rear tub that ties the two sides with the rear.
So by design, sticking a bar across the hatch gap will strengthen the chassis.

On cars similar to the 2g, like the Supra, 350z, Tiburon, Integra, S13 and other cars with a Fastback style, the benefits depend on where the bar is placed.

With chassis strengthening bars, they are usually placed in the area of highest strength of the B C or D pillars. Noticeably these spots usually happen to correspond to the placement of the upper seatbelt anchors.

From my research on automotive chassis design, this is now where individual automobile design takes over and the black and white area of uniform placement tends to grey. On some variations, the C-pillar tends to be wide (distance from frame to roof based on B-pillar height and relation to the angle (theta) formed by rear strut towers) and the placement of the bar is much higher up into the passenger space then height of the rear seat.
On other variants where the sweep is longer and the b-pillar height is smaller, the placement is lower than or equal to the height of the rear seat.
On these longer sweeps, the placement is noted to be within 6 inches of the rear strut towers.

The further away from the rear shock towers you get the more an addition brace may help.

As the rear angle opens and B pillar height becomes longer, the length of the top of the C-pillar shortens. Either this is where the end of the glass is and the trunk starts (coups, saloons, notchbacks), or you have reached the end of the car (fastbacks, liftbacks, and hatchbacks)

As this angle increases, the role the lower unibodys impact on rigidity decreases and the more important the thickness of pillars and rigidity of the roof come into play. Coups and saloons work well here because they have frame component that spans the end of the C-pillar and strengthens the frame.

On a race chassis, anything goes. On cars without tube frames and extensive caging (most road race chassis), I found examples of 3 4 and 6 point rear braces. These triangulated the C-pillar placement, shock tower, and the rear frame depending on how many points. Most beyond 3 were custom made.


References:
The Automotive Chassis - Engineering Principles (2nd Edition 2001) J. Reimpell
Engineer to Win: Understanding Race Car Dynamics, (2007) Carroll Smith
High-Performance Handling Handbook, (2002) Don Alexander