splinter138:

First introduced in 1985 by SunTour, the 3-pully derailleur only lasted a couple of years. Intended for touring purposes and as one of SunTour’s early mountain bike designs, the 3-pully derailleur was designed to wrap a lot of chain and act as a long-cage derailleur while offering better ground clearance.This design was intended for bikes with triple-ring cranksets and could wrap up to 42t of chain in order to eliminate irritating chainslap.

Initially Nishiki came up with the design and licensed SunTour to build the 3-pully rear derailleur. It was exclusive to Nishiki bikes for the first year and later available for other bikes for the remainder of its short life. These derailleurs were offered in several different models, Lepree, AX, Cyclone and XC.

Although largely unnecessary and overbuilt, the 3-pully derailleur is still cool and unique in the cycling community.

Below is a link to a 3-pulley derailleur in action. http://www.youtube.com/watch?v=5HKmDrBuMrQ

(via chirosangaku)

Source: splinter138