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Chassis bracing for convertibles?


S7MS

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After six years with "Shaky Jake" - my 2007 GT500 Convertible- I attempted to do something about the less than stellar chassis rigidity.

 

Since day one I have been annoyed by the wibbly-wobbly nature of my vert. Sure, the S197's are tighter than the Fox body Mustangs, but a wet shoe box has less flex than a Fox body. There has always been an eccentric "pulsing" to the wheel, with a rapid back and forth oscillation even on smooth roads. More disconcerting are the tremors that pulse through the floor and wheel when traveling over even slightly uneven roads. I have tried different tires, alignments and balancing and re-balancing the wheels but to no effect. I have read some opinions that say chassis braces work wonders and others that say they do nothing at all.

 

So throwing caution to the winds, I had Stiffler's (http://buystifflers.com) "Fit System" chassis stiffeners installed - to the tune of $660.00 including installation. This "System" is a set of square steel tubes with triangulated pads welded directly to the floor pan at numerous points. It was supposedly designed by an aerospace engineer. Laying on the ground, it does not look very impressive and the complete set only weighs about twenty pounds, so I was not expecting much. It took six hours for the guys at GTR Autosport in Rancho Cucamonga to weld it in.

 

After driving the newly "stiffened" vert over familiar roads, I wish to report that the "Fit System" has very little to no effect on the cowl shake inherent in the convertible. Stiffler advises that "Stiffer chassis reduces or eliminates interior rattles and extends life of vehicle". This may be technically true, but the car with the "Fit System" installed still rattles like a soda can filled with rocks. In fact, other than a nebulous sense that the car is a hair more stable (which may be the placebo effect) I have to say that I can perceive no difference in the behavior of the car post "stiffening".

 

In short, I do not believe this may be the best use for $660.00.

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As you have found out - too stiff of a chassis on a convert is a cause of " cowl shudder " not a cure . Technically speaking . Interested in doing a test ? Remove the front sway bar and take it for a drive - post your findings as far as "shake/shudder" goes .

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My verts--in order '72 Mustang (heavy, lots of lost motion--but it was a 70's car), 92 LX 5.0 5 speed (lots of cowl shake and shimmy), 97 Sebring JXi (very little cowl shake, then again it was a front driver), 07 GT 500 (easily the most stable of them all--very little cowl shake, very few rattles/squeaks/etc., tight vert, in my opinion). I seriously considered box framing the '92, but never got around to it. Most sources I read when I bought the 07 did not encourage frame stiffening........

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My convertible is tight, could be other issues...

 

What other issues would you suggest? I welcome any and all advice.

 

Because the car has been so annoying since day one, I have read every article I could lay my hands on regarding the S197 verts. Turns out there are two distinct opinions - the first is that they are rock solid and the second is that they are cowl-shaking champs. How can this be on mass produced vehicles? Shouldn't they all be one or the other? Perhaps some verts are mis-assembled in some way. Perhaps some drivers are less sensitive to the shake. The BMW vert I had previous to the GT500 was smooth as glass. On the other hand, the Z28 vert I had in the 90's could shake the fillings out of your teeth. I'm at a loss, but I'd give anything to have one of those rock-solid GT500 verts.

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And then I drive by a local detail shop and there's a full-sized late '70's Caddy 2-door with the top chopped off and a "custom" boot covering where a top should be. There's a couple of those big rascals running around town and I know they've got to flex in the middle like a twisted coke can! With huge rims and ultra low profile rims I don't see how they keep from collapsing in the middle!

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As you have found out - too stiff of a chassis on a convert is a cause of " cowl shudder " not a cure . Technically speaking . Interested in doing a test ? Remove the front sway bar and take it for a drive - post your findings as far as "shake/shudder" goes .

 

 

I can confirm this, especially with my roll bar installed. When my '07 was stock I rarely noticed cowl shake. Only on the worst country roads (which we have plenty). After my 40th package with the Eibach suspension and 20" wheels I noticed it right away. I ended up needing to use hub centric rings and have my wheels rebalanced and that took care of it but it is easier to get it to shake on the rough roads than when it was stock. I now have a bolt-in 6 pt roll bar that really stiffens up the chassis and also brings on the cowl shudder. It will shudder @ 70 mph on uneven highway with the roll bar installed. Something about going 70 that brings on a harmonic or something.

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