Planetary Gear Crankset
The Schlumpf Mountain Drive is a planetary geared crankset spider that acts like two chainrings on the front of your bike. In high gear, it uses a 1:1 ratio and gives you a gear of whichever chainring size you have installed. In low gear mode, it bumps the chainring size down by a factor of 2.5 for a 2.5:1 gearing ratio.
For example, with a 50T chainring, the Schlumpf Speed-Drive makes it seem like a 22T chainring when operating in geared mode.
The Schlumpf Mountain-Drive gives any bike an ultimate granny gear by gearing down the existing drivetrain. Without a front derailleur, The Mountain Drive in conjunction with a conventional rear cassette, gives the rider a huge gear range with almost no gear overlap. With a front derailleur, the gear overlap helps ease transitions between lower and higher gearsets.
With a 20" rear wheel using an 11-34T rear cassette and two front chainring sizes of 53T and 39T, you can expect a gear range of approximately 8.6 - 90 gear inches, with a fair bit of overlap (that is NOT a typo).
The standard Mountain-Drive comes with a chainring. One or two side chainguards / trouser-guards can be added. The standard bolt drilling is 130bcd (pcd), but the Speed-Drive can be ordered with a 110 bcd bolt patter to fit the smaller 110 pcd chainring sizes.
Althought Rohloff officially recommends against it, recumbent trike riders looking for a huge gear-range while eliminating the fuss of derailleurs can effectively use the Schlumpf MountainDrive in conjunction with the Rohloff Speedhub. On a trike with 20" wheels, the following gearing ranges would be reasonable (Chainring-53/Cog-13 -> 8.5-112 gear inches or Chainring-65/Cog-15 -> 9.1-119)