Always competing as an amateur, Stig Mårtensson was one of Sweden’s greatest road cyclists in the 1950s. He was first Swedish junior champion in the time trial in 1946, and he later won nine Swedish national championships. He was road long-distance champion in 1950. In 1951 he won three titles, taking the time trial, road title, and team time trial championship. He repeated on the road and in the individual time trial in 1952. He was part of the Swedish champion team time trial champions in 1956, and in both 1955 and 1956 was on the team champion in the road race.
Sweden