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Ross = 40/44 rings
15t fixed cog wheel
17/21 double-fixed cog
19t cog on 2-speed hub – The 14 in the chart is the equivalent high gear from the auto-hub.
I want to build my new Torpedo Automatic hub into a wheel I can use on my Ross (frenchified touring bike), and my Quickbeam (british-style rough-stuff bike). I realized that replacement cogs are a lot cheaper than replacement chainrings, and easier to install. The same three-spline cogs work on Sturmey-Archer, Sachs (F&S), and Shimano internal gear hubs, and seem readily available: $8 some places, $15 from Harris, who has a comprehensive selection of sizes (14 to 23 teeth in both chain widths).
A 36 tooth ring would work perfectly with the 19 tooth sprocket the hub came with. Luckily it’s 3/32″ width, as well, rather than the 1/8″ bullshit BMX/track width. However, the chainring setup I have is perfect for each bike, and getting a 36 tooth chainring would almost necessitate a new bike. Or building the hub into a 26″ wheel, which just isn’t going to happen.
Since the rings on both bikes work great with the 3 fixed wheelsets I already have, one of which I JUST built up… the Torpedo cog was up for re-consideration.
My existing wheelsets have 15t cogs and Surly “dingle” cogs (really unfortunate name – I say ‘dingle,’ you say… ‘berry’). The double-fixed cogs work great on the Quickbeam for road and steep singletrack with the 32 and 40 tooth rings, and also on the Ross, with 40 and 44 tooth “Rocket Rings” from Harris Cyclery. You can see from the chart above the gear inches I can use.
After a lot of cogitation, mathematics, and plugging numbers into Sheldon Brown’s gear calculator, I decided that the best cog for the Torpedo Automatic… was the one it came with! A 19 tooth 3/32″ dished cog.
I think that will give me the most fun and versatility with each bike. The ‘road gear’ on the Quickbeam is crazy-low at 62 inches, but it also usually has larger tires (35 to 42 mm), which knocks it up an inch or two. On the Ross, the high gear is 77 inches, and the low gear is 57″, both higher than I prefer, but very usable.
There’s no way I would be doing this right now and considering it fun, if it weren’t for Sheldon Brown. Viva Sheldon!
I can't even figure out where I was going with this. I'm putting a 36t ring on the Quickbeam, and see how it works.
With 40mm tire adn 170mm cranks, a 36t ring and a 19t cog on the Torpedo Automatic gives:
low = 51.8" gear
high = 70.44" gear.
Perfect.
The high is calculated by multiplying the low gear by 1.36, since the high gear on the Automatic and Duomatic is 36% higher than the low.