The Schlumpf Mountain drive would give broader dynamic range than the Speed Drive.
It might give you too much torque throught he hub, though there were faqs out there from SRAM on how to mate the S7 to the Mountan Drive.
A Schlumpf with track/fixie rear hb could give you a 2 speed as well.
Sheldon Brown acquired an ASC hub in 2005 and mated it to a Schlumpf to give him a 6 speed.
The mountain drive has way too big a step between ratios to work well with the range of the S3X or ASC IMO. Ity would give two three speed ranges with a large gap between them. The step between low and high is 250%.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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