These photos are provided by Mike Krnc.
This section highlights the BMW Motorsport E46 2-door chassis
roll cage layout. A bare E46 shell with cage installed can
be purchased from BMW Motorsport (as of 2002-2003) for conversion
to race car for any number of racing series.
The E46 cage is notable in that BMW moves away from extending
the roll cage all the way back to the top of the C-pillars (as was the case
on the late E30 and all E36 factory cages). A very unique feature are the twin
buttresses extending down from mid-roof to the dash. These
buttresses first appear to be "double front down bars" with a large
perforated gusset connecting the two. However, BMW enthusiast
Kurt Johnson, who has examined one of these cages up close, points
out that the design is actually standard downtubes tucked into the A-pillars
with a thin, bent sheet metal gusset forming the rest of the buttresses.
Certainly a very light and rigid layout.
The rear part of the cage is now quite conventional in design. This is especially
true in light of the previous E36 factory roll cage, which had a very complex rear
layout. It is a fact that the factory "road" chassis on modern cars is
becoming ever stiffer, and it is thus likely that cage designers have come to rely
more on this inherent rigidity of the bare shell in contributing to overall race car
chassis stiffness. Also, as pointed out by K. Johnson (based on conversations with PTG),
the rear of the Motorsport cage is purposely kept simple to allow the end-user to customize
it according to the specific rules of the racing series in question. This eliminates the need
for BMW Motorsport to make several different types of cages for the various racing series
around the globe (as was the case with the E36 chassis).