The roof of a conventional car is essential to the stiffness of the chassis. The roof helps keep the car from twisting and bending. Building a car without a roof is a bit like building a suspension bridge without the cables. As such, the bottom part of the structure of a convertible has to be stiffened considerably. Heavy reinforcing brackets have to be added to the body of the car. This is why convertibles often weigh more than their fixed-roof counterparts.