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.
2 thoughts on “Gear ratios for Torpedo 2-speed hubs”
Hi Philip. I have a Dahon Mu Uno single speed. I would like to install one of these Sachs Torpedo Automatic rear hubs. Any idea what they sell for and where I might buy one? Thanks Mike
I bought mine on eBay. It wasn’t very well titled, so not many people bid on it.
I bought a new driving ring from Jens Hansen, in Germany [hansen(at)scheunenfun.de] – I know he buys Automatics and Duomatics and sells replacement parts, but I don’t know if he sells entire hubs.
His site is in frames, but in Chrome you can open each frame in its own tab, and Google will translate it for you. If you don’t open the frames in tabs, it SAYS it’s been translated, but it ain’t! :^)
Actually… I just looked, and he has a red-band (what you’d want) on his homepage for 150 Euros:
Here’s a Sachs Automatic on German eBay for 99 Euros (~$130), and there’s another one up there with a Buy It Now of 200 Euros!
A request at Hubstripping.Wordpress.com might get you a good recommendation, as well.
Good luck – that Dahon looks cool! Have you considered one of the new Sturmey-Archer S2C kickback hubs?