The SRAM Dual Drive (DD) is the
outcome of the evolution of the previous Sachs 3x7. Both
feature an internally geared hub with a Shimano-compatible cassette driver to accept regular Shimano cassette sprockets. The internal hub offers a 36
per cent increase in gearing in overdrive and a reduction by the same amount in
low speed. This enables you to achieve
gears both higher and lower than a plain rear hub with multiple chainrings. The DD is standard equipment on the
GTT. Other models can be upgraded or
Disc version, $394
Twist shifter, $40 .
Also available as a trike upgrade at time of order:
SRAM Dual Drive User Manual
in English; more languages
Hardcore tech page, almost 7Mb.