The SRAM Dual Drive

 

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 gto, gtr, gtt, gte, gts and gth. Other models can be upgraded or retro-fitted.

Prices in AUD unless otherwise stated. 

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Standard hub,  $357 (USD$310).
Disc version,  $394 (USD$330).
Twist shifter,  $40 (USD$30).
Clickbox,  $43 (USD$36).
Also available as a trike upgrade at time of order: 
Models gt3/gt5, x3/x5  $299 (USD$230).

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SRAM Dual Drive User Manual in English; more languages available.
Hardcore tech page, almost 7Mb.