Like the Truvativ HammerSchmidt, and predating it by about 10 years, is a Swiss bottom-bracket two-speed planetary gear changer. It comes in several types (1:1.65; 2.5:1; 1:2.5).
Can I use the Schlumpf with a fixed gear?
Yep. Ever since late 2009, according to their website. The older ones no, the newest ones, yes. The internals are symmetrical, so they engage as well forward as backwards.
You can use a Schlumpf for a two-speed fixed setup, or mate it to a 3-speed fixed gear hub for six fixed gears.
The SpeedDrive has the same 1:1.65 gear ratio as the HammerSchmidt, so the same nice spread can be had on a road-going fixed-gear. There are lots of chainrings available (9, vs 2 for the SRAM HammerSchmidt): 27, 28, 30, 32, 34, 36, 38, 40 or 42. The rings are bigger, so your cogs are going to be bigger to get the same gears. That’s a bonus, since bigger drivetrain parts last longer.
A 34t ring and a 21t cog would give you a 44″ low and a 73″ high gear. I like that.
If you’d like to play with the numbers, Sheldon Brown’s gear calculator has a dropdown for the Schlumpf. Of course it does!
Can I slap this on my bike and go?
Nope. Just like for the HammerSchmidt you’ll have to modify your frame. The Schlumpf needs to have a 45° angle beveled into your bottom bracket to set against. Schlumpf says they can rent you a mitering machine “in most countries.” The bevel doesn’t keep you from reinstalling a normal bottom bracket on your frame, so it’s non-destructive.
What’s with the crazy names?
All the best high-performance, sturdy internal gear and niche-market hubs have intense chunky German names: Rohloff, Schmidt, Fichtel und Sachs, Schlumpf. I’m sure that’s why SRAM made up “HammerSchmidt.” It sounds bad ass.