November 17, 2009

What Lies Beneath: Audi UFO Brakes

If you hang around foreign car mechanics for long enough, particularly those familiar with VW/Audi, you may very well hear the term “UFO brakes” come up – and most likely with a hint of disdain. This gem of 1980s automotive technology has nearly all but been forgotten and is mostly regarded as a curiosity now, however, it raised quite a few eyebrows when debuting on Audi’s V8 sedan of 1988. Technically, these “UFO brakes” are called internal caliper brakes. The rotor is carried on the hub by a flying saucer like casting which allows the caliper to grab the rotor’s surface from the opposite angle of a typical disc system. Confused yet?

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:


Anonymous said...

Actually, I see mention of inner calipers from time to time in engineering publications. With larger wheels, large rotors gain too much mass. Inner calipers and large ring shaped rotors help solve this problem.

So we may see similar systems on mainstream vehicles in the near future.

Anonymous said...

urS4 never used UFOs. "Audi used a unique set of front brakes on the 1991 200TQ 20V and on the 1990-91 Audi V8. After introduction, Audi would retrofit the G60 dual piston calipers and conventional rotors from the Audi S4, along with new strut assemblies on customers cars that complained about the shimmy problem. Front Caliper change on some '89-90 200TQ was ATE single piston calipers. The 1992 and later Audi V8 and Audi S4 used the dual piston G60 calipers and Audi discontinued the use of these UFO brakes." [from ]

I. R. Rothwell said...

I've checked into this and you may be right, although I've read conflicting information saying that the UrS4 used UFOs till 1993... That being said, however, I have personally only ever seen them on 200s and V8s.

Ian said...

Do the wheels use lug bolts? The pictures seem to hint that, otherwise it seems like you's have a hell of a time fitting the rotor.

I. R. Rothwell said...

Excellent photo, Ronan...

Ian, yep - same five bolt pattern as all other Audis of the time.

Anonymous said...

Nice write-up on UFO brakes. For all those that hated the UFO brakes, there are people that love them. They provided incredible stopping power. Just think- a 310mm rotor on a 1991 car.

domulis said...

UrS4 used UFOs only in first production years, until 1993. I know some people that own urS4 with
UFO brakes. I had them on my 1990 200 20v, but I made a conversion to 314mm HP2 brakes from 1996 Audi S6, still having 314mm and fitting them on original Audi 200/V8 BBS R15(!) ET35 wheels, but to fit that you need to have calipers without additional 'weights' on them.

Paulv560 said...

Im having the same problem on my 91' 200 the inside of the rotor is the thickness of the igniton key!Domulis,how hard was the conversion to the s6 brakes?

thanks paul

domulis said...

Hey Paul,
we made it in one and a half day. The second part of second day we have spent on solving the problem of wheel fitment, and it turned out to be only 5 min solution. Be ready to fight with some 20yr old screws, and you'll probably need to have spring mounting kit to press it.
Email me to get more infomation.

paulv560 said...

Whats domulis your email address it wont let me email you? Let me know paul.

domulis said...

You can contact me at domulis at gmail dot com