In 1969, Auto Union merged with NSU, based in Neckarsulm, near Stuttgart. In the 1950s, NSU had been the world's largest manufacturer of motorcycles, but had moved on to produce small cars like the NSU Prinz, the TT and TTS versions of which are still popular as vintage race cars. NSU then focused on new rotary engines based on the ideas of Felix Wankel. In 1967, the new NSU Ro 80 was a car well ahead of its time in technical details such as aerodynamics, light weight, and safety. However, teething problems with the rotary engines put an end to the independence of NSU. The Neckarsulm plant is now used to produce the larger Audi models A6 and A8. The Neckarsulm factory is also home of the "quattro GmbH" (from November 2016 "Audi Sport GmbH"), a subsidiary responsible for development and production of Audi high-performance models: the R8 and the RS model range.