My 2 cents worth about the convertible braces: I definitely could tell the difference in handling with braces vs. without them on my street driven, modded S197.
The long version: I’ve built a number of semi serious street machines over the past 4 decades and have always built them from the ground up. After I purchased my first S197 in Jan 2020 (103,000 mi on a V6 premium convertible) I soon realized that improving the handling was necessary. First mod: I decided on the Stiffler’s Fit System weld in frame system (installed at their factory in Indianapolis). The owner has a background in Indy car racing and in addition to the frame system they installed an improved design of the convertible braces, the Stiffler heavy adjustable panhard bar and the Stiffler K-member front brace. Convertibles come with a factory front brace but Brian pointed out the significant differences in the size & mounting points of the Stiffler front K-brace. Regarding the rear Convertible braces, Brian improved the design by changing the box orientation of the brace. This not only effectively stiffened the brace but got rid of an attachment point that was designed by the factory to eliminate a harmonic in the flimsy factory braces. Although my factory rear braces appeared to be in pretty good shape, once they were off it was evident that corrosion had taken their toll in one hard to see area which likely compromised their usefulness. I am guessing that Stiffler’s has added the improved convertible braces in their catalog for sale but you may need to call them if you are interested.
What I can tell you is that adding those Stiffler chassis parts to my convertible (with no sway bar / spring / strut changes) was a night and day difference. Just pulling out of the Stiffler parking lot and “stepping over” the curb was dramatically different.
So, how much difference did removing the improved convertible braces make AFTER the frame was stiffened up?
The next set of mods that I planned was brakes and mild lowering. Because of their great tech support, I bought springs and a bunch of front end parts from J&M Products: 1.25” Vogtland lowering springs, tall ball joints, bump steer tie rod kit, Koni yellow adjustable struts and caster/camber plates. For the front brakes I refitted front 4 piston FoMoCo calipers from a 2015 up S550 on 14” slotted rotors. For the rear brakes I decided on the large rotor (13.98”) GT500 setup which meant I needed to pull axles to replace the caliper mounting brackets (I did not want to use the caliper spacers). This meant 2 things: I had a long weekend of wrenching AND that I had to take those pesky convertible braces off to get the rear springs in.
Needless to say, by the time I finished the parts install (the tall ball joints sucked), adjusted the panhard, measured/adjusted the bump steer and used the lasers to do my own alignment in the driveway (in the dark) It was too late (read I was too damned tired) to put the convertible braces back on. I drove it for a week without the braces and finally put them back on the following weekend.
Bottom line: The handling took 2 full steps forward with the Stiffler’s chassis mods alone. The car became a lot more fun to drive and was significantly more predictable for spirited street driving.
I would give the lowering springs, Koni struts and all the associated/necessary geometry correction crap (without the improved Stiffler rear Convertible braces installed) a score of 1.25 steps forward. The notable part of this stage of the mods (besides the brakes on this non-ABS system are now friggin’ unbelievable) is that the body roll is dramatically reduced without replacing the stock front sway bar. Lastly, returning the beefy Stiffler Convertible braces to the setup was noticeable (add 0.25 to 0.375). Since I don’t track the car I can’t give you a lap time difference like the serious guys on this forum can yet my butt-o-meter tells me it’s a real improvement. That difference leads to me to believe that a stock convertible chassis can benefit from the braces and even more so from the redesigned Stiffler’s S197 convertible braces.
FYI - I don’t work for Stiffler’s or J&M products - they just offered lots of support to my rookie S197 efforts. And don’t kid yourself: my mods are cute (read fun for street driving) but I have no misconception that I would have my butt handed to me by the more serious track prepped cars/drivers... although I did weld up some front brake cooling ducts so I might be able to go just a little deeper into those corners if I ever make it to the track =;^)
