The 425 S had a twin-shock swinging arm rear suspension and a top speed of . It had wheels with full-width drum brakes to give it better acceleration and braking than the plunger model. Initially the 425 S had 7.2:1 compression and produced 14 bhp; in 1961 this was increased to 8.3:1 which increased power output to 15.5 bhp. Simson made the 425 S from 1955 until 1962 and a total of 85,000 were built.
