Interesting problems arise when sedan cars are reengineered as roadsters or convertibles. Removing the roof to create an open cockpit typically lowers a vehicle's torsional rigidity by at least half, leading to concurrent losses in handling "crispness" and ride. The challenge - and opportunity - is how to add stiffness back without paying too high a weight or cost penalty.
Automobili Lamborghini S.p.A. (Sant'Agata Bolognese, Italy) faced this dilemma when designing a roadster version of its popular V-12/AWD Murcièlago Coupé. To compensate for removal of the roof structure, this open cockpit supercar was significantly redesigned to increase torsional stiffness below the beltline by adding new structural members and stiffening critical components. Interestingly, the component that contributed the greatest torsional stiffness (»50%) was a unique engine subframe, which spans the entire rear engine bay.The initial subframe was a multi-piece welded tubular-steel assembly similar to that used for the vehicle's space frame.