In the struggle to keep weight down, the sheetmetal on some cars today is just thick enough to hold stampings, and no thicker. Walk on the metallic roof membrane that stretches over a contemporary coupe, and you'll wind up with a top whose contours resemble the South Dakota Badlands.

But when that same thin roof span is removed from the structural equation, rigidity is likely to suffer. The latest Mustang convertible provides, sorry to say, a good illustration of this phenomenon. Ford claims a gain in torsional rigidity for the 2011 model—via a standard strut-tower brace, an extra brace connecting the chassis crossmembers, crossmember grace gusseting, and stiffened A-pillars—but the first pavement ripples we encountered provoked a festival of wiggles and shakes in the cowl and the steering column. The GT coupe is impressively stiff; the topless version is disappointingly flexible.

Mustang GT convertibles have the same spring rates as the coupes, but the shock damping is more relaxed, the rear anti-roll bar is softer, and the ragtops with the $1695 Brembo brake package roll on Goodyear Eagle F1 Supercar tires rather than the GT coupe's Pirelli P Zeros. The Goodyears and that softer suspension are nominally there for ride quality, but their underlying function may be to mitigate chassis tremors. If that's the case, they ain't workin'. In addition to being annoying, the quivers and shakes afflicting the droptop's chassis add jitters to its responses in quick transitions, particularly on pavement with warts. Attention autocrossers: You want a Mustang with a hard roof.

The Goodyears and that softer suspension are nominally there for ride quality, but their underlying function may be to mitigate chassis tremors.