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Because it's a unibody the A pillars flow down into the uprights that the door hinges attach to on either side of the car then that flows into the rockers, on the inside of the floor pan/firewall is the transmission tunnel that ties it all together.  You could also add a direct connection between the two and some triangulation.



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Grannyknot's explanation is on the right path and identifies a key part of the S30's structure whose importance is often overlooked: the transmission tunnel.  Your question seems to be focused on braking loads (i.e. rear-directed loads that occur in the horizontal plane) and your supposition is that the rocker structures are the only elements of the structure available to absorb these loads.  Were that true, than the braking loads would, indeed, need to be transferred to the rockers by the firewall panel.  Not an ideal situation, as you've guessed.

In fact, the braking loads are largely absorbed into the transmission tunnel (note: 'largely', not 'entirely'), which then distributes some of those loads out into the floor pans.  The pictures below will, hopefully, help you to see this...

S30 - Underside, stripped - full chassis view.jpg

S30 Engine Bay details - stripped and painted.jpg

S30 - Underside, stripped 5.jpg

The rocker structures may eventually take up some of the braking loads, too, but I don't think it's a very large component.  The rocker panels' job in the structure ure that everything was kosher.

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8 hours ago, ennisdavis said:

Yes but the A pillar rocker connection is outboard of the front frame rails. I believe its just the firewalll doing the work. And in my case, its rusted out.

All I'm saying is you can't think of any part of the body as an individual piece, because it isn't. In your case with the fire wall rusted out the stresses have to take other routes which isn't good.  There are lots of little things you can do to a 240-280z body to improve stiffness but most of them aren't practical unless the body is stripped and on a rotisserie.

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11 hours ago, ennisdavis said:

Grannyknot, great idea to tie the frame rail into the rockers. How does it look with fenders on. ie can you see the support. Alot of unitized structures tie these together.

Thanks but I just borrowed it from the Z guys that came before me, no you can't see it unless you get on your hands and knees and it doesn't come near tire.

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