We don’t really think much about sills. On some cars they were not even visible, as in the 1978-1993 Saab 900. It’s a case of the missing shutline.
Admittedly this example is rather dented. Looking past that, notice that the door comes all the way down to where the sill or rocker panel is normally visible. There is a sill there, but it is about ten centimetres in-board, with a thick rubber seal to close the gap between the body and the door.
The 1968 Saab 99 had the same construction principle. Citroen’s DS and CX also have the same design and, as far as I can tell most other cars did it the usual way. If anyone can think of another car with doors that wrap under the car, let me know.
Why would Saab do this? First, it makes the car’s body side look simpler. There is one less element to manage. Second, it makes the car easier to get into as you do not have to step so far past the body to get to the seat. Third, it may be easier to construct. The sill, which is a load-bearing member can be designed to be as strong as needed without having to worry about how it looks. The door can cover it up.
For the replacement Saab 900 of 1993, Saab gave up this little detail and went with an approach Audi also used, a sill or rocker panel that is located below the doors, with an unbroken horizontal line from front to rear.
Above is a more conventional approach. There is a small strip of metal running under the door. Much the same can be seen here:
However, Audi have decided not to weld the sill and body as did BMW. There is a shutline running from the rear of the front wheel arch backwards. The body-in-white is then covered with another rocker panel which seems to be painted black. Audi used this concept until the 2008 Audi A4 (B8) when they then decided to have the sill and rear wing made from what looks like one pressing.
I will be coming back to consider the arrangements of other rocker panels and sills in the near future.
[Editor’s note: The text has been amended to correct an erroneous reference to the Saab 900. It now correctly refers to the Saab 99. See comments below.]