top frame tube was activated by a cable from the front brake lever. This arrangement was supposed to protect the master cylinder in the event of a crash; but later "/7" machines adopted a conventional handlebar-mounted Brembo master cylinder. There were three series of the R90S: model year 1974: September 1973 to August 1974 (6,058 units) model year 1975: June 1974 to September 1975 (6,413 units) - A strengthened crankshaft has a new main bearing to prevent flex. model year 1976: August 1975 to June 1976 (4,984 units) - Engine casings are improved in anticipation of the forthcoming R100S. Other differences distinguish the three series: After the first series, brake discs were drilled. The first series kept the "/5" series kick start lever. The later 17mm diameter front axle was stronger than the first series 14mm axle. The original R90S had "/5" switchgear, but this was changed in 1975 to a new non-standard design that proved unpopular. The engine crankcase breather was a phenolic disc-&-spring item which made a 'popping' sound as the breather operated. The modified "/7" breather was a reed valve design, which could be retro-fitted to "/6" models to replace their noisy breathers. Production numbers From 1973 to 1976, 17,455 R90S models were sold. The R90S was succeeded in 1977 by the R100S, which retained the bikini fairing with altered paintwork, and whose engine capacity was increased to 1,000 cc. The R100S reverted to using 40 mm Bing CV carbs. The BMW flagship mantle passed to the R100RS, which now sported a full fairing. Other factory variants included the R100CS of 1983–1984. The R90S always had "/5" type "round" valve covers, but the R100CS had black-painted "squared" valve covers. The other bikes in the BMW "/6" range were the new R90/6, the R75/6, and the R60/6. The 500cc R50/5 was discontinued and no "R50/6" version was produced. Legacy A later example of the R259 oilhead BMW, the R850/1100R, sported a similar two-tone smoke grey paint scheme in homage to the original BMW 'superbike' color scheme. Two R90S bikes placed first and second at the very first AMA Superbike race at Daytona International Speedway in 1976. On the first place bike was Steve McLaughlin and on the second was Reg Pridmore. The first place bike is now in the BMW Mobil Tradition Collection. The first United States AMA Superbike Championship was won in 1976 by British rider Reg