Although a convertible is derived from a standard car, there's more to it than simply removing just the roof. In addition to the space needed for the roof, the car-body also has to be strengthened.
How it works
Almost all the same safety standards applying to vehicles with a solid roof, must be fulfilled, at least those that are legally required. Apart from the few rare vehicles with welded tubular frames, most convertibles are
derived from saloon-cars or, even better still, from coupes. However, only very few body parts can be adopted without modification. The modifications start with the door sill metal plates. They are generally considerably
higher and are made up of up to three interlaced box profiles. They can also contain strengthening stays, the same as in the doors. Nowadays the windows are frameless. If this was not already the case with the
(basis) coupe, then thicker windows and a different mechanism for the window-winders are necessary. Even the motor must sometimes be strengthened in the case of electric windows.
Now, of course, the windscreen comes into direct contact with the A-pillar, so much so, that the pillar must not only be strengthened but also has to be remodelled for the convertible. Even the front side plates are often
changed because of the altered door sills. All that is now left is the front section and the bonnet. In rear, the boot lid must be changed, at any rate, at least the upper part. The vehicle floor, from the A-pillar onwards, is
the most radically changed section anyhow. Should neither a roof case nor roll bar be planned, considerable strengthening must be built into the vehicle floor. Sometimes the room at the bottom of the boot has to be
reduced to achieve the necessary lateral stability.
In the case of self-supporting car bodies, it is valid to say, the more stabilising parts that are left out, the heavier they become by strengthening. To keep the changes to a minimum, coupes and convertibles are
generally planned simultaneously. If, however, the manufacturers simply equip an available convertible with a solid roof and then offer it as a coupe, it is, as far as the driving dynamics and the fuel consumption are
concerned, the worst possible.
The building of the roof is continually becoming more and more complex. Even the construction of the roofing-frame is an art itself. The target here is clearly to lend a special appearance to the convertible also with the
roof closed, even though, in advertising, convertibles are shown mostly with the top down. Because the windscreens are becoming steeper and steeper and the vehicle rear higher, folding roofs are also possible.
Nowadays, to be able to integrate them rationally into the boot, the rear windows are sometimes even rotated by 180°. (See the Mercedes
500 SL, for example) 09/08