the remainder of its decade-long lifespan. The Imperial Crown convertible was listed at US$5,598 ($ in dollars ) and 1,167 were manufactured. Unlike the rest of the Chrysler Corporation makes (Chrysler, De Soto, Dodge, and Plymouth) that began unibody construction for 1960, the Imperial retained separate full perimeter frames for rigidity through the 1966 model year. These substantial frames had a box cross-section with strong crossmembers connecting the frame. The convertible had extra strength built into its crossmembers that formed an "X". The driveshaft passed through a hole in the "X" frame. The parking brake gripped the driveshaft and was not connected to the rear drum brakes prior to the 1963 model year. The Imperial, and all Chrysler-built cars, incorporated "Torsion-Aire" suspension for 1957. This was an indirect-acting, torsion-bar front suspension system that reduced unsprung weight and shifted the car's center of gravity downward and rearward. Torsion-bar suspension on the front combined with multi-leaf springs on the rear provided a smoother ride and improved handling. Imperial's handling was better than its competitors of the time due to its stiffness from the torsion-bars combined with a thick anti roll bar in the front. Pillarless hardtops, in both two and four-door configurations, received the Southampton designation. The Imperial served as the basis for the Hongqi CA72 Chinese state limousine. In 1966 the car was modernized as the Hongqi CA770 however it used a 340 which was not available in the Imperial. The car continued production in various guises until the mid-1990s. After this, it was replaced by a range of licensed produced vehicles based on the Audi 100. 1957 (Series IMI-1,2,4) The 1957 model year was based on an even greater degree on Virgil Exner's "Forward Look" styling (also used on other full-size Chryslers of the period). It featured a "biplane" front bumper, a full-width egg-crate grille, and quad headlights (where legal). Taller tailfins now encompassed the trademark gunsight taillights and framed a downward tapering decklid that met the rear bumper. Curved side glass was employed for the first time in a U.S. production car. The Hemi engine with a displacement enlarged to was standard for 1957-58. Power seats and dual exhaust were made standard across the line. A convertible was available for the first time on an Imperial and only offered in the mid-range Crown series. Sales were helped by Exner's "ahead of the competition" styling, with 1957 becoming the best-selling