On pure performance. BMW's R90S was not the fastest superbike of the mid-1970s. Nor was it the quickest around a racetrack: at least, not often. But for a rider with a long distance to travel on straight roads and twisty ones, there was nothing on two wheels that would prove faster or more comfortable than the uniquely well-equipped flat twin. 

The BMW's distinctive smoked paint scheme was perfectly in keeping with its image as a very refined and expensive sports-tourer. In many respects, the R90S was the best all-round superbike of its day. Certainly, no other production machine could match its combination of I25mph (2()lkm/h) top speed, relaxed high-speed cruising ability, fine handling, reliability and impeccable finish. 

BMW had plenty of practice in building horizontally opposed twins, and it showed. The R90S came from a line of fiat twins stretching back to Max Fritz's original R32 of 1923. The German firm had been building competent, comfortable and conservative tourers for many years. But this bike, shaped by noted stylist Hans Muth. had a distinctly more aggressive personality.  The basics were typical BMW. Like the naked R90/6. which was launched at the same time in 1973. the R90S owed much to the previous year's R75. Enlarging the 745cc R75's bore from 82 to 90mm while retaining the 70.6mm stroke increased capacity to 898cc. BMW also made a few other updates, including a revised gearchange mechanism, more powerful 280W alternator and stronger bottom end.  There was more to the S-bike than just its fairing.

The engine was given a higher compression ratio. 9.5:1 against the 9:1 of the R90/6. and breathed in through big 38mm Dell'Orto carbs with accelerator pumps. Maximum power was 67bhp at 7()00rpni. an increase of 7bhp over the standard model. The R90S also had a steering damper in its cockpit, a larger fuel tank and a second front brake disc.  The extra power gave the S model a worthwhile boost at high revs, without hindering either its torquey mid-range response or its smoothness at most engine speeds. Better still, the fairing meant the engine's performance could be fully exploited by the rider, who was free of the wind-blast generated by every rival superbike. Other manufacturers would soon follow BMW's lead, but for the moment the fairing put the R90S in a class of its own.

Source Fast Bikes by Roland Brown

Motorcycle-USA review

As the 1960s departed and the new, flower-tied, flared-jeaned and pink-shirted 1970s were born, BMW hit the biking world with a whole range of truly outstanding motorcycles. And at the very top of this new, shiny tree was the R90S.

The R90S was one of the cleverest motorcycles ever to be produced and was proof positive that deep in southern Germany BMW's marketing team really understood the problem they faced. In addressing the factory's image of staid, dull reliability they managed to do that smartest of all three card tricks: retain their existing customers, bring on new ones - and do so cost-effectively.

Technically, the R90S was light years behind the best of the Japanese and was nothing more than a modest technical step up from earlier Boxer Twins. Thus, the shaftdrive was retained, as was the two-valve, horizontally opposed, push-rod twin-cylinder engine. This was not state-of-the-art engineering by any stretch of the imagination. But what an engine the Germans produced! The new powerplant, with its five-speed gearbox, still looked, sounded and felt like previous BMWs, but this was a motor which had been to the gym for a serious work out.

The three aces the R90S had to play were usability, fitness for purpose and utter, totally, unburstable reliability. With at top speed of less than 120 mph, the R90S lost out to the true hyper sports bikes of the day and was, on paper, no faster than Triumph's 750 Bonneville or the Honda CB750. The big difference was that the BMW's 120-mph top speed equated to a solid 115-mph cruising performance.

This point needs stressing. Riding an R90S, it was comfortably, practically and realistically possible to cover 90 miles in an hour and 150 plus in two hours. Nothing else in the bike world, and very few cars of the time, could offer this supreme distance-devouring performance. Miles didn't disappear beneath the BMW's wheels - whole countries simply zipped past in a satisfying blur.

Critics of the motor complained, justifiably, that the gearbox was dreadful compared with the Kawasaki Z1 - which it was. BSA group Triple owners boasted that their bikes offered vastly better handling - and this too was true. And the supremely refined Honda CB750 was infinitely smoother. But when the BMW rider looked at his map and saw that there was 1500 miles of hard riding ahead, there was always going to be only one winner.

The chassis complemented the motor wonderfully. The R90S was never the hyper sports bike which BMW claimed but it was a supremely confident high-speed performer. With long-travel suspension, superb rider ergonomics and a large fuel tank, the bike allowed the rider to make use of the motor's abilities. As pilots of modern sports bikes know all too well, there is no point in having limitless performance if the riding position causes agony after 50 miles.

There was also a delightful attention to detail which inculcated a sense of pride in owning a BMW. As an example, the discs were drilled not only for lightness but also to enhance their performance in wet weather. So far, so good. What was really clever was that BMW first cadmium plated the disc so that the drilled holes wouldn't rust and then surface ground the rotors. Knowledgeable observers stood back, looked at the BMW and nodded sagely at the outstanding fit and finish. The R90S was expensive but looked to be an out-and-out bargain when compared with its British and Japanese opposition.

And last but not least, there was the R90S' appearance. In a deliberate attempt to put the black and dour images behind them, BMW produced their new flagship in a range of stunning air-brushed colors ranging from burnt orange to a magnificent air burst grey.

The impact of the styling was immediate and dramatic. Suddenly, for the wealthy, the R90S became the bike to have. Film stars, Chief Executives and Wall Street Bankers became BMW owners, and in doing so opened up a whole new market for the Bavarian bikes.

Yet the really clever thing was that hardcore BMW enthusiasts still stayed with the factory because of the bike's mechanical prowess: truly, the hardest thing for any company to pull off.

Source Motorcycle-USA