SunTour was the most popular brand of derailleur on bicycles sold in the USA for about a 10 year period, from 1975 to 1985. They had to the forefront with good performing, value oriented derailleurs, when the Europeans could supply the demand during the peak years of the early 1970s bicycle boom. Then, Shimano perfected index shifting with SIS in 1985 on Dura-Ace and trickled it down to the mid-range and entry levels in 1986. It was a huge marketing success, allowing Shimano to surpass SunTour. SunTour rushed their index system, Accushift, to market in 1987 but it was inferior to SIS and SunTour's reputation took a huge hit from which they never recovered. Shimano followed SIS with other shifting improvements including Hyperglide and STi which allowed them to further increase their popularity over other derailleur manufacturers. Game over for SunTour. For more background and details, read
Sunset for SunTour ,