The first feature most people noticed was that, instead of the usual, staid, black-or-white BMW finish, the R90S had smoke-effect paintwork that made it seem either avant-garde or, depending on your opinion, the victim of a cellulose shortage.
The first BMW to receive the attentions of a dedicated stylist, the R90S owed its appearance to a former Ford designer called Hans Muth. With its kick-tail seat and revolutionary bikini fairing, this upstart BMW looked capable of providing a bit more fun than the boxer plodders everyone had come to know and love, a suggestion backed up by the presence of sporty Dell'Orto carbs in place of the usual Bings and not just one, but two disc brakes up front.
To further enhance its image as a bit of a tearaway, the R90S was even adorned with attractive females in skin-tight leather for advertising purposes – another first for the Bavarian manufacturer.
But the R90S was go as much as show, thanks to a combination of those new carbs and high-compression heads which boosted power to 67bhp, sufficient to push the bike to almost 125mph despite the weight of the Cardan shaft drive which remained BMW's favoured form of getting the power to the back wheel.
Shaft drive or not, it was still more than 50lb lighter than the other hot 900cc machine of the time, Kawasaki's Z1. Despite the Japanese bike's extra 15bhp, the BMW could easily show it a clean pair of heels through the bends.
It all added up to a fun package, yet the R90S retained the reliability and comfort as a long-distance tourer for which its less sporty brethren had become renowned. If it could prove itself as a racer, then its place in the motorcycling hall of fame would be assured.
Enter Helmut Dähne, archetypal amateur road racer and famed in Germany during the early 1970s for his efforts in the Isle of Man TT. In 1974, on a bike that sported the R90S smoked paint scheme but was actually an earlier R75 with a host of R90 upgrades and other tweaks, he managed second in the production race to fellow German Hans Butenuth.
The following year, in the 226-mile, six-lap production race, Dähne was holding a commanding lead over Chas Mortimer on a TZ Yamaha when the Achilles Heel of all old BMW race bikes – the protruding rocker covers of the flat twin engine – let him down. A stone punched through the road-abraided cover and the oil drained out.
Nevertheless, the world loves a hero (and his motorcycle) and, although the R90S was superseded by the more sophisticated R100 models of 1976, it remains the first BMW to truly rock.
Low-mileage examples still wearing their original smoky paint scheme and bikini fairings are now highly sought-after and can command more than £10,000 in mint condition. Less pristine models are cheaper and still worth hunting down, not least because built-in BMW reliability means they make great classic bikes for regular use.
Just be sure to keep those rocker covers off the deck...