Scuttle shake is a phenomenon whereby a
car - particularly an open-topped car, and particularly an open-topped car that has been created from saloon
underpinnings - wobbles and shakes when going over bumps. This is due to a lack of
torsional rigidity in the car's structure. It is a
Bad Thing - apart from the noise and vibrations, it leads to
metal fatigue and causes the car to handle unpredictably.
Manufacturers can combat scuttle shake by reinforcing the car's chassis. This makes the car more expensive than it would be otherwise, and the increased mass of reinforcement usually obliterates any weight saved by the lack of a roof.
For cabriolets and convertibles that are intended to be driven slowly in such a fashion as to attract attention from members of the opposite sex, this does not matter a great deal.