Schlumpf double-fixed

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.

HammerSchmidt double-singlespeed gearing

Does the HammerSchmidt crankset work with a fixed wheel?

Nope. The cranks themselves freewheel, so the bike would no longer be fixed. If you put Hammerschmidt cranks on a fixed-gear, it will turn it into a freewheeling-enabled double-singlespeed bike.

But, yep. Nothing would be damaged, it just wouldn’t be a fixed wheel. And… you could run a superlight singlespeed rear wheel. A Shimano or White freewheel weighs a lot more than a 15t (or 12t Phil!) fixed cog.

Can I use this thing on a road bike?

Sure. With a little help from a framebuilder. You’d need the proper “ISCG 03 or ISCG 05 tabs” (whatever those are) retrofit to the bottom bracket.

The HammerSchmidt only has two chainring sizes available: 22t and 24t, which forces you to choose a very small cog if you want to use it on the road.

Road Gearing

I had thought the spread was too big, until I did the number-crunching. I like a wide range, because of where I like to ride. A normal gear for most times, and a low low for climbing.

  • 24/15 = 44″ and 70″ gears – I’d run this setup any day.
  • 24/14 = 47″ and 75″ gears – If it was a fixed drivetrain, it would be ideal.
  • 24/12 = 54″ and 87″ gears (for time trialing over mountain passes?)

Offroad XC (non-Downhill riding)

  • 22/15 = 40″ and 64″
  • 24/16 = 41″ and 65″

I calculated the low gears with Sheldon’s gear calculator, then multiplied by 1.65 to get the high gears.

I could have (should have) just chosen Schlumpf Speed Drive Bottom Bracket from the “internal gear hub” dropdown, since the multiplier is also 1.65. The numbers aren’t the same, but they’re close.

These gears and opinions are theoretical in nature – follow your own folly.
I would really like to hear about (and see) any road bikes set up with HammerSchmidt crankset. And if Truvativ wants me to test one, I think I can add the tabs…

Gear ratios for Torpedo 2-speed hubs

Figuring your gears is a little tricky with a two-speed hub that shifts by itself. The low gear is 1:1, the high gear is 1:1.36.

The low gear is direct-drive. There’s no fussing; the gear you calculate on Sheldon’s gear calculator is the actual low gear. It is the 1:1 gear.

The high gear is 36% larger. I plugged in larger cog sizes until I got ones about exactly 36% larger. I just noticed that Sheldon’s calculator will let you add fractional cogs!

I’m a double idiot – I just noticed that Sheldon’s gear calculator lets you choose a “Sachs Duomatic / Automatic” hub in the Internal Gears dropdown. This way is a little easier to read, though.

I have a 19t sprocket, and a 22t sprocket. I also have 39, 40 and 42 tooth chainrings I think might work for this project.

The “high gear equivalent” for the 19t is 13.97 (let’s call it a 14). The “high gear equivalent” for the 22t sprocket is 16.18. Again, 16 is plenty close.

With a 19t cog and a 39t chainring, I get a low gear of 56″ and a high of 76″

19t low
56.1 57.5 60.4
36.0 %
high 76.3 78.2 82.1

With a 22t cog and a 42t ring, I get my ideal setup:  52″ low and 71″ high gear.

39 2.6 % 40 5.0 % 42
22t low 48.4 49.7 52.2
36.0 %
high 65.9 67.5 70.9

The Sachs Torpedo 2-speed gives a slightly wider ratio than my double fixed setups that use a 17/21 Surly dingle cog and a four-tooth chainring difference.