Renovated JR Jackson

Renovated JR Jackson, originally uploaded by capnahabnick.

Man, that’s cool. Look at that machine. Click the picture and look at the whole set. Clearance to spare, hubs, rims, flared drops. Kickback(?) Fichtel & Sachs hub. White hoods, beautiful old saddle.

And you probably missed it, but if you look closely you can see the barber pole seat tube. Too good. How many other bikes could carry that off? (The answer is “none.”)

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.