The small capacity twin AT90 and its derivates.

The AT90 arrived in Dec. '65 when the Japanese market peaked at 2.2 million units. It had a YG1 type frame with the smallest twin in production slung underneath, just 90 cc. Bore stroke was 36,5 x 43 mm giving 8,2 hp at 8000 rpm. it was a cross between a small capacity street sportster and an utility bike. It was redesigned into the (2) HS1 in Dec. '68, same engine but with 89 kg 10 kg less than the AT90. (3) is the HS90 from Dec. '70, followed by the HX90 (Aug. '71) which was the last of these 90 cc models. This bike was the base for the later RD125 series. The RD90 (Apr. '74) looked as the HX90 but had a single cylinder engien and already a hydraulic front brake. In Mar. '77 the machine got more angular looks andreceived also the 7-port engine, giving 10,5 hp at 8.000 rpm. (7) RX125 is a 125 cc export version of the RD90 with enclosed chaincase making it very practical for commuting.

p84 Yamaha AT90, Dec. '65