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″
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|
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.