This gear caused me distress and dismay when it appeared. SunTour seemed to have abandoned the best design in the world (the slant parallelogram) and copied the geometry of the much despised 1976 Huret Eco, which was, in turn, the bastard child of the 1963 Huret Svelto.

The design did have some interesting details. SunTour had adopted a ‘Light Action’ style sprung cable clamp (in imitation of one of Shimano’s better features). The top pulley was dramatically off-set from the lower pivot, in an attempt to get the Huret geometry to provide a decent change over a wide freewheel.

Despite these touches SunTour’s decision to adopt this geometry was like Ferrari deciding to copy a Lada.

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• Derailleur brands: SunTour
• Categories: SunTour - Frank Berto and the curse of Duopar
• Country: Japan
• Date of introduction: 1982?
• Date of this example: 1982 (two letter date code is YG)
• Model no.: 5100
• Weight: 308g including hanger plate
• Maximum cog: 34 teeth (SunTour assuming 26mm dropout)
• Total capacity: 20 teeth (SunTour assuming 26mm dropout)
• Pulley centre to centre: 60mm
• Index compatibility: friction
• Chain width: 3/32”
• Logic: top normal
• B pivot: unsprung
• P pivot: sprung
• Materials: largely steel, aluminium top knuckle