I believe the ?leathers? xBMWx is referring to are the traditional German garb called ?Lederhosen?.

Perhaps you?ve seen those German/Austrian short-pants with the built in suspenders oft seen during Oktoberfest? My understanding is the shorts are made of leather (Lederhosen), traditionally worn with no under-garments and are never, never washed!

Perhaps the only allowable way to wash them is to stand in the rain?

 

I wonder if this is the result of the E60's slick aerodynamics?

The smooth, laminar boundary layer of air flows over the entire length of the car, and as it separates at the tail it becomes turbulent. This creates an area of low pressure right there, just under the lip of the trunk down to the bumper surface. This actually is smart work - keeping the boundary layer attached to the body all the way. If the boundary layer detached any sooner you would have even more drag on the surface.

However, all that dang dirt that was flowing along smoothly gets deposited on the rear bumper. Have you ever noticed that just after you wash the car and drive it even for a few klicks the bumper is all dusty?

Disclaimer: I was usually really hung-over for my 8:20 am Fluid Dynamics class, so this could be the beer talking.