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.]
Driven To Write 
The 1968 Saab 900 had the same construction principle
That will be the 99. It was also meant to keep dirt out of the car.
Hi: that was a typo. I meant to write 99.
Thanks for dropping by. We have other Saab items including Archie Vicar’s legendary review of the 99 somewhere here.
What happened to P Thomas?
After a bit of a hiatus some more 900s have emerged in my area.
Hi: That sounds marvellous. You can post photos here if you are so moved. Was it a full body respray? I had my 20 year old car repainted but left the doors on: I couldn’t imagine they’d go back on correctly.
Picture:
How do I post a picture 🙂
I find it a very simple and beautiful solution, although doesn’t it mean, that the lower edges of the door are more prone to rust due to contact with all the salt in the winter and all that? Correct me if I’m wrong.
“What happened to P Thomas?”
Alive and kicking. Waiting to collect my freshly painted 99 🙂
Several Citroën CX and GS I know have problems with rust on this area. But also on a lot of other, less exposed spots. And other older cars also have perforated door bottoms…
I remember that the CX had a sort of rubber flaps or strips mounted inside the door bottom edge. I’m not sure if these serve to protect the doors from stones or to trap water and cause even more corrosion.
Indeed, the CX and 900 have a similar solution to the door to body seal.
Kajetan: I’m not sure if it matters what’s at the lower edge of the body. Both sill and door are equally prone to rust. A cover panel over the actual sill was Audi’s solution though.
Of course, though I think it’s easier to replace a rusty sill than the whole metal door.
I note that many new cars replace the metal sill with a plastic piece. A sensible move, especially as styling kits become ubiquitous.
The plastic sill has a lot of advantages. It´s more resistant to dings than metal and doesn´t rust. What´s underneath still rusts though! Have you ever seen the way Mercedes and Renault handled these in the late 80s?
Was it basically a plastic trough for storing salted soil against metal?
Yes, with some nice plastic seals that work their way free.
Kajetan: I’ve had rusty sills, just behind the front wheel arch. It was a nuisance. It seems cars rust most near the road. I wonder if this is a fundmental weakness that no engineering can stop but only mitigate.
These days plastic linings fill the whole wheel arch and sit flush with the outer metal panel. Indeed, manufacturers make a styling feature of them on CUVs, a thick border of black plastic around the arches giving them a robust look.
The shutline theme seemed inexhaustabe. Today I found myself mystified by the shutlines on an Aventador. The roof/c-pillar shutlines are hard to understand. It´s panel a-go-go.