Hi,

I am about to getting my '91 to welding shop, as there are some common rust problems underside. As the interior is out, there would be perfect time to weld some reinforcements too.

The common way to lessen the body flex are SFC-s. I read an article that claims it is not most proper and best way to do it. The underbody stiffens too much and other weak spots will broke. And subframe connectors do not let the unibody work as one piece too - the A pillars that should take amount of torsional forces does not work anymore like they should and there is pretty big possibility to get cracks to some uncommon places.

I tried to figure out why these cars have so great underbody flex and I think I found the reason. There seem to be no reinforcement panels between inner and outer rocker panels and the rockers are tied to the subframes too weakly! I know that old Mustang hardtops get often convertible inner rocker reinforcements and torque boxes that make the body more stiffer. I tried to search, but I didn't find any answers about F-Body convertibles - what kind of reinforcements are used for making them stronger. As most 3-gen convertibles are aftermarket conversions, I suspect that these have some SFC-s. But what kind of reinforcements are under 4th gen cars?

I have a plan to spot weld another rocker to the inner panel (like Mustang convertibles have), there seem to be plenty of room to do it. The only questionable place is around the seatbelts, I haven't figured out yet, how to make the reinforcements there. Then these inner reinforcements would be connected to the subframes with some beefier torque boxes.

Any comments and ideas would be appreciated

PS. Sorry for my rusty English, haven't had much practice lately Hope that you got my point...

 

there is nothing wrong with subframe connectors, use a good perimeter style set, and maybe even add a set of inner ones as well. A cage would finish the job, if you want to go that far.

I dont see what other weak points would pop up if you added sfc's.

as for the verts, the 3rd gen verts had extra pieces rivited to the pinchrail area down in the rocker area for exrta support.

 

I would be curious about this article you read about sfc's. People have been putting those on our cars for well over a decade Im sure and Ive never heard anyone say 'oh I wish I hadnt installed those 10 years ago' due to some weakening of another point on the car.

 

That article was about 60's Mustangs, but I think that it is true for any unibody car. I would try to find the magazine where I did see the article and scan it.

The second reason why it is so hard to use SFC-s - I have a custom dual cat exhaust that takes pretty all of the room under passenger legs. It looks like that:



Strenghtening without SFCs would give me a possibility to rebuild the exhaust to full dual going at the stock location without any clearance isssues.

 

You could always make your own sfc a straight line is best could even make a 2nd rail tie them in to the rockers with some cross pieces.Only time i have seen cracks in a pillars is on t tops and convertibles most car makers never brace them enough.

 

If the tech article you reference was written about mustangs,
I can assure you its erroneous to assume the same principles
will apply to the GM f-bodies.

Ditto the previous comment about sfc's reputation in the
thirdgen community. Some tell-tale effects of f-body flex
is cracks in the roof structure where the side windows roll up,
and dashboards squeeking.

Your idea sounds like an interesting experiment, but I wouldn't
be surprised if the results were... disappointing.
If you go that route, keep us posted.

As far as constraints of exhaust components,
well pick your poison.

 

Well, I have seen those cracked roofs on about every second F-Bodied car in Estonia. Many of our streets are not in the best shape and are pretty harsh for car chassis and body. So that is not a tell-tale

I compared early stangs and F-Bodies and the construction and weak points are pretty similar. Mostly rockers. I think there should be reason, why factory does not build 'verts with SFCs but use rocker reinforcements...

 

early stangs were total crap buckets, im sure adding SFC would find weak points.

I have never seen an f-body receive any damage from installing SFC's.

with that crazy cat setup, you will have to build your own sfc's

 

Comparing the unibody build quality of a 60s car to a 90s car....There just isn't much comparison there.

SFCs are always a good thing, but require a little more planning, if you have a custom dual cat setup, vs a stock dual cat setup.

If the interior is out, you could spot weld EVERY seem, even inch or so, to make the unibody seams more solid. You could try to stitch weld them, but you also risk the possibility of warping, unless your welding skills are good.

Welded seams AND SFCs would be even better. Just the spot welds are easier to hide, if your trying to stay 110% stock LOOKING.

 

Yesterday I looked under my ***** daily driver, as it feels like 10 times stiffer than Firebird and what did I see? The car has factory mounted subframe connector like structures (a 1.75x1.75" box shaped reinforcements). So I am not so concerned that these are overkill anymore.

I found some 60x30 mm mild steel that I could use and try to make my own SFC-s. I think, that I have to make them semi thru-floor type, as my cats take pretty much of passenger side room under the floor. I have an idea to get them running directly from rear framerail to the point where front framerail and the torque box (I think this front reinforcement from subframe to the rocker is called so) join. Is it OK if I cut a V shaped slots, bend the connectors to follow the contours of the floorpan and then weld the gaps? Or should I reinforce the bend somehow? Should I tie them with something more (like transmission crossmember or rocker panels) or is it ok to just connect the subframes?

 

I readily admit that I can't visualize the SFC structure you're describing.
But here's my
In order to save some exhaust system components, you're working
really hard at constructing something, that w/ respect to GM F-bodies,
you aren't really well versed. For that reason, I would be surprised if
the results will be worth the effort involved.
Do some cost benefit analysis of your time and materials, then double
it, because you never get the first one right. At least I haven't,
and that's born of years of experience that probably approach your life-time.
And don't forget, the SFC's will probably outlive those exhaust components.
BUY some SFC's, then modify the exhaust comp's as needed.

Good luck, however you choose to proceed.

 

Fixing one week point, point more stress elsewhere. It is somewhat true.

But its like building a motor. You fix an airflow restriction in one place, it makes the next place a weak spot. Where do you stop?

I would say adding some subframes, and maybe welding a few spots.

As for the verts and what do they have. As mentioned some bracing under the gfx. But also some formed pieces in the rear floor board, one elsewhere (brain farting right now), plus the brace that the torsion bars attach to for the lid and trunk.

 

My Kenny Brown Double Diamonds. Bought them because "they came off my 3rd gen Trans Am", only to find pics later of the EXACT SAME SFCs, on a 4th gen!

 

Well, I had in mind some Alston style connectors, that directly connect front and rear subframe. If I use that boxed tubing, then it would work with my crazy exhaust pretty well - I need to remove the heatshield, but it rattles anyway. I would like to use some thermatec heat barrier material directly on the floorboard. So it is not a big problem.

Maybe that kind of subframe connectors are not the best kind, but it is street drivin' car and for that matter these are better than nothing. If I would plan to use it for racing - then the things would be different