Official Audi technical illustration of the set-up:
The concept may seem overly complicated, but the Germans had some decent reasoning for the design. Audi wanted to improve the braking in their sedans, which were becoming increasingly faster, yet wanted to retain a 15-inch wheel size. Rear brakes on these cars maintained a conventional disc brake design, but the front wheels were treated to a totally new and unique ATE built system. By mounting the caliper inside of the rotor, the rotor itself can be larger since there doesn’t need to be a gap between the rim for the caliper to wrap over it. While the actual surface area of the rotor doesn’t change significantly, the swept area of the pads increases with diameter. Not only does this make it mechanically easier for the brakes to slow the spinning wheel, but also allows heat to dissipate off the rotor more quickly. Well, that was the idea anyway.
Below: Configuration of typical disc brake set-up (A) and internal caliper set-up (B) with caliper in red and rotor in gray*
The problems with the UFO brakes ultimately became too much. They warped easily, particularly in stop-and-go traffic when the rotors were not allowed to cool down thoroughly. They were also very complicated, much to the dismay of Audi mechanics and the car owners paying the bills. Audi (at least in the US) started recommending to it’s customers an aftermarket replacement of the UFO brakes with more conventional Girling dual-piston calipers and traditional rotors years later. This however, requires completely changing the front assembly due to different caliper mounting points. Many cars equipped with the UFO units have been converted, but the originals are still out there. Unfortunately, one can expect to pay $200 per rotor for replacements and a caliper rebuild will likely cost a small fortune as well.
Above and Below: Caliper and rotor mounted to strut, shown from side and behind
The Audi V8s all sported the UFOs, as well as the C3 generation 200 and even the first “S4” badged car which was based on the C4 generation 100, also known as “Ur S4”. By 1995, the controversial concept’s services were no longer deemed necessary and the internal caliper system faded into the automotive history books.
Above: Audi 200, Below: Audi V8
* Vector illustrations by I.R. Rothwell 2009
Ian, allow me to add this photo snapped at the junkyard: