ORIGINAL: Norm Peterson

As far as the convertible is concerned, there isn't a lot of benefit to be gained by bracing the rear seatback plane. Reason being that assuming that you could push some loads up to the top of the channel shape (looking in rear view at the side/trunk floor/side "U") - there is nothing to carry those loads (some combination of torsion, bending, and shear) across the door openings. The reality is that without continuous structure from C pillar to A pillar, loads will not be attracted along any new bracing in this particular application. You'll get a little in-plane stiffness (seatback plane), for whatever that's worth, butlittle else. Loads go where there is a continuous path in accordance with the stiffness of that path relative to any other available paths.

By the same token, there is a bit more to be had with the same mod in a coupe, as those loads are fed into the roof structure and carried across the door and window openings there.

It's an occasional mod with the hard-core handling fanatics within the G-body (GM) world. Even though the S197 is probably much stiffer in torsion, the effect is still present to some extent.

The benefit comes when you start tuning the suspension. The stiffer the chassis, the more effective any given spring/sta-bar/shock tuning becomes, meaning that you need less stiffness change in any of those coomponents, front or rear, to effect a given handling change, and you'll have a wider range of tuning via the front:rear distribution of lateral load transfer as well. Less motion is "lost" in the chassis.

I'm pretty sure that absolute chassis stiffness numbers are not the be-all, end-all here. The whole analysis is a "springs in series" sort of thing, so the roll resistances of the front and rear suspensions matter. Statically and dynamically, the latter including shock/strut damping effects.


Norm
(does piping stress analysis - a form of structural analysis - for a living)

ORIGINAL: Norm Peterson
As far as the convertible is concerned, there isn't a lot of benefit to be gained by bracing the rear seatback plane. Reason being that assuming that you could push some loads up to the top of the channel shape (looking in rear view at the side/trunk floor/side "U") - there is nothing to carry those loads (some combination of torsion, bending, and shear) across the door openings. The reality is that without continuous structure from C pillar to A pillar, loads will not be attracted along any new bracing in this particular application. You'll get a little in-plane stiffness (seatback plane), for whatever that's worth, butlittle else. Loads go where there is a continuous path in accordance with the stiffness of that path relative to any other available paths.

By the same token, there is a bit more to be had with the same mod in a coupe, as those loads are fed into the roof structure and carried across the door and window openings there.

It's an occasional mod with the hard-core handling fanatics within the G-body (GM) world. Even though the S197 is probably much stiffer in torsion, the effect is still present to some extent.

The benefit comes when you start tuning the suspension. The stiffer the chassis, the more effective any given spring/sta-bar/shock tuning becomes, meaning that you need less stiffness change in any of those coomponents, front or rear, to effect a given handling change, and you'll have a wider range of tuning via the front:rear distribution of lateral load transfer as well. Less motion is "lost" in the chassis.

I'm pretty sure that absolute chassis stiffness numbers are not the be-all, end-all here. The whole analysis is a "springs in series" sort of thing, so the roll resistances of the front and rear suspensions matter. Statically and dynamically, the latter including shock/strut damping effects.

Norm
(does piping stress analysis - a form of structural analysis - for a living)

ORIGINAL: F1Fan
...Second, all you have to do is drive an S197GT convertible around the block on the street and you will know instantly that the GT convertible chassis is not nearly as stiff as the GT coupe’s chassis even on stock 17� wheels and tires. This is why I bought a coupe, well I also liked the roofline of the coupe but the chassis stiffness was a deal breaker for me. As you may know, I am a major handling and suspension hound so there was no chance that I would buy the very pretty and amazingly attractive Windveil Blue GT convertible I drove even though I loved the colors and would disappoint my daughter tremendously.

Knowing all of this information let me state that based on my own stress testing IMO there is no need for any front or rear strut or damper tower tie bars. There is just not enough movement in the strut towers or rear damper mounting areas that these parts when bolted on could measurably improve the handling or grip performance of the S197GT coupes IMO.

The situation for a GT convertible is much less clear because based on what I know of suspension and chassis design there are no convertible specific chassis stiffeners that can restore the lost chassis torsional rigidity due to the loss of the top half of the coupe’s structure. If you wanted to build chassis stiffeners for a convertible that would improve on the structural loss in torsional strength, you would need to add an awful lot of weight at the same time reducing the interior space and access to the rear seats. Even then, I seriously doubt you would likely achieve the same torsional strength as the GT coupe unless your additional stiffeners included a weld in roll bar with at least 8 points welded into the chassis and a fair amount of triangulation along with additional gusseting and bracing added around the rear cowl over the rear damper mounts. In terms of appearance and weight, the GT coupe chassis is clearly going to be the winner of this contest.

I hope this helps clarify what I have posted before and clears up any vague comments I may have made previously.

Cheers!