Monday, July 2, 2018

How Audi’s Out-of-this-World UFO Brakes Worked (and How They Also Kinda Didn’t)

By: Evan Williams

If you need more brakes on a car today, you have two main choices. Call up the wheel department and fit some monster wheels to cover those pizza pan rotors or switch to carbon-ceramics. But in 1988, big wheels and fancy composites weren’t an option. So Audi developed a braking system that was out of this world. Audi UFO brakes. How (and why) do they work?
First, though let’s dive in to why bigger brakes matter. Larger diameter brake rotors have a few benefits over smaller rotors. The larger diameter gives the brakes a longer lever for stopping the car. Like using a longer wrench to tighten a nut, only in this case, you’re stopping the nut not tightening it.
Bigger diameter rotors also give you more pad area without designing some strangely long caliper that curves around the hub. Bigger pads can mean more clamping force.
Ultimate braking force is limited by the tires, though. It doesn’t matter how hard you can clamp down if the tire has been locked up since your foot hit the pedal. Or if the ABS has been pulsing and pounding away under your foot. So brakes that are too big won’t stop you any faster. They do have another benefit.
Bigger brake rotors and larger pads handle heat better. They can absorb more and they can better dissipate it. If you’ve driven a car with small drum brakes, you might notice them start to overheat and fade in just one long downhill stretch. Rotors take longer to overheat, but hard and high speed stops on the Autobahn or winding through a canyon can still overheat discs.
The 2018 Audi A8 uses big wheels. 19-inches and up. Underneath those are great big 14.0-inch rotors. Take a look through the spokes of the wheel and the brake caliper is nearly touching the inside of the wheel. Big brakes to stop a big sedan.
But in 1988, 19-inch tires were unheard of. Or 18-inch. Even 17-inch wheels and tires were massive. Audi’s flagship of 1988 was the V8. The big sedan needed big brakes. But it needed to make do with 15-inch wheels.
You can fit big-ish brakes under 15-inch wheels if you have to–the Porsche 911 from that year uses an 11.12-inch brake in the front–but that’s not exactly massive stopping power, and it’s not going to like the heat of a big, hard-driven sedan.
Audi wanted bigger brakes, but couldn’t use bigger wheels. So it was time to fill the empty space with brake. And where is there empty space inside the car wheel? That ring between the rotor and wheel where the caliper lives. Look at all that space! Just look at it!
So where you put the caliper? After all, the caliper lives outside of the brake rotor. Bigger rotors mean the wheels won’t fit. So how about on the inside of the brake disc? Instead of the outside.
It’s an idea so crazy it just might work. So Audi engineers set out to turn the disc brake inside out.

By mounting the caliper inside the rotor circle instead of outside, Audi was able to fit bigger brakes. 12.2-inches! That’s more than an inch more rotor than a Porsche 911 from the same year, and it means significantly more braking surface!
It was a strange design. With a hollow center section instead of the solid section of a normal rotor, it had a strange UFO-like cage that wrapped around the outside and attached the rotor to the hub. The caliper was similar to a regular brake caliper but flipped so that it could fit around the rotor.
Seems like a perfect fix, right? Well, there’s a reason why you’ve probably never heard of them. And it’s not because the Audi V8 wasn’t sold in North America. The internal caliper brake made it to the C3 generation of the Audi 200 as well as the Ur S4.
There were a couple of issues. The system was more complicated, mostly because it was unusual. And the big cage around the rotor and the lack of space around the wheel meant that they did still overheat. Reports from the time say they were prone to warping as well. The fancy rotors were expensive, and since none of the parts were shared with other cars they stayed expensive. The front brake pad for a 2010 Civic, for example, is used on Hondas and Acuras from 1987 all the way to 2015, and on some other brands’ cars too.
The expense and difficulty of finding parts means that many of them were converted to regular, if slightly smaller, brakes from other Audi models.
The idea wasn’t exactly new. It was used by some aircraft before and has been found on some bicycles and motorcycles since. But this looks like the only time it’s been tried on a car. And it didn’t quite work.
That’s because not every fancy tech fix works. Some hit big and solve problems you didn’t know you had, but some fade to history. Even the failed ones can still be cool. Like this one, Audi’s UFO internal caliper brakes.

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